ON THE ROLE OF VOCAL EMOTIONS FOR VERBAL MEMORY

ON THE ROLE OF VOCAL EMOTIONS FOR VERBAL MEMORY: AN INVESTIGATION OF NEURAL AND PSYCHOPHYSIOLOGICAL MECHANISMS CHAN PEI LING, KAREN (B.Soc.Sci., Psychology, NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTERS IN SOCIAL SCIENCES DEPARTMENT OF PSYCHOLOGY NATIONAL UNIVERSITY OF SINGAPORE 2012 i Acknowledgments This entire page is dedicated to all who have helped in some way or another in different stages of my thesis preparation and production. During the course of my academic pursuit I have formed new friendships, particularly with folks at the Brain and Behavior Lab as well as fellow Masters students. Amongst them are Shimin, Eric, Darshini, Nicolas and Cisy, from whom I have learnt much about the process of doing good research. I thank Eric for his positive support and always giving constructive criticisms and suggestions that spurs me on to improve my research, Shimin for her helpfulness and her contagious readiness to see things from a different perspective, Darshini for her spontaneity in organizing outdoor activities that has helped me lead a healthy, all-rounded student life, Nicolas and Cisy for the enjoyable discussions with brilliant ideas, humor and also technical knowledge like professional researchers. I’d also like to thank many others for their guidance and provisions. To Dr. Annett Schirmer, who has taught me much about the research arena – thanks for your guidance. Also to Dr. Trevor Penney, I enjoyed the interesting conversations and am always amazed by your insights pertaining to different aspects of life. I’m grateful to Prof Michael Chee, who has summoned much expertise during my data collection period - Soon for his help in the initial piloting phase, Weiyan for conducting the fMRI scans. Thanks also to Christy and April for helping in the data collection. Last but not the least, I owe all to my fiancé, Calvin, who has always been around for me rain (when the machine broke down) or shine (when I get new insights to my findings). His patience and encouraging spirit has given me a huge leap forward in this journey. A big Thanks to all of you! ii Summary The present study explored the neural and psychophysiological substrates underlying the influence of vocal emotions on verbal memory. Heart rate (Experiment 1) and fMRI data (Experiment 2) were acquired while participants performed a verbal memory task comprising an encoding phase and a test phase. In the encoding phase, participants were asked to memorize a series of words spoken with either a neutral or sad prosody. During the test phase, participants were presented with written words and indicated whether they had previously been studied. During encoding, attending to sad prosody as compared to neutral prosody elicited greater heart rate (HR) deceleration and greater activity in the bilateral superior temporal gyrus, superior temporal sulcus and right transverse temporal gyrus. At test, words previously heard with a sad prosody were remembered less accurately than words previously heard with a neutral prosody. Moreover, the former were rated more negatively than the latter. While the encoding effects observed here failed to predict test effects, there was a correlation between HR acceleration and memory. Specifically, a greater HR acceleration to words with sad as compared to neutral prosody was associated with a reduced memory deficit for sadly as compared to neutrally spoken words. This may be mediated by the relationship between sympathetic arousal and memory. Implications of current findings are discussed in relation to vocal communication and future directions proposed to further elucidate the complex relationship between prosody and verbal memory. Keywords: vocal, emotion, prosody, verbal, memory, superior temporal gyrus, sadness, heart rate, cardiac. iii Table of Contents Acknowledgement ....................................................................................................... i Summary ..................................................................................................................... ii Table of Contents ...................................................................................................... iii List of Tables ...............................................................................................................v List of Figures ............................................................................................................ vi Chapter 1: Introduction Prosody and emotional expression ................................................................................1 Effects of speaker prosody on verbal memory .............................................................4 Heart rate studies on emotion and memory ..................................................................6 fMRI studies on emotional processing ..........................................................................8 Individual variation in neural and heart rate responses to emotional stimuli ......................................................................................................................................10 fMRI studies on verbal memory .................................................................................11 Thesis objectives ..........................................................................................................12 Chapter 2: Experiment 1 Objectives ...................................................................................................................15 Methods .......................................................................................................................15 Results ..........................................................................................................................22 Discussion ....................................................................................................................23 Chapter 3: Experiment 2 Objectives ...................................................................................................................25 Methods .......................................................................................................................26 iv Results ..........................................................................................................................30 Discussion ....................................................................................................................34 Chapter 4: General Discussion Heart rate and neural correlates of prosody encoding .................................................36 Effects of prosody on verbal memory performance ....................................................38 Effects of prosody on word valence judgment ............................................................42 Limitations and future directions .................................................................................43 Conclusions ..................................................................................................................44 Bibliography ................................................................................................................45 v List of Tables Table 1. Table illustrating means and standard deviations for heart rate deceleration and acceleration in response to words spoken with the neutral and sad prosody (Experiment 1) ..................................................................................................................21 Table 2. Table illustrating means and standard deviations of mean dprime scores valence ratings for words spoken with the neutral and sad prosody (Experiment 1) .......22 Table 3. Table illustrating means and standard deviations of dprime scores for words spoken with the neutral and sad prosody (Experiment 2) ..................................................29 Table 4. Table illustrating peak activations for hitemo > hitneu contrast for the study phase (Experiment 2) ........................................................................................................30 Table 5. Table illustrating peak activations for hits > correct rejections contrast for the test phase (Experiment 2) ..................................................................................................33 vi List of Figures Figure 1.1. Figure illustrating the sequence of stimulus presentation during a study phase ..................................................................................................................................17 Figure 1.2. Figure illustrating the sequence of stimulus presentation during a test phase ..................................................................................................................................18 Figure 2. The QRS complex . ............................................................................................19 Figure 3. A time-series plot illustrating how maximum HR deceleration and acceleration were computed for each participant ..............................................................20 Figure 4. Figures illustrating regions that show greater activity for words spoken in negative as compared to neutral intonation . .....................................................................31 1 Chapter 1: Introduction The utility of verbal memory is ubiquitous in our daily lives. It serves the mastery of many tasks including those linked to academic and classroom performance. Hence, understanding the processes that enable verbal memory is of general interest and has spurred much research in the area of cognitive psychology. The present thesis extends this research by scrutinizing a factor that has hitherto been neglected. Specifically, it reports two studies that explored whether and how a speaker’s vocal tone or prosody influences listener memory for communication content. In the following, I will shortly introduce prosody as a means of emotional expression and detail behavioral, psychophysiological and neurological research on prosody processing and verbal memory. Prosody and emotional expression Prosody is defined by the suprasegmental features of an utterance. These features include pitch (or fundamental frequency), amplitude, rhythm and voice quality among others. By affecting the vocal apparatus (e.g., rate of breathing, muscular tension), emotions induce changes in these acoustic features and thus prosody. Given basic regularities in the way emotions affect vocalizations, individuals can use prosody to make inferences about a speaker’s emotional state (Banse & Scherer, 1996; Scherer, 1986). Research suggests that such inferences can be fast and automatic and guide listener attention. Evidence to this effect comes from behavioral, electrophysiological and neuroimaging research as detailed below. Behavioral evidence for automatic prosodic processing and attention capture comes from a range of studies (Brosch, Grandjean, Sander, & Scherer, 2008; Schirmer, Kotz, & Friederici, 2002; Schirmer & Kotz, 2003). In one of these studies, 2 the authors employed a cross-modal dot-probe paradigm in which participants indicated whether a dot appeared on the left or right side of a screen. The authors paired dots with task-irrelevant nonverbal exclamations that sounded angry on one ear and neutral on the other. Participants were faster at responding to a dot if it appeared on the side of an angry as compared to a neutral exclamation (Brosch, Grandjean, Sander, & Scherer, 2008). Thus, one may infer that emotional prosody is processed even if it is task-irrelevant and that it modulates spatial attention. Other existing work corroborates this inference using male and female voices as well as happy, angry, sad and neutral prosody (Schirmer, Kotz, & Friederici, 2002; Schirmer & Kotz, 2003). Electrophysiological studies provide further evidence that emotional prosody is automatically processed and captures attention. Additionally, they outline a temporal course for its influence (Schirmer, Striano, & Friederici, 2005; Schirmer, Escoffier, & Simpson, 2007). In an auditory event-related potential (ERP) study by Schirmer and colleagues (2005), the authors employed an oddball paradigm in which participants were presented with an auditory sequence consisting of rare ‘deviants’ and a series of frequent ‘standards’. The auditory sequence was played in the background while participants watched a silent movie with subtitles. In one experimental block, the deviant was an emotionally spoken syllable, while the standard was a neutrally spoken syllable. In another experimental block, the deviant was a neutrally spoken syllable, while the standard was an emotionally spoken syllable. The authors measured the mismatch negativity (MMN), an ERP component that presumably reflects pre-attentive change detection (Näätänen & Alho, 1995; Näätänen, 2001). It can be visualized by subtracting the ERP elicited by standards from those elicited by deviants. When performing such subtractions, Schirmer and colleagues found a greater MMN in response to emotionally spoken ‘deviants’ as compared to the neutral 3 ‘deviants’. This suggests that listeners can discriminate tone of voice pre-attentively. Moreover, given that the MMN amplitude is thought to indicate the likelihood of attention capture (Näätänen & Alho, 1995), one may infer that emotionally spoken material is more likely than neutral material to engage attention that is initially directed elsewhere. Finally, support for the preferential processing of emotional relative to neutral prosody comes from neuroimaging studies. For example, a study by Grandjean and colleagues (2005) examined brain responses to meaningless utterances pronounced with either emotional or neutral prosody and found that emotional prosody elicited enhanced responses in the superior temporal sulcus (STS) relative to neutral prosody. This was accompanied by greater activity in the right amygdala regardless of whether sounds were task-relevant or irrelevant (Grandjean, Sander, Pourtois, Schwartz, Seghier, Scherer, & Vuilleumier, 2005). Other neuroimaging work is in line with this (for a review see Schirmer & Kotz, 2006). Together behavioral, electrophysiological and functional neuroimaging work indicate that emotional prosody is processed automatically and guides listener attention. Thus, one might ask whether it could benefit the encoding and memory storage of concurrently presented verbal information. Moreover, given that prosody is typically tied to a verbal message, the existing work raises the possibility that the storage of such a message depends on whether prosody is emotional or neutral. Two aspects of memory storage have been investigated in this respect. First, researchers have looked at whether emotionally spoken material is better retained in memory than neutrally spoken material. Second, researchers have examined the effect of emotional prosody on the emotional connotation of verbal information maintained in long-term memory. In the following section, I will present their findings. 4 Effects of speaker prosody on verbal memory Effects of emotional prosody on verbal memory have been examined by Kitayama (1996). The author tested the effects of emotional prosody on memory under different memory load conditions. In his study, participants performed a memory span task which required them to memorize either two (low load) or four (high load) two-digit numbers for 20s. During the 20s interval, a sentence was presented as a distraction stimulus. Participants were told to ignore this distractor so that they could perform at their best for the memory span task. They were then given a surprise recall and recognition test of the sentences. Prosody effects were assessed by comparing the percent recall for sentences spoken with the emotional and neutral prosody. First, the free recall protocols were coded with a gist criterion. A recalled item was coded correct if it was uniquely identified with any one of the 24 sentences. Percent recall was then computed for each condition. Findings revealed that when the task was demanding (high load), verbal memory (recall) was better if the message was delivered in an emotional tone of voice than if it was delivered in a neutral prosody. In other words, when the task is demanding, emotional prosody enabled participants to recall more sentences. However, when participants had to memorize only two twodigit numbers (low load), memory tended to be worse when the sentences were spoken with an emotional prosody than when they were spoken with a neutral prosody. Kitayama had also replicated his findings in a second study using recognition as an additional dependent measure. This time round participants were not only given a surprise free recall test but were also asked to select old from among new sentences and to indicate their level of confidence in this selection. When the memory load was low, results for recognition memory paralleled that of the free recall memory in that memory for sentences spoken with the neutral prosody was better than that for 5 sentences spoken with the emotional prosody. In contrast, when memory load was high, memory for both types of sentences were comparable. Findings suggest that emotional prosody can either improve or impair memory for verbal content, and that the effect of emotional prosody on memory depend largely on memory load and the retrieval method employed at test (Kitayama, 1996). A recent study by Schirmer (2010) also explored the effect of speaker prosody on the memory representation of words. In this study, participants performed a crossmodal verbal memory paradigm. During encoding, participants heard words spoken with either an emotional or neutral prosody that were presented at intervals of one second. At recognition, words were presented visually and participants made an oldnew judgment. Memory recognition performance was comparable for words spoken with emotional and neutral prosody (Schirmer, 2010). Together with the findings by Kitayama, this suggests that any memory benefit for emotionally as compared to neutrally spoken material may show for free recall only. However, verbal recognition may not benefit and, according to the results by Kitayama, potentially suffer from emotional prosody. At present it is still unclear what determines the relationship between prosody and verbal memory. However, it is clear that this relationship is more complex than what has been observed for the relationship between prosody and attention. Although encoding prosody does not seem to have a consistent effect on subsequent word recognition, there is evidence that it reliably modulates another aspect of long-term verbal memory (Schirmer, 2010). Specifically, Schirmer found that encoding prosody significantly modulated listener’s attitudes towards verbal information (Schirmer, 2010). Correctly recognized words that were previously heard with a sad prosody were subsequently rated as more negative compared to words 6 encoded with a neutral prosody. The reversed pattern was found when the author compared happy and neutral prosody. Interestingly, these effects were independent of the listener’s ability to consciously recollect speaker prosody suggesting that they reflect changes in the words’ stored affective representations rather than the conscious retrieval of encoding prosody. Moreover, they indicate that the valence of words stored in memory is not fixed and can be adjusted dynamically based on the emotional context in which these words are encountered. A recent study using electroencephalography (Schirmer et al. in preparation), replicated these results and further outlined the time course of prosody encoding processes that underlie the observed change in affective memory. The present thesis was aimed to extend this work by studying the role of emotion related autonomic changes and the involvement of neuroanatomical substrates. The former was achieved by measuring event-related changes in heart rate. The latter was achieved by measuring event-related changes in brain activity using fMRI. In the following, I will review both measures and their utilization in previous studies on emotion and memory. Heart rate studies on emotion and memory Heart rate can be measured as sustained heart rate, heart rate variability and eventrelated heart rate, with the latter being of interest here. An event-related heart rate (HR) response is a change induced by a stimulus lasting a few seconds (Jennings, 1981); this change is typically triggered within less than a second following stimulus onset and may last up to several seconds thereafter. The event-related heart rate requires the observation of individual heart beats and is generally assessed by interpolating and averaging beat-to-beat intervals across trials. 7 In the past, researchers have measured HR responses to stimuli that vary with respect to valence. Bradley and Lang (for a review see Bradley & Lang, 2000a), for instance, presented participants with pleasant and unpleasant pictures and found that both elicited an initial HR deceleration followed by a HR acceleration. Moreover, the initial deceleration was greater for both pleasant and unpleasant as compared to neutral pictures. These results were replicated with other stimuli such as environmental sounds (Bradley & Lang, 2000b; Palomba, Angrilli & Mini, 1997, 2000) leading researchers to argue that HR deceleration reflects the emotional intensity of a perceived stimulus. This and related work also inspired the idea that HR deceleration is linked to stimulus intake or an orienting response, which promotes attention to information of high survival value. The HR acceleration that typically follows an initial deceleration has been linked to cognitive processing effort (Lacey & Lacey, 1979; Barry, Robert, Tremayne & Patsy, 1987). Its role in emotional processing is still equivocal. In a study by Harrison and Turpin (2003), the authors examined whether individuals who were high on anxiety show a bias to threat-related material. Heart rate measures were obtained while participants performed a memory task consisting of an encoding phase and a test phase. During encoding, participants viewed threat and non-threat words. At test, they were presented with word stems and asked to complete these words on a response sheet. Upon completion, each word was rated based on the level of threat associated. An initial HR deceleration and subsequent HR acceleration was observed. The authors found a greater HR deceleration in response to threat stimuli as opposed to non-threat stimuli for all participants. However, they found also that non-threat stimuli induced a greater subsequent HR acceleration as compared to threat stimuli and that they were better remembered (Harrison & Turpin, 2003). Somewhat different 8 results were observed by Buchanan and colleagues (2006), who presented participants with neutral-unrelated words, school-related words, moderately arousing unpleasant words and highly arousing taboo words. Participants were told to attend to the words and remember as many as possible for a subsequent recall and recognition test. The authors noted greater HR deceleration in response to unpleasant words that were subsequently remembered as compared to those that were forgotten. In addition, highly arousing taboo words were found to induce greater HR acceleration as compared to moderately arousing unpleasant words (Buchanan, Etzel, Adolphs, & Tranel, 2006). Although both studies found a greater HR deceleration for threatening words and taboo words, there seems to be a discrepancy with respect to HR acceleration. These may stem from the nature of the stimuli and call for further investigations. fMRI studies on emotional processing The last century has seen an explosion in the number of studies that used noninvasive techniques such as functional magnetic resonance imaging (fMRI) to examine the neural processes that underlie psychological phenomena. fMRI is a brain imaging technique that measures changes in blood oxygenation that appear to be linked to neural activity (Ogawa, 1990). There are several reasons for using fMRI as a tool for studying neural processes. One reason is that unlike X-ray Computed Tomography (CT) or Positron Emission Tomography (PET) scans, fMRI is a noninvasive technique. Another reason is that fMRI provides a relatively high spatial resolution. Hence, fMRI is an appropriate technique for identifying the brain structures that support mental processes. The fMRI technique has been used extensively to study the brain structures that 9 support emotion and memory. With respect to emotions, numerous studies report enhanced neural activity in response to emotional as compared to neutral stimuli in a range of modalities including audition (see reviews by Vuilleumier, Armony, & Dolan, 2003; Costafreda, Brammer, David, & Fu, 2008; Fusar-Poli et al., 2009). These enhancements are typically seen in regions associated with sensory processing and perceptual encoding (Dolan & Vuilleumier, 2003; Kensinger, 2004; Grandjean et al., 2005; Schirmer & Kotz, 2006; Wildgruber et al., 2005; Sander & Scheich, 2001; Fecteau et al., 2007). As reviewed above, in the case of prosody, researchers observed greater activity in auditory regions or ‘voice-selective areas’ (see Belin, Zatorre, Lafaille, Ahad, & Pike, 2000) along the superior temporal sulcus (for a review see Schirmer & Kotz, 2006), specifically regions in the superior temporal sulcus, superior temporal gyrus and transverse temporal gyrus (Mitchell et al., 2003, Ethofer et al. 2006; Beaucousin et al. 2007; Wildgruber et al., 2005; Ethofer et al., 2006). Apart from enhancing sensory and perceptual processes, emotional stimuli have been found to activate a range of other regions. Foremost among them is the amygdala, an almond shaped structure in the medial temporal lobe. Emotion effects in this structure have been observed in studies that used faces (Breiter et al. 1996; Morris, Frith, Perrett, Rowland, Young, Calder, & Dolan, 1996; Critchley, Rotshtein, Nagai, O'Doherty, Mathias, & Dolan, 2005; Hariri, Bookheimer, & Mazziotta, 2000; Vuilleumier, Armony, & Dolan, 2003), images such as pictures of emotional scenes Ohman & Mineka, 2001; Adolphs, 2002; Vuilleumier, Armony, Clarke, Husain, Driver, & Dolan, 2002), written words (Kensinger & Corkin, 2004; LaBar & Cabeza, 2006; Sommer, Gläscher, Moritz, & Büchel, 2008; Mickley & Kensinger, 2008), nonverbal exclamations (Fecteau et al., 2007; Phillips, Young, Scott, Calder, Andrew, Giampietro, Williams, Bullmore, Brammer, & Gray, 1998; Sander & Scheich, 2001) 10 and to a lesser extent words or sentences spoken with emotional prosody (Sander et al., 2005; but see Schirmer, Escoffier, Zysset, Koester, Striano, & Friederici, 2008; Ethofer et al., 2006; Kotz et al., 2003; Mitchell et al., 2003; Morris, Scott & Dolan, 1999, Wiethoff, Wildgruber, Kreifelts, Becker, Herbert, Grodd, & Ethofer, 2008). Based on this work, it has been proposed that the amygdala serves as a “relevance” detector – that is an emotion unspecific region that is activated by any stimulus of intrinsic relevance for the individual (Sander, Grafman & Zalla, 2003). Meta-analyses of neuroimaging work on emotions suggest that the involvement of overall brain activation patterns depend on the specific emotions evoked by the stimuli (Vytel & Hamann, 2010). Specifically, regions apart from the amygdala and basic sensory and perceptual processing are activated in an emotion-specific fashion. For instance, sadness consistently activated the middle frontal gyrus and head of the caudate/subgenual anterior cingulate cortex. Individual variation in neural and heart rate responses to emotional stimuli Verbal memory tasks have been demonstrated to elicit cortical activation that shows good intra-subject reproducibility but significant inter-individual variation in spatial location and extent (Miller et al., 2002). Some neuroimaging studies have also found individual variability in the extent of neural activation evoked by emotional stimuli. For instance, in a study by Canli and colleagues (2002) where participants were shown happy facial expressions, the authors found that subjects exhibited highly variable responses in the amygdala, such that the average group response was not statistically significant. However, it was subsequently found that this variability was strongly correlated with subjects’ degree of extraversion (Canli, Sivers, Whitfield, Gotlib, & Gabrieli, 2002). The greater the degree of extraversion, the greater the 11 extent of amygdala response to the happy faces. Hence, it appears that a certain amount of variability in neural response to emotional stimuli exists and this variability may provide vital cues in elucidating the underlying brain mechanisms. It would thus be prudent to examine the change in cortical activation in response to emotional stimuli within each individual and how this change may vary across individuals. Such individual variability in neural correlates could then be related to behavioral correlates such as memory performance. Despite a considerable amount of literature devoted to the study of emotional memory and it neural correlates, there are few studies that examined individual variability in neural / physiological changes evoked by emotional stimuli. One of the aims of the present study is to examine how such individual variability in neural / physiological differences may relate to verbal memory. fMRI studies on verbal memory Apart from highlighting structures implicated in emotion, fMRI research has also provided insights into the brain systems that support memory or the storage of semantic information (Buckner, Koutstaal, Schacter, Wagner, Rosen, 1998; Eldridge, Knowlton, Furmanski, Bookheimer, & Engel, 2000; Konishi, Wheeler, Donaldson, Buckner, 2000; Chee, Goh, Lim, Graham, & Lee, 2004; Henson, Hornberger, & Rugg, 2005). An early study by Buckner and colleagues (1998) found that word retrieval as compared to viewing a fixation on the screen activated the extrastriate cortex, motor cortex, dorsolateral prefrontal cortex, anterior cingulate, parietal cortex, thalamus, anterior insular cortex and several other regions (for a complete list of regions see Buckner et al., 1998). Subsequent studies have found a similar set of regions. For instance, in a study by Chee and colleagues (2004), participants performed an incidental word encoding task (living/non-living judgments) and were subsequently tested using an old/new recognition paradigm. In this paradigm, 12 participants saw words from the encoding task together with new words and indicated for each word whether it had been previously seen or whether it was new. Relative to correctly recognized new words (correct rejections), correctly recognized old words (hits) elicited greater neural activity in left middle frontal gyrus, left inferior frontal gyrus, left inferior temporal gyrus, left anterior cingulate, left parietal region, thalamus and insular cortex (Chee, Goh, Lim, Graham, & Lee, 2004). Together these studies highlight a network supporting the successful recollection of previously studied items that comprises the middle frontal gyrus, inferior frontal gyrus, middle temporal gyrus, inferior temporal gyrus and cingulate gyrus (Chee, Goh, Lim, Graham, & Lee; Henson, Hornberger, & Rugg, 2005). Of interest for the present study is whether these memory effects are enhanced for words previously heard with an emotional as compared to neutral prosody. Thesis Objectives As outlined above, this thesis was inspired by previous behavioral work that identified an effect of emotional prosody on the accuracy (Kitayama, 1996) and affective connotation (Schirmer, 2010) of verbal memory. Moreover, it sought to further investigate these effects by assessing their autonomic and neural correlates. These two aspects were addressed in Experiments 1 and 2, respectively. These experiments comprised a study and a test phase. In the study phase, participants listened to a series of neutral words spoken with neutral or sad prosody. In the test phase, participants saw previously studied words together with new words and indicated for each word whether it was ‘old’ or ‘new’. Experiment 1 recorded heart rate responses to words presented in the study phase. Based on previous work (Bradley & Lang, 2000b; Palomba, Angrilli & Mini, 1997; 13 Buchanan, Etzel, Adolphs, & Tranel, 2006), I predicted greater heart rate deceleration to words spoken with sad as compared to neutral prosody. If, as previously suggested, this HR response reflects orienting to the eliciting stimulus as a whole (Lacey & Lacey, 1979), it should predict subsequent memory. Specifically, it should correlate with potential differences in the recognition accuracy of visually-presented test words previously heard with sad as compared to neutral prosody. It might also explain condition differences in subsequent word valence rating. In line with previous work (Schirmer, 2010), words studied with a sad prosody should be rated more negatively than words studied with a neutral prosody and this difference might be enhanced for individuals with a greater HR deceleration effect. However, if the relationship between HR deceleration and stimulus processing goes beyond a simple orienting response as suggested by Harrison and Turbin (2003) then HR deceleration may not predict verbal memory and valence. Instead, such effects may arise from HR acceleration. Thus, apart from investigating HR deceleration, Experiment 1 also aimed to elucidate potential emotion effects on HR acceleration. Experiment 2 recorded brain activity both during the study and test phases. Previous neuroimaging studies have implicated perceptual (superior temporal sulcus, superior temporal gyrus and transverse temporal gyrus) and emotion-specific (amygdala) regions in processing emotional information. Therefore, during the study phase, I expected greater activity in the amygdala, superior temporal sulcus, superior temporal gyrus and transverse temporal gyrus for words spoken with sad as compared to neutral prosody. During the test phase, I expected greater activity in the middle frontal gyrus, inferior frontal gyrus, middle temporal gyrus, inferior temporal gyrus, cingulate gyrus and anterior cingulate for hits relative to correct rejections. Moreover, based on previous research indicating an influence of emotion on memory (for a 14 review see Phelps & LeDoux, 2005; Murty et al., 2010), I hypothesized this memory effect to be greater for negatively as compared to neutrally spoken words. Finally, I explored whether study and test emotion effects on neural activity predict behavioral performance. 15 Chapter 2: Experiment 1 Do prosody encoding effects predict differences in verbal memory performance and subsequent word valence judgments? Objectives Experiment 1 explored the effect of prosody on subsequent word memory and valence. Moreover, of interest was whether such effects relate to autonomic changes triggered by the prosody during word encoding. Based on work by Schirmer (2010), no significant differences in word memory were expected at the group level for words studied with sad and neutral prosody. However, as such differences could exist at the individual level, I intended to explore such individual differences and their relationship to heart rate changes. A second objective was to replicate the ‘valence shift effect’ found by Schirmer (2010) and to examine whether words successfully encoded with a sad prosody were subsequently rated more negatively than words successfully encoded with a neutral prosody. Furthermore, I hoped to determine whether this shift in valence was linked to heart rate correlates. More specifically, I predicted this valence shift effect to be greater for individuals with a greater prosody effect (emotional – neutral) on heart rate. Methods Participants Forty-seven participants (23 female) aged 21 to 27 took part in the experiment. Participants reported normal hearing and normal or corrected to normal vision. Informed consent was obtained prior to the start of the experiment and participants were reimbursed S$10 per hour. 16 Materials The materials for this research were taken from a previous study by Schirmer (2010). It comprised a list of 240 neutrally valenced words. The words were selected from among 500 words, which were rated by 30 independent raters (15 female) on emotional valence and arousal. Raters were required to rate the emotional valence of each word on a 5-point scale ranging from -2 (very negative) to +2 (very positive) and its arousal ranging from 0 (non-arousing) to 4 (highly arousing). The words selected for the experiment had a mean valence of 0.16 (SD 0.20) and a mean arousal of 0.58 (SD 0.24). All selected words were spoken with neutral and sad prosody by a female native speaker of English. Words were recorded and digitized at a sampling rate of 44.1 KHz. Word amplitude was normalized at the root-mean-square value using Adobe Audition 2.0. The average duration of words produced by the speaker was 1132.4 ms (SD 245.5) for sad prosody and 777.6 ms (SD 149) for neutral prosody. The speaker was selected from among four speakers with drama experience who were invited to speak 15 neutral words in anger, sadness, happiness and neutrality. These words were presented to a group of 30 volunteers who indicated whether the speaker was in an angry, sad, happy, neutral or other emotional state not listed. Additionally, they rated each word on a five-point scale ranging from -2 (very negative) to +2 (very positive) for prosody valence and from 0 (non-aroused) to 4 (highly aroused) for prosody arousal. The speaker who produced the material for the present and previous work (Schirmer, 2010) portrayed sadness (identification accuracy = 88%, valence = -1.45, arousal = 2.92) and neutrality (identification accuracy = 89%, valence = 0.06, arousal = 0.79) better than the other speakers. 17 Procedure Participants were tested individually. A participant visiting our lab was first asked to read and sign the experimental consent form. Then s/he was brought into a room and asked to sit in a comfortable chair facing a computer screen. Heart rate (HR) was measured by two Ag/AgCl electrodes attached to the left and right forearm, respectively. The data were recorded at 256 Hz with the ActiveTwo system from Biosemi. The difference between the two electrodes was computed and the resulting bipolar recording processed using Matlab (Schirmer & Escoffier, 2010). The present study used an old-new recognition paradigm comprising two study phases each followed by a test phase (as illustrated in Figures 1.1 and 1.2 respectively). Prior to the task, participants attempted a short practice to familiarize themselves with the mapping of required responses and response buttons. During the practice session, participants were presented with 10 words spoken in either a neutral or sad prosody and asked to memorize the words. Subsequently, participants viewed 20 words and were told to indicate whether these words were ‘old’ or ‘new’. The experiment was conducted using Presentation® software (Version 13.0, www.neurobs.com). A CRT monitor of 18 inches was used for visual presentation. Sounds were presented using Etymotic ER 4 MicroPro in-ear earphones. Study phase During the study phase, participants listened to a series of words spoken with either a neutral or sad prosody. They were instructed to study the words for a subsequent memory test. Each trial began with a fixation cross that was presented for 0.2 s in the center of the screen, followed by a spoken word simultaneously presented with a fixation cross, the latter lasting 2.3 s. The trial ended with a blank screen 18 marking the onset of the intertrial interval (ITI). The ITI was jittered from 12 to 15 s in one second steps. Each study phase consisted of 60 trials. Half of the trials consisted of words spoken with sad prosody; the other half consisted of words spoken with neutral prosody. The sequence of words presented was pseudorandomized such that no more than four consecutive trials were of the same prosody. A sample trial is shown in Figure 1.1. Figure 1.1. Figure illustrating the sequence of stimulus presentation during a study phase. Test phase During the test phase, participants viewed 120 words half of which were previously studied (old) and half of which were not previously studied (new) on the screen. Each test trial began with a fixation cross that lasted 0.2 s, followed by a word on the screen for 1 s. Next, a prompt appeared, instructing participants to indicate whether the word was an ‘old’ or a ‘new’ word. Participants who had to press the left button for old words and the right button for new words, were prompted with the word ‘OLD’ on the left and the word ‘NEW’ on the right of the screen. Participants 19 with the opposite button assignment saw the reversed prompt. The button assignments were counterbalanced across participants. Once participants made an old/new judgment, the prompt disappeared and a second prompt appeared, instructing participants to rate the same word in terms of its emotional valence on a 5-point scale ranging from -2 (very negative) to +2 (very positive). Participants now saw this rating scale and were instructed to move a cursor (↑) to the appropriate point on this scale and press a key to confirm their response. The rating scale then disappeared and the screen remained blank for a period jittered from 0.5 to 1.25 s. After the first test phase, participants took a short break before continuing with the experiment. The test procedure is illustrated in Figure 1.2. Figure 1.2. Figure illustrating the sequence of stimulus presentation during a test phase. 20 Data analysis Heart rate data was processed off-line. To remove slow drifts and high frequency noise, a digital band pass filter was applied with a high frequency cutoff of 0.8 and a low pass frequency cutoff at 40 Hz. QRS complexes in the recorded signal were then detected using a pattern matching algorithm as implemented in the Biosig toolbox (Nygards & Sornmo, 1983). The QRS complex (Einthoven, 1901) is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG). It corresponds to the depolarization of the right and left ventricles of the human heart. The algorithm takes into consideration the QRS complex shape and detects the R peak (see Figure 2). This technique has been shown to be more accurate and sensitive than a thresholding technique solely based on amplitude (Berntson et al. 1997). The heart rate data was then plotted on a time series and visually corrected for potentially erroneous R-peak detection. Instantaneous HR was computed from interbeat intervals and re-sampled at 4 Hz using linear interpolation (Berntson et al., 1995). Figure 2. The QRS complex. 21 Next, event-related time courses of inter-beat-intervals were computed over a 12 s interval after stimulus onset for each condition and participant. To eliminate the possibility of random pre-stimulus differences between conditions as a potential confounding factor, heart rate data for each condition was normalized against a prestimulus baseline. To this end, the data 1 s prior to stimulus onset was averaged and subtracted from each data point in the 12 s epoch. HR deceleration and acceleration were identified by selecting the HR minimum between 0 and 3 s and the HR maximum between 1 and 9 s from stimulus onset for each condition and participant, respectively (Figure 3). Change in heart rate across time nstudy estudy 5 change in heart rate (beats/min) 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 12 -1 -2 -3 time lapse from onset of stimulus (s) Figure 3. A time-series plot illustrating how maximum HR deceleration and acceleration were computed for each participant. 22 Results Study phase First, I examined the influence of emotion on heart rate during the study phase. A paired-sample t-test was performed to compare HR deceleration for words spoken with neutral and sad prosody (refer to Table 1). Results revealed that words spoken with sad prosody (M = -0.867, SD = 0.932) elicited a greater HR deceleration than words spoken with neutral prosody (M = -0.584, SD = 0.769), t(46) = 2.692, p < 0.05. A statistical comparison of HR acceleration was non-significant (p > 0.1). Table 1. Table illustrating means and standard deviations for heart rate deceleration and acceleration in response to words spoken with the neutral and sad prosody (Experiment 1). HR deceleration HR acceleration M SD M SD Neutral -0.584 0.769 1.803 1.287 Sad -0.867 0.932 1.676 1.307 Vocal emotion Test phase Next, I examined the influence of vocal emotion on verbal memory. To this end, d’ scores were computed by subtracting the z-score obtained from the probability of ‘new’ being falsely recognized as ‘old’ (false alarms) from the z-score obtained from the probability of ‘old’ words being correctly recognized as ‘old’ (hits) (Wickens, T., 2002). A paired samples t-test revealed higher d’ scores for words encoded in neutral (M = 1.950, SD = 1.082) relative to sad (M = 1.825, SD = 0.951) prosody, t(46) = 2.489, p < 0.05 (Table 2). I also compared the mean valence rating of words encoded in the neutral and sad conditions. A paired-samples t-test yielded significantly lower 23 mean ratings for correctly recognized ‘old’ words previously heard with sad (M = 0.283, SD = 0.348) as compared to neutral (M = 0.371, SD = 0.320) prosody, t(46) = 2.644 , p < 0.05. Table 2. Table illustrating means and standard deviations of mean dprime scores valence ratings for words spoken with the neutral and sad prosody (Experiment 1). Dprime scores Valence rating M SD M SD Neutral 1.950 1.082 0.371 0.320 Sad 1.825 0.951 0.283 0.348 Vocal emotion Correlational analyses To examine the possibility of a relationship between heart rate effects during study and subsequent behavioral effects at test, I computed an emotion sensitivity index (ESI) for heart rate and behavioral measures. To this end values obtained for the neutral condition were subtracted from values obtained for the sad condition for HR deceleration, HR acceleration, d’ scores and mean valence ratings. The resulting indices were then subjected to the following two-tailed Pearson correlation analyses. First, I tested the relationship between the HR deceleration ESI and the d’ ESI. This analysis was non-significant (p > 0.1). Next, I tested the relationship between the HR acceleration ESI and the d’ ESI and observed a significant positive correlation (r = 0.287, p = 0.05). Correlations between cardiac responses and the valence rating were non-significant (ps > 0.1). Discussion The current study explored the influence of vocal emotions on heart rate during verbal encoding and whether such influences predict subsequent verbal memory. 24 Previous studies have reported heart rate changes in response to emotional stimuli (Bradley & Lang, 2000b) and linked such changes to modulations in memory (Palomba, Angrilli & Mini, 1997). The goal of the present study was to extend this work to the context of emotional prosody. First, words spoken with a sad prosody were expected to elicit greater heart rate deceleration as compared to words spoken with a neutral prosody. Second, the prosody effect (emotional - neutral) on heart rate during study was expected to predict emotional differences in subsequent memory and word valence at test. As predicted, results revealed a greater HR deceleration in response to words spoken with sad prosody as compared to words spoken with neutral prosody. However, results also revealed better performance for words encoded in the neutral as compared to sad prosody, which was not found previously in Schirmer’s study (Schirmer, 2010). Critically, results of the current study successfully replicated the word valence effect found previously (Schirmer, 2010). Words correctly recognized as ‘old’ were rated as more negative when they were spoken with the negative prosody than the neutral prosody. HR deceleration failed to correlate with memory performance and changes in perceived word valence. Although, across participants, HR acceleration was comparable for words with sad and neutral prosody, individual variation in HR acceleration between sadly and neutrally spoken words (HR acceleration ESI) correlated positively with the emotional difference in memory performance (d’ ESI). Thus, in line with work by Harrison and Turpin (2003), results seem to suggest that HR acceleration during encoding predicts subsequent memory performance. The valence shift effect observed was not related to HR effects. Current findings will be discussed in further details in the General Discussion. 25 Chapter 3: Experiment 2 Objectives Does emotional prosody engage preferential neural processing over neutral prosody? What are the regions associated with successful memory recognition (hits relative to correct rejections) and does encoding prosody modulate activity in these regions? Do prosody effects on neural activity predict verbal memory performance? The objectives of Experiment 2 were fourfold. First, I aimed to replicate and extend the existing literature on auditory emotional processing. This literature has implicated the amygdala, superior temporal sulcus, superior temporal gyrus and transverse temporal gyrus and I thus expected to find these regions more strongly activated while participants listen to sad as compared to neutral prosody. Second, I aimed to replicate and extend the literature on memory retrieval. This literature holds that middle frontal gyrus, inferior frontal gyrus, middle temporal gyrus, inferior temporal gyrus, cingulate gyrus and anterior cingulate are more strongly activated for ‘hits’ relative to ‘correct rejections’. Thus, I expected to observe similar results here. Third, I hoped to see activation differences between words previously studied with sad and neutral prosody. Given differences in the recognition and perceived valence of these words (Experiment 1), one would expect that they also differ in their recruitment of brain structures. Moreover, such recruitment differences may overlap with potential processing differences between sadly and neutrally spoken words during study. Finally, I examined whether prosody effects on neural activity during study and test predict behavioral performance. 26 Methods Participants Fifty-one participants were recruited. Ten participants’ data were lost due to technical problems (image stripes inherent in the MR scanner). Data for forty-one participants (20 female) ranging in age from 21 to 27 were eventually used for this thesis. They had no history of neurological disorders and had either normal or corrected to normal vision. They reported normal hearing. All participants signed informed consent and were reimbursed S$40 for their participation. Pre-scan memory test Due to the expenses associated with booking the MRI scanner, I wanted to ensure that participants in the experiment perform well enough for their data to be retained. Hence, I invited everyone to an initial memory screen. The paradigm used for this screen was identical to the behavioral paradigm used in the fMRI experiment, with the exception of a different list of words used. Only participants who scored a minimum d’ of 1.5 were allowed to proceed to the fMRI experiment. fMRI experiment Procedure The fMRI experiment used a similar old new paradigm as the one used for Experiment 1. A short practice session was followed by two study/test phases. The few differences between Experiment 1 and 2 are highlighted in the following. First, because the experiment took place in an MRI scanner, participants had to lie down rather than sit in front of a computer screen. Visual images were projected onto a screen at the back of the MRI scanner and participants viewed the screen using a 27 mirror that was attached to the head coil of the magnet and the position was adjusted such that it was directly in front of their eyes. Sounds were presented using NordicNeuroLab headphones (NNL, Bergen, Norway). During the study phases, participants studied only 40 words as opposed to 60 words and the test phases consisted of only 80 words. Moreover, the interval between words was jittered from to 5 to 9 s in steps of 1 s and each trial lasted 12 s. In the test phases, old-new decisions were no longer followed by a word valence rating. The response window was set to 5s and the blank interval between the response and the next stimulus was jittered from 1 to 4 s in steps of 1 s. These procedural changes were introduced in adaptation to the fMRI sparse sampling technique and to ensure a feasible experiment duration (~45min). All the remaining aspects of this experiment were comparable to Experiment 1. Image Acquisition All images were acquired on a 3T Siemens Tim Trio (Siemens, Erlangen, Germany). A T1-weighted anatomical image was first acquired with a 3D MPRAGE sequence (192 slices, TR = 2530 ms, TE1 = 1.64 ms, TE2 = 3.5ms, TE3 = 5.36ms, TE4 = 7.22ms, TI = 1200 ms, flip angle = 7°, FOV = 256mm, voxel size = 1 mm × 1 mm × 1 mm) for co-registration purposes. Functional images for the study phase were obtained by using a standard gradient-echo EPI sequence (TE = 30ms, flip angle = 90º, FOV read = 192mm x 192mm, matrix = 64 x 64). Twenty-eight oblique slices aligned parallel to the AC-PC line with a slice thickness 4 mm and a distance factor of 0.4 mm were acquired. A sparse-sampling EPI sequence was used during the study phase to ensure that scanner noise would not interfere with the audibility of words played over the headphones. Apart from the imaging parameters that were identical to the standard EPI sequences (parameters mentioned above), the sparse-sampling EPI 28 sequence employed a TR of 12 s and a TA of 2 ms. The interval between the onset of each EPI pulse and stimulus (i.e., Pulse-Stimulus Interval, PSI), was jittered from 3 to 7 s to allow maximum coverage of the hemodynamic response. Functional images for the test phase were acquired using a similar EPI sequence with the exception of the TR being 2 s. Image analysis The fMRI data was preprocessed and analyzed using the Statistical Parametric Mapping software (SPM8, Wellcome Trust Centre for Neuroimaging, University College London). Functional images obtained from the scanner were converted to NIFTI formatted images for further processing. A within-subject registration of image time series was performed. The time series of functional images were realigned using a least-square minimization and a six-parameter (translations and rotations in the x, y and z planes) rigid-body spatial transformation. All functional images were realigned to the first image in the series. Next, images were co-registered by aligning the anatomical image (MPRAGE) to the reference image (mean functional image averaged across the series). The anatomical image was then segmented to obtain a bias field corrected MPRAGE for overlaying purposes. Previously realigned functional images were normalized to the MNI space, interpolated to 3 mm × 3 mm × 3 mm voxels and then spatially smoothed with a Gaussian kernel of 8 mm FWHM. Finally, a high-pass filter of 128 s was applied to the data to remove slow signal drifts. Statistical analysis was performed on individual and group data by using the general linear model. Study phase For the study phase fMRI analysis, a general linear model with four experimental 29 predictors was used: subsequently remembered words previously spoken with neutral (study_hitneu) and sad (study_hitemo) prosody, and subsequently forgotten words previously spoken with neutral (study_missneu1) and sad (study_missemo) prosody. Motion parameters were included as covariates. T-contrasts of interest were generated for each participant: Contrast #1: study_hitemo > study_hitneu, Contrast #2: study_hitneu > study_hitemo, Contrast #3: study_emo > study_neu, Contrast #4: study_neu > study_emo. A statistical threshold of p < 0.05 (family wise error corrected) and a cluster size of > 10 continuous voxels was used in the creation of activation maps. To elucidate regions that showed a greater neural activity in response to ‘hit’ words spoken with emotional as compared to neutral prosody, a random effects analysis was conducted across participants (Contrast #1: study_hitemo > study_hitneu). To determine whether the expected prosody effect on neural activity during encoding predicts an emotional memory effect, a regression analysis using the Contrast #1 as the predictor and d’ ESI (emotional - neutral) as the criterion was generated. Test phase For the test phase fMRI analysis, a general linear model with six experimental predictors was used: remembered words previously spoken with a neutral (test_hitneu) and sad prosody (test_hitemo), forgotten words previously spoken in a neutral (test_missneu) and sad prosody (test_missemo) and new words correctly recognized as ‘new’ (test_newcr) and incorrectly identified as ‘old’ (test_newfa). The following t-contrasts were generated for each participant: Contrast #1: test_hitemo > test_hitneu, Contrast #2: test_hitneu > test_hitemo, Contrast #3: hit > cr, Contrast #4: 1 Misses were entered in case they explained variance but not used for subsequent generation of contrasts due to an insufficient number of trials. 30 cr > hit, Contrast #5: test_emo > test_neu, Contrast #6: test_neu > test_emo. A statistical threshold of p < 0.05 (family-wise error corrected) and a cluster size of > 10 contiguous voxels was used in the creation of activation maps. To determine regions that show a memory effect (a greater neural activity in response to ‘hits’ as opposed to ‘correct rejections’), a random effects analysis was generated across participants using Contrast #3. To examine whether recognition memory differs as a function of the encoding prosody, I compared activity for negative ‘hits’ and neutral ‘hits’. Finally, to determine whether the prosody effect for neural activity at test predicts the emotional difference in memory performance, a regression analysis using the prosody effect for neural activity (Contrast #5: test_emo > test_neu) as the predictor and d’ ESI (emoneu) as the criterion was generated. Results Behavioral analyses To examine the effect of emotion on memory performance, I subjected the d’ scores (refer to Experiment 1 results for how the d’ scores were computed) to a paired-samples t-test with emotion as the factor of interest. Memory performance was better for words previously spoken with a neutral prosody (M = 2.593, SD = 0.984) than for words spoken previously spoken with a sad prosody (M = 2.403, SD = 0.890), t(40) = 2.961, p < 0.01 (Table 3). Table 3. Table illustrating means and standard deviations of dprime scores for words spoken with the neutral and sad prosody (Experiment 2). Dprime scores M Vocal emotion Neutral Sad 2.593 2.403 SD 0.984 0.890 31 fMRI analyses Study phase At encoding, sadly spoken words elicited greater activity than neutrally spoken words in the bilateral superior temporal gyrus and the right transverse temporal gyrus (Figure 4). A similar effect was found in the inferior frontal gyrus only with a lower significance threshold of p < 0.001 (uncorrected). No regions showed greater activity for words spoken in neutral as compared to sad prosody. To determine whether there is a relationship between the neural correlates of prosody encoding and memory performance, I conducted a separate whole-brain regression analysis with the contrast obtained for the emotional difference in neural activity (Contrast #1) as the predictor and the emotional difference in memory performance (d’ ESI) as criterion. This regression analysis did not reveal any regions that showed a relationship between memory performance and neural activity. Table 4. Table illustrating peak activations for hitemo > hitneu contrast for the study phase. Region Left Superior Temporal Gyrus / Superior Temporal Sulcus cluster Left Superior Temporal Gyrus / Superior Temporal Sulcus cluster Left Superior Temporal Gyrus / Superior Temporal Sulcus cluster BA Talairach coordinates Peak T-statistic Peak Z-score BA 41 -48 -15 2 11.760 7.690 BA 41 -48 -24 5 10.320 7.170 BA 41 -45 -32 16 9.390 6.780 Right Transverse Temporal Gyrus Right Superior Temporal Gyrus / Superior Temporal Sulcus cluster Right Superior Temporal Gyrus / Superior Temporal Sulcus cluster BA 41 53 -24 10 10.520 7.240 BA 22 50 -10 1 9.230 6.720 BA 22 45 4 -12 4.880 4.300 Right Inferior Frontal Gyrus* BA 44 54 20 19 4.770 4.220 Right Inferior Frontal Gyrus* BA 44 50 17 11 4.570 4.070 32 Left superior temporal gyrus / superior temporal sulcus cluster Right superior temporal gyrus / superior temporal sulcus cluster Right transverse temporal gyrus Figure 4. Figure illustrating regions that show greater activity for words spoken in negative as compared to neutral intonation. Peak activations at left superior temporal gyrus / superior temporal sulcus cluster, right superior temporal gyrus / superior temporal sulcus cluster and right transverse temporal gyrus. 33 Test phase I first examined the regions with greater activity for the correct recognition of ‘old’ words (hits) as compared to the correct rejection of ‘new’ words. Such an effect was observed in left superior frontal gyrus, bilateral middle frontal gyrus, left middle temporal gyrus, left anterior cingulate, bilateral inferior frontal gyrus, bilateral cingulate gyrus, bilateral sub-gyral and right middle occipital gyrus (Table 5). I then examined whether neural activity in response to test words previously successfully encoded with the emotional prosody (hitemo) was greater than those encoded with the neutral prosody (hitneu). Results did not reveal any regions that showed greater activity for this contrast. Finally, I examined whether the emotional difference in memory performance (d’ ESI) is greater for individuals with a greater prosody effect on neural activity at test by conducting a regression analysis using the contrast generated for the emotional difference in neural activity at test as the predictor and d’ ESI as the criterion. This regression analyses yielded no significant voxels. 34 Table 5. Table illustrating peak activations for hits > correct rejections contrast for the test phase. Region BA Left Superior Frontal Gyrus - Left Sub-Gyral Left Inferior Frontal Gyrus Left Cingulate Gyrus Left Cingulate Gyrus Left Superior Frontal Gyrus Left Middle Frontal Gyrus Left Sub-Gyral Left Middle Temporal Gyrus Left Anterior Cingulate Right Cingulate Gyrus Right Middle Occipital Gyrus Right Sub-Gyral Right Inferior Frontal Gyrus Right Middle Frontal Gyrus Right Inferior Frontal Gyrus Right Middle Frontal Gyrus BA 23 BA 24 BA 10 BA 18 BA 9 BA 6 BA 47 - Talairach coordinates -6 -48 -3 -3 -27 -33 -30 -50 -9 6 30 39 50 33 36 39 5 53 -33 -46 39 9 23 -23 27 3 26 54 -4 57 4 43 4 -44 9 37 10 3 26 -85 0 -62 -4 12 28 4 50 16 -3 32 19 Peak T-statistic Peak Z-score 17.870 Inf 17.010 14.400 11.640 7.050 8.310 7.200 6.080 7.100 6.390 7.430 11.020 8.790 10.270 9.190 9.080 7.280 Inf Inf 7.650 5.650 6.300 5.730 5.080 5.680 5.270 5.850 7.430 6.520 7.150 6.700 6.650 5.770 Discussion The current study examined the influence of emotional prosody on the encoding of spoken words and determined whether neural activity in response to prosody during encoding predicts subsequent recognition memory during test. Words spoken with a negative prosody were expected to evoke greater activity in the amygdala, superior temporal sulcus, superior temporal gyrus and transverse temporal gyrus. Greater activity was evoked by negative prosody relative to neutral prosody only in the superior temporal gyrus and transverse temporal gyrus, but not in the amygdala. No regions showed a correlation between the emotional difference on encoding activity and the emotional difference in memory performance. The effect of emotion 35 was also examined for recognition memory. Similar to the findings from previous studies, results yielded significantly greater activity for words successfully recognized as ‘old’ (hits) than for words successfully recognized as ‘new’ (correct rejections). However, further analysis did not reveal significant differences in the memory effect (hits relative to correct rejections) as a function of emotion. In addition, the emotional difference in memory performance seemed unrelated to the emotional difference in neural activity both during study and test. The significance and implications of these findings will be discussed in the General Discussion. 36 General Discussion The overarching objective of the current thesis was to examine the heart rate and neural correlates underlying the influence of emotional prosody on verbal memory. My findings have, in general, successfully replicated and extended prior work in this realm. In the following, I will discuss in greater detail the effects of encoding prosody on verbal memory and heart rate / neural correlates and the extent to which these prosody encoding effects predict verbal memory. Finally, I discuss the limitations of the current thesis and propose future directions in investigating the multifaceted relationship between prosody and verbal memory. Heart rate and neural correlates of prosody encoding During encoding, I observed a greater HR deceleration in response to words spoken with sad as compared to neutral prosody. This finding is in accord with previous studies that reported greater HR deceleration in response to emotional as compared to neutral stimuli (Palomba, Angrilli & Mini, 1997; Hamann, Ely, Grafton, & Kilts, 1999; Bradley & Lang, 2000b). A decrease in heart rate was thought to reflect an orienting response towards an incoming stimulus (Sokolov, 1963; Graham & Clifton, 1966; Lacey & Lacey, 1969). Thus, the greater HR deceleration observed here may reflect a greater orienting of attention to sad as compared to neutral prosody. Interestingly, such an enhanced deceleratory effect was observed previously across different types of emotional stimuli. For instance, in a study by Palomba and colleagues (1997), pleasant, neutral and unpleasant picture slides were used. The authors observed a greater HR deceleration in attending to unpleasant slides than neutral or pleasant slides. In a study by Kreibig and colleagues (2007), the authors observed a greater decline in heart rate when participants viewed sad as compared to 37 neutral films (Kreibig, Wilhelm, Roth & Gross, 2007). Bradley and Lang (2000) used acoustic stimuli that were rated pleasant and unpleasant (such as the sound of a crying baby or bees buzzing). They observed greater HR deceleration for unpleasant sounds relative to pleasant sounds. Finally, evidence also stems from studies that used other acoustic stimuli such as music excerpts (Krumhansl & Carol, 1997; Etzel et al., 2006). For example, Etzel and colleagues (2006) presented subjects with music excerpts that conveyed happiness, sadness, and fear while heart rate data was recorded. Results revealed a heart rate deceleration during sadness induction (Etzel et al., 2006). Taken together, these findings seem to suggest that emotional and in particular negative stimuli evoke greater heart rate deceleratory responses as compared to neutral stimuli, and that this effect holds for a wide range of stimuli including pictures, sounds and musical excerpts. The results of the first experiment conducted in this thesis suggest that the HR deceleratory effect extends to words spoken with sad prosody. They imply that words spoken with a sad prosody elicit greater orienting of attention than words spoken with a neutral prosody. The fMRI study conducted in the second experiment supports this assertion. Compared to words spoken with a neutral prosody, those spoken with a sad prosody elicited greater activity in the superior temporal gyrus and the transverse temporal gyrus. That encoding emotional prosody would engage the superior temporal gyrus and superior temporal sulcus to a greater extent than neutral prosody is not surprising granted the array of studies that have found a similar pattern in prosody perception (for a review see Schirmer & Kotz, 2006). For instance, in an fMRI study that also examined the difference in activity in processing emotional and neutral prosody, Wildgruber and colleagues (2005) found greater activity in the superior temporal gyrus when participants attended to prosodic excerpts of multiple 38 emotions (sad, happy, fearful, angry and erotic) as compared to neutral excerpts. The current experiments show that such an effect also holds when sad prosody is examined individually. Effects of prosody on verbal memory performance Memory performance for words spoken with the neutral prosody was better as compared to those spoken with the sad prosody in both experiments. This finding is in discord with that of Schirmer (2010), where memory performance was found to be comparable for both conditions. The extended inter-stimulus-intervals used in the current study as compared to the short ISIs used in Schirmer’s study could possibly account for the discrepant findings. The greater time interval available for memorizing words in the present study has likely reduced memory load and task difficulty. That this produces a memory benefit for neutrally intoned speech is in line with evidence from Kitayama’s study (Kitayama, 1996) which demonstrated that for a task that involves low memory load, recognition performance for distracting sentences spoken with an emotional prosody was worse than that for their counterparts spoken with a neutral prosody. But when task difficulty increases with an increase in memory load, the effect of prosody on memory diminishes. Hence, one might equate memorizing words at shorter intervals to a task of greater difficulty and memorizing words at longer intervals to a task that is less demanding. Thus, in the case of recognition memory, when the task is relatively demanding or involves a high memory load, the vocal emotion poses neither a threat nor advantage to the verbal content. However, when the memory load is low, emotional prosody may disrupt verbal recognition memory relative to neutral prosody. This argument is further supported by evidence for higher mean d’ scores obtained in both experiments 1 and 2 (mean d’ ~ 1.8) as compared to the relatively lower d’ scores reported in Schirmer 39 (2010) (mean d’ ~ 1.5). Although one should be careful about the implications in comparing the abovementioned studies since they differed in terms of subject pool and the number of trials used, the memory paradigm employed in all three studies were nevertheless comparable. Hence, one may speculate that the memory task in Schirmer’s study was indeed more demanding than the one used in the current thesis. As previously mentioned, this difference in task difficulty may stem from a differential amount of time available for encoding words. Perhaps the enhanced memory performance for words encoded with neutral prosody as compared to emotional prosody can be better understood by examining the match between conditions at encoding and retrieval. Such encoding-retrieval conditions have been studied by multiple researchers. Recent researchers have suggested that in optimizing memory performance, it is not enough to simply ensure a good match between encoding and retrieval conditions; the relative diagnostic value of the encoding-retrieval match is also crucial (Nairne, 2002; Goh & Lu, 2010). The diagnostic value refers to the presence of features that help discriminate the target from competing foils. For example, if a target word “nail” is encoded and stored in the context of the word “finger”, the words “human body part” would make a better encoding-retrieval match and thus a more effective retrieval cue than the word “tool”. However, if other words such as “toe” and “hand” were included in the encoding list, the effectiveness of the cue “human body part” may diminish. Hence, memory enhancement by encoding-retrieval match is contingent on the number of competing foils subsuming the retrieval cue. In the current study, although participants clearly retained emotional information embedded within emotionally encoded words (as evident from the more negative ratings for words encoded with sad relative to neutral prosody), they might have found it difficult to use this information at recognition as 40 several other words were also encoded in the same context (i.e. sad prosody). Hence, the visual cue presented at recognition had little diagnostic value for words encoded with the emotional prosody. Instead, the essentially more neutral cues (visually presented words) at recognition provided a better match with words encoded with the neutral prosody than those encoded with the emotional prosody, giving rise to a better memory performance for the former. The present research is in line with Schirmer’s work in that it failed to find a memory advantage for words spoken emotionally. This finding is supported by current neuroimaging results that revealed no significant contribution of the amygdala in processing emotional prosody. Multiple studies have demonstrated that the emotional enhancement of memory is contingent on amygdala (Canli, Brewer, Gabrieli, & Cahill, 2000) activity and interaction between hippocampal and amygdala activity (Cahill & McGaugh, 1998). Moreover, the engagement of the amygdala in vocal emotional processing has been demonstrated to vary based on individual traits such as social orientation (Schirmer et al., 2008) and to be less reliable than for emotional stimuli from other modalities (Schirmer & Kotz, 2006). Despite differences in the recognition of words previously studied with neutral and sad prosody, the present study found no accompanying differences in brain activity at test. Two reasons could account for this null finding. First, it is possible that there is an immense amount of inter-individual variation in how prosody may affect recognition memory so that effects cancel each other out. For instance, while some participants may show emotionally enhanced neural activity in regions concerning memory recognition, others may display the opposite pattern. Hence, future studies may need to explore such inter-individual variation. Second, sad prosody, being arousal relatively weak emotional stimulus as compared to vocal 41 screams or other emotions sounds, may have caused only small changes in the memory representation of the verbal content that can only be detected with a larger sample size. Apart from studying encoding and test differences as a function of prosody, I was interested in examining a potential relation between the two. Specifically, I asked whether prosody encoding effects predict recognition accuracy. Although results revealed no significant correlation between the HR deceleration effect at encoding and subsequent word recognition, individual variation in the HR acceleration effect at encoding predicted subsequent word recognition. A greater HR acceleration to sadly spoken words relative to neutrally spoken words was associated with a smaller memory decrement for the former relative to the latter. This is in line with the proposal that HR acceleration reflects cognitive effort and enhances stimulus processing (Lacey & Lacey, 1980; Barry, Robert, Tremayne & Patsy, 1987). It also accords with research suggesting a benefit for the retention of information that triggers an increase in bodily arousal (Jennings and Hall, 1980; Kahneman & Peavler, 1969). However, the fact that, overall, neutrally spoken words were more readily recognized than sadly spoken words suggests that the retention of verbal information is a complex process and is subjected to a range of factors including whether emotional information is expressed verbally or vocally and what the demands are on memory retention (Kitayama, 1996). Contrary to expectations, encoding activity in the superior temporal gyrus and superior temporal sulcus did not appear to predict subsequent verbal memory performance. Infact, no regions showed a significant relationship between encoding activity and d’ scores. One possible explanation for this null finding is that there is a sizeable amount of inter-individual variation in how prosody affected verbal memory 42 performance. While most of the participants showed better memory performance for words encoded with neutral in comparison to negative prosody, a subset of them either showed no difference or performed better with emotional prosody. Hence, memory processes may differ amongst these individuals and complicate the relationship between neural activity and verbal memory performance. Effects of prosody on word valence judgment Albeit showing differences in the role of prosody for recognition memory, the present study has successfully replicated the finding of prosodic influences on word valence by Schirmer (2010). Words successfully encoded with the sad prosody were subsequently rated more negatively than words encoded with the neutral prosody. We can see that this valence shift effect is indeed very robust even at different intervals between words. In other words, the tone of voice that speech is delivered in can reliably influence the affective connotation of its verbal content. However, the word valence shift effect observed above did not seem to be related to HR correlates. Individuals who reacted more strongly to emotional as compared to neutral prosody in terms of cardiac deceleration and acceleration were not necessarily the ones who perceived sadly spoken words and neutrally spoken words differently during test. Several reasons could account for this finding. First, the valence shift effect observed may be due to consolidatory mechanisms that extend beyond the encoding phase. Hence, the heart rate measures obtained during encoding may not be appropriate for predicting subsequent valence judgment. A second reason for why there is no relationship between HR and subsequent word valence is that the measurement of this relationship was inappropriate. It comprised an average of HR changes and valence across many items and thus failed to account for item specific 43 co-variation. Additionally, it assumed linearity for a relationship that may well be more complex. Limitations and future directions The present study has several limitations. First, the stimuli employed words spoken with sad prosody. Findings observed with this “withdrawal” emotion may not apply to other “approach” emotions such as anger. Future studies need to include other negative emotions such as fear, anger and disgust to determine whether implications of current findings can be extended to other types of negative emotions. Other vocal stimuli such as vocalizations (e.g. wail or cough) could be employed to see if a similar pattern of memory impairment may be observed. In addition, the current study had not considered individual subject traits that may contribute to the inter-individual differences in memory performance and thus, may have underestimated the complexity of the relationship between prosody and verbal memory. For instance, factors such as the extent to which one is socially orientated can influence vocal processing (Schirmer et al., 2008) and may affect the relationship. Other biological factors such as estrogen have been known to affect vocal processing (Schirmer, Escoffier, Li, Li, Strafford & Li, 2008). Future studies need to factor in these variables to better characterize the interplay between prosody and verbal memory. Finally, as discussed previously the interval between word presentations seems to affect the relationship between encoding prosody and verbal memory performance, at least for recognition memory. Future studies could attempt to include the interval between words as a variable of interest to understand how “pauses” inserted between verbal information can modulate the effect of prosody on verbal memory. 44 Conclusions To conclude, the current study has successfully replicated Schirmer’s finding that the affective connotation of a word in memory can be altered by changing its spoken prosody. It has also demonstrated that this valence shift effect is indeed a robust effect that is relatively independent of the time interval between to-be-encoded words. Current findings also point to a link between individual variation in heart rate acceleration and prosody effects on verbal memory. Individual variation in heart rate acceleration between sadly and neutrally spoken words correlated positively with the emotional difference in memory performance (d’ ESI). 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Neuroimage, 39, 885-893. [...]...4 Effects of speaker prosody on verbal memory Effects of emotional prosody on verbal memory have been examined by Kitayama (1996) The author tested the effects of emotional prosody on memory under different memory load conditions In his study, participants performed a memory span task which required them to memorize either two (low load) or four (high load) two-digit numbers for 20s During the 20s interval,... select old from among new sentences and to indicate their level of confidence in this selection When the memory load was low, results for recognition memory paralleled that of the free recall memory in that memory for sentences spoken with the neutral prosody was better than that for 5 sentences spoken with the emotional prosody In contrast, when memory load was high, memory for both types of sentences... that emotional prosody can either improve or impair memory for verbal content, and that the effect of emotional prosody on memory depend largely on memory load and the retrieval method employed at test (Kitayama, 1996) A recent study by Schirmer (2010) also explored the effect of speaker prosody on the memory representation of words In this study, participants performed a crossmodal verbal memory paradigm... obtained for the neutral condition were subtracted from values obtained for the sad condition for HR deceleration, HR acceleration, d’ scores and mean valence ratings The resulting indices were then subjected to the following two-tailed Pearson correlation analyses First, I tested the relationship between the HR deceleration ESI and the d’ ESI This analysis was non-significant (p > 0.1) Next, I tested the. .. with a fixation cross that was presented for 0.2 s in the center of the screen, followed by a spoken word simultaneously presented with a fixation cross, the latter lasting 2.3 s The trial ended with a blank screen 18 marking the onset of the intertrial interval (ITI) The ITI was jittered from 12 to 15 s in one second steps Each study phase consisted of 60 trials Half of the trials consisted of words spoken... deceleration for threatening words and taboo words, there seems to be a discrepancy with respect to HR acceleration These may stem from the nature of the stimuli and call for further investigations fMRI studies on emotional processing The last century has seen an explosion in the number of studies that used noninvasive techniques such as functional magnetic resonance imaging (fMRI) to examine the neural... tested the relationship between the HR acceleration ESI and the d’ ESI and observed a significant positive correlation (r = 0.287, p = 0.05) Correlations between cardiac responses and the valence rating were non-significant (ps > 0.1) Discussion The current study explored the influence of vocal emotions on heart rate during verbal encoding and whether such influences predict subsequent verbal memory 24... based on the emotional context in which these words are encountered A recent study using electroencephalography (Schirmer et al in preparation), replicated these results and further outlined the time course of prosody encoding processes that underlie the observed change in affective memory The present thesis was aimed to extend this work by studying the role of emotion related autonomic changes and the. .. response Functional images for the test phase were acquired using a similar EPI sequence with the exception of the TR being 2 s Image analysis The fMRI data was preprocessed and analyzed using the Statistical Parametric Mapping software (SPM8, Wellcome Trust Centre for Neuroimaging, University College London) Functional images obtained from the scanner were converted to NIFTI formatted images for further... word ‘OLD’ on the left and the word ‘NEW’ on the right of the screen Participants 19 with the opposite button assignment saw the reversed prompt The button assignments were counterbalanced across participants Once participants made an old/new judgment, the prompt disappeared and a second prompt appeared, instructing participants to rate the same word in terms of its emotional valence on a 5-point scale ... of verbal information maintained in long-term memory In the following section, I will present their findings 4 Effects of speaker prosody on verbal memory Effects of emotional prosody on verbal. .. indicate their level of confidence in this selection When the memory load was low, results for recognition memory paralleled that of the free recall memory in that memory for sentences spoken with the. .. button for new words, were prompted with the word ‘OLD’ on the left and the word ‘NEW’ on the right of the screen Participants 19 with the opposite button assignment saw the reversed prompt The