NEUROLOGICAL FOUNDATIONS OF COGNITIVE NEUROSCIENCE - PART 2 pot

Anjan Chatterjee of the object itself or to axes intrinsic to the environment Attention and Intention Attention and intention are tightly linked The extent to which perception and actions are coordinated in the formation and sustenance of spatial representations is remarkable The actions themselves, whether they are eye movements, head movements, or limb movements in space, are also related to notions of different kinds of reference frames Attention and Perception Attention and perception may not be as distinct as is often thought Processing of relatively early stages of perception seems to be modulated by attention, although the precise boundaries between the two remain to be worked out Unresolved Issues Despite this convergence of ideas, I would like to mention some issues that in my view warrant further consideration Some questions involve research in neglect directly and others involve the relationship of findings in neglect and other approaches Contralesional Hyperorientation in Neglect Why patients with right brain damage sometimes “hyperorient” into contralesional space, rather than neglect contralesional space? We are used to thinking of neglect as the tendency to orient toward or act in ipsilesional space However, in some cases patients seem to be drawn contralesionally The most robust of these contralesional productive behaviors is the crossover phenomenon, in which patients bisect short lines (usually less than cm) to the left of the midline However, there are other dramatic instances of contralesional hyperorientation (Chatterjee, 1998) Some patients bisect long lines in contralesional space (Adair, Chatterjee, Schwartz, & Heilman, 1998a; Kwon & Heilman, 1991) Some patients will point into contralesional 18 space when asked to indicate the environmental midline (Chokron & Bartolomeo, 1998) What has happened to left-sided representations or to motor systems directed contralesionally to produce this paradoxical behavior? Memory, Attention, and Representation How does memory interact with attention to affect online processing of stimuli in neglect? Functional imaging studies and neurophysiological studies suggest that there is considerable overlap between circuits dedicated to spatial attention and spatial working memory Monkey lesion studies indicate an important role for spatial memories in online processing (Gaffan & Hornak, 1997) We recently reported that memory traces of contralesional stimuli might have a disproportionate influence on online representations in patients with neglect (Chatterjee et al., 2000) A conceptual framework that relates spatial memory and attention in influencing online perception remains to be articulated Frontal and Parietal Differences How different are the roles of the frontal and parietal cortices in spatial attention? The notion that parietal neglect is attentional and frontal neglect is intentional has great appeal Unfortunately, the empirical evidence for such a clear dichotomy is mixed at best It is not even clear that these distinctions make conceptual sense, since what has been called “attentional neglect” involves eye movements and what has been called “intentional neglect” involves limb movements Single-cell neurophysiological studies suggest that neurons within both parietal and frontal cortices mediate spatial actions It may be the case that the actions are more clearly segregated in the frontal cortex than in the parietal cortex However, it is not clear that one should expect clean behavioral dissociations from lesions to the frontal and parietal cortices Perhaps eye and limb movements may be coded within the same array of neurons, as suggested by Andersen and colleagues (Andersen, 1995a) and Pouget and Neglect Sejnowski (1997) for the coding of visual reference frames If that were the case, it is not clear how lesions would bias behavior toward different forms of neglect Furthermore, the ways in which frontal and parietal areas interact based on their interconnections is not well understood In humans, damage to the posterior superior longitudinal fasciculus and the inferior frontal fasciculus is associated with more severe and long-lasting neglect Similarly in monkeys, transection of the white matter underlying the parietal cortex is also associated with greater neglect Distinctions within the Parietal Cortex What are the roles of different regions within the posterior parietotemporal lobes? Lesion studies in humans suggest that damage to the inferior parietal lobule or the superior temporal gyrus produces the most consistent and profound disorder of spatial attention and representation Lesion studies in humans suggest that damage to the inferior parietal lobule or superior temporal gyrus produces the most consistent and profound disorder of spatial attention and representation By contrast, functional imaging studies activate more dersal regions within the intraparietal sulcus and the superior parietal sulcus most consistently Why this discrepancy? Perhaps the greater dorsal involvement in functional imaging studies is related to the design of the studies, which emphasize shifts of visual attention Perhaps experimental probes emphasizing the integration of both “what” and “where” information would be more likely to involve the inferior parietal cortex Recent functional imaging data suggest that the temporal-parietal junction may be preferentially activated when subjects detect targets, rather than simply attend to locations (Corbetta et al., 2000) Monkey lesion studies may not be able to resolve the discrepancy for two reasons As mentioned below, the appropriate anatomical monkey–human homologs are not clear, and neglectlike symptoms occur only transiently following parietal lesions in monkeys 19 Monkey and Human Homologs What are the appropriate anatomical homologs between humans and monkeys? Human lesion studies focus on the inferior parietal lobule It is not clear that an analogous structure exists in monkeys (Watson et al., 1994) Both human functional imaging studies and monkey neurophysiology emphasize the role of the intraparietal sulcus However, it is not clear that these two structures are homologous across species In summary, we know a great deal about spatial attention and representation Across the varied disciplines there is a remarkable convergence of the kinds of questions being asked and solutions being proposed However, many questions remain A comprehensive and coherent understanding of spatial attention and representation is more likely with the recognition of insights gleaned from different methods Acknowledgments This work was supported by National Institutes & Health grout RO1 NS37539 I would like to thank Lisa Santer for her critical reading of early drafts of this chapter References Adair, J., Chatterjee, A., Schwartz, R., & Heilman, K (1998a) Ipsilateral neglect: Reversal of bias or exaggerated cross-over phenomenon? 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at the age of 54 and produced infarction in the right parietal lobe and a small lesion in the right cerebellum He recovered from a transient left hemiparesis and left hemispatial neglect The second stroke, in March 1992, involved the left parietal lobe and left him functionally blind Five months after the second stroke, he was referred to a neurologist for headaches At that time, neurological examination revealed a classical Bálint’s syndrome without any other deficits of cognitive, motor, or sensory function The patient had normal visual acuity; he could recognize colors, shapes, objects, and faces and could read single words He suffered severe spatial disorientation, however, and got lost easily anywhere except in his own home Although he was independent in all activities of daily living, he could not maintain his own household and had to be cared for by his family He had to be escorted about the hospital When shown two objects, he often saw only one When he did report both, he did so slowly and seemed to see them sequentially Depth perception was severely impaired and he could not judge the distance of objects from him or tell which of two objects was closer to him Optic ataxia was pronounced He could not reach accurately toward objects, and was unable to use a pencil to place a mark within a circle He could not make accurate saccades to objects and he could not make pursuit eye movements to follow the most slowly moving object Visual acuity was 20/15 in both eyes Perimetry at the time of the initial neurological exam revealed an altitudinal loss of the lower visual fields Two years later, however, visual fields were full Contrast sensitivity and color vision were normal Three-dimensional experience of shapes in random dot stereograms was preserved and he experienced depth from shading His headaches were controlled with amitriptyline, and anticoagulation treatment with warfarin was instituted to prevent further strokes By June 1995, the patient was able to live independently in a duplex next door to his brother’s daughter, and needed only intermittent help in his daily activities He was able to take unescorted walks in his neighborhood, to get about in his own house without help, watch television, eat and dress himself, and carry on many activities of daily living He was slower than normal in these activities, but was able to lead a semi-independent life A magnetic resonance imaging (MRI) scan in 1994 with three-dimensional reconstruction revealed nearly symmetrical lesions in each parieto-occipital region (FriedmanHill, Robertson, & Treisman, 1995) The lesions were concentrated primarily in Brodmann areas and 39, and possibly included some of areas and 19 In addition, there was a small (volume