Localization of the human cortical visual area MT based on computer aided histological analysis.

We describe human area MT histologically based on the observer independent analysis of cortical myeloarchiteture, multiple complementary staining techniques and 3-D reconstruction. The topography of an architectonic field that presented constant structural characteristics across specimens was studied in relation to the sulcal geography of the occipito-temporal region. Objective and semi-automated analysis of local microstructure revealed a distinct cortical architecture and matched topographically the localization of MT derived from functional imaging. MT was localized by the histotopographic method in relation to definite macroscopic landmarks. This study demonstrates a new set of distinguishing architectonic features of human MT that permit localization on structural grounds and suggests that the characteristic laminar structure of this area may be related to its unique pattern of connections and to its role in visual perception.

The neural substrates of visual implicit memory: do the two hemispheres play different roles?

Identification of visually presented words is facilitated by implicit memory, or visual priming, for past visual experiences with those words. There is disagreement over the neuro-anatomical substrates of this form of implicit memory. Several studies have suggested that this form of priming relies on a visual word-form system localized in the right occipital lobe, whereas other studies have indicated that both hemispheres are equally involved. The discrepancies may be related to the types of priming tasks that have been used because the former studies have relied primarily on word-stem completion tasks and the latter on tasks like word-fragment completion. The present experiments compared word-fragment and word-stem measurements of visual implicit memory in patients with right occipital lobe lesions and patients with complete callosotomies. The patients showed normal visual implicit memory on fragment completion tests, but essentially no visual priming on standard stem completion tests. However, when we used a set of word stems that had only one correct solution for each test item, as was true of the items in the fragment completion tests, the patients showed normal priming effects. The results indicate that visual implicit memory for words is not solely dependent upon the right hemisphere, rather it reflects changes in processing efficiency in bilateral visual regions involved in the initial processing of the items. However, under conditions of high lexical competition (i.e., multiple completion word stems), the lexical processes, which are dominant in the left hemisphere, overshadow the visual priming supported by the left hemisphere.

Selective activation of a parietofrontal circuit during implicitly imagined prehension.

It is generally held that motor imagery is the internal simulation of movements involving one's own body in the absence of overt execution. Consistent with this hypothesis, results from numerous functional neuroimaging studies indicate that motor imagery activates a large variety of motor-related brain regions. However, it is unclear precisely which of these areas are involved in motor imagery per se as opposed to other planning processes that do not involve movement simulation. In an attempt to resolve this issue, we employed event-related fMRI to separate activations related to hand preparation-a task component that does not demand imagining movements-from grip selection-a component previously shown to require the internal simulation of reaching movements. Our results show that in contrast to preparation of overt actions, preparation of either hand for covert movement simulation activates a large network of motor-related areas located primarily within the left cerebral and right cerebellar hemispheres. By contrast, imagined grip selection activates a distinct parietofrontal circuit that includes the bilateral dorsal premotor cortex, contralateral intraparietal sulcus, and right superior parietal lobule. Because these areas are highly consistent with the frontoparietal reach circuit identified in monkeys, we conclude that motor imagery involves action-specific motor representations computed in parietofrontal circuits.

Speculations on the neural basis of islands of blindsight.

Blindsight, residual visual function in the absence of conscious awareness, can sometimes be found within the scotomas of patients with lesions to primary visual cortex. However, cases in which blindsight is distributed across large regions of a scotoma are extremely rare. In contrast, blindsight is relatively frequent within small islands of residual visual function. We review the evidence for the existence of these islands. We argue that blindsight is likely to depend on vestiges of geniculostriate function, and that in humans the secondary retinotectal pathway has little functional utility in the absence of geniculostriate support. To account for the frequency of blindsight within residual islands of function, we speculate that patients may be unaware of such islands precisely because they are islands, which are isolated from the integrated network of neural activity that represents visual space. The relationship of blindsight to the hemispatial neglect is considered in this context.

The Functional Magnetic Resonance Imaging Data Center (fMRIDC): the challenges and rewards of large-scale databasing of neuroimaging studies.

The Functional Magnetic Resonance Imaging Data Center (fMRIDC) (http://www.fmridc.org) was established in the Autumn of 1999 with the objective of creating a mechanism by which members of the neuroscientific community may more easily share functional neuroimaging data. Examples in other sciences offer proof of the usefulness and benefit that sharing data provides through encouraging growth and development in those fields. By building a publicly accessible repository of raw data from peer-reviewed studies, the Data Center hopes to create a similarly successful environment for the neurosciences. In this article, we discuss the continuum of data-sharing efforts and provide an overview of the scientific and practical difficulties inherent in managing various fMRI data-sharing approaches. Next, we detail the organization, design and foundation of the fMRIDC, ranging from its current capabilities to the issues involved in the submitting and requesting of data. We discuss how a publicly accessible database enables other fields to develop relevant tools that can aid in the growth of understanding of cognitive processes. Information retrieval and meta-analytic techniques can be used to search, sort and categorize study information with a view towards subjecting study data to secondary 'meta-' and 'mega-analyses'. In addition, we detail the technical and policy challenges that have had to be addressed in the formation of the Data Center. Among others, these include: human subject confidentiality issues; ensuring investigator's rights; heterogeneous data description and organization; development of search tools; and data transfer issues. We conclude with comments concerning the future of the fMRIDC effort, its role in promoting the sharing of neuroscientific data, and how this may alter the manner in which studies are published.

Hemispheric processing asymmetries: implications for memory.

Recent research has demonstrated that memory for words elicits left hemisphere activation, faces right hemisphere activation, and nameable objects bilateral activation. This pattern of results was attributed to dual coding of information, with the left hemisphere employing a verbal code and the right a nonverbal code. Nameable objects can be encoded either verbally or nonverbally and this accounts for their bilateral activation. We investigated this hypothesis in a callosotomy patient. Consistent with dual coding, the left hemisphere was superior to the right in memory for words, whereas the right was superior for faces. Contrary to prediction, performance on nameable pictures was not equivalent in the two hemispheres, but rather resulted in a right hemisphere superiority. In addition, memory for pictures was significantly better than for either words or faces. These findings suggest that the dual code hypothesis is an oversimplification of the processing capabilities of the two hemispheres.

Visual implicit memory in the left hemisphere: evidence from patients with callosotomies and right occipital lobe lesions.

Identification of visually presented objects and words is facilitated by implicit memory for past visual experiences with those items. Several behavioral and neuroimaging studies suggest that this form of memory is dependent on perceptual processes localized in the right occipital lobe. We tested this claim by examining implicit memory in patients with extensive right occipital lobe lesions, using lexical-decision mirror-reading, picture-fragment, and word-fragment-completion tests, and found that these patients exhibited normal levels of priming. We also examined implicit memory in patients with complete callosotomies, using standard and divided-visual-field word-fragment-completion procedures, and found that the isolated left hemisphere exhibited normal priming effects. The results indicate that the right occipital lobe does not play a necessary role in visual implicit memory, and that the isolated left hemisphere can support normal levels of visual priming in a variety of tasks.

Failure to remap visuotactile space across the midline in the split-brain.

We examined the effect of posture change on the representation of visuotactile space in a split-brain patient using a cross-modal congruency task. Split-brain patient J.W. made speeded elevation discrimination responses (up versus down) to a series of tactile targets presented to the index finger or thumb of his right hand. We report congruency effects elicited by irrelevant visual distractors placed either close to, or far from, the stimulated hand. These cross-modal congruency effects followed the right hand as it moved within the right hemispace, but failed to do so when the hand crossed the midline into left hemispace. These results support recent claims that interhemispheric connections are required to maintain an accurate representation of visuotactile space.

Representation of visuotactile space in the split brain.

Recent neurophysiological research in the monkey has revealed bimodal neuronal cells with both tactile receptive fields on the hand and visual receptive fields that follow the hands as they move, suggesting the existence of a bimodal map of visuotactile space. Using a cross-modal congruency task, we examined the representation of visuotactile space in normal people and in a split-brain patient (J. W.) as the right arm assumed different postures. The results showed that the congruency effects from distracting lights followed the hand around in space in normal people, but failed to do so in the split-brain patient when the hand crossed the midline. This suggests that cross-cortical connections are required to remap visual space to the current hand position when the hand crosses the midline.

Within grasp but out of reach: evidence for a double dissociation between imagined hand and arm movements in the left cerebral hemisphere.

What roles are played by the cerebral hemispheres in planning object-oriented reaching and grasping movements? In an attempt to address this question, we compared the abilities of the left and right hemispheres of commissurotomy patient J.W. to imagine hand manipulation (i.e., grasp) or arm transportation (i.e., reach) movements. A graphically rendered manipulandum (dowel) was briefly presented to the left (LVF) or right (RVF) visual fields in a variety of different orientations. In the grasp selection task (experiment 1), J.W. was required to determine which side of a dowel his thumb would be on if he were to engage the stimulus in a power grip using either his dominant (right) or non-dominant hand. In the reach selection task (experiment 3), J.W. judged which end his elbow would be on if he treated the dowel as an armrest for his dominant or non-dominant forearm. No actual movements were allowed in either task. Movements selected in the imagery tasks were compared with those chosen during actual motor control under comparable circumstances. These comparisons revealed a left hemisphere advantage for representing grasping movements involving the right hand, and reaching movements involving the left arm. The right hemisphere, by contrast, displayed moderate accuracy when representing grasping movements with the left hand, but appeared incapable of imagining reaching movements with either arm. The double dissociation between imagery for hand and arm movements in the left cerebral hemispere is consistent with the hypothesis that grasping and reaching components of prehension involve dissociable planning mechanisms.

Anterior and posterior callosal contributions to simultaneous bimanual movements of the hands and fingers.

In order to study the role of the corpus callosum in two-handed coordination we tested callosotomy subjects while they attempted to initiate simultaneous discrete movements with both hands. We observed four split-brain patients, including one pre- and post-operatively, as well as normal and epileptic control subjects. Split-brain patients made button presses that were less synchronous than either normal or epileptic controls. Although split-brain patients' average performance did not always differ from control subjects, callosotomy resulted in a 3-fold increase in the variability with which 'simultaneous' movements were initiated. The one subject tested pre- and post-callosotomy showed distinct changes in movement initiation synchrony after both the anterior and the posterior stages of the surgery. These changes suggest that anterior and posterior callosal fibres may make unique contributions to bimanual synchronization, depending on whether responses are self-initiated or in reaction to a visual stimulus. This study demonstrates that neural communication across anterior and posterior fibres of the corpus callosum strongly influences the temporal precision of bimanual coordination. Specifically, callosal transmission affects the degree of bilateral synchrony with which simple simultaneous hand and finger movements are initiated.

Neuroscience. Regional differences in cortical organization.

Although there are elegants maps of the human brain that reveal differences in cellular architecture between different cortical regions, there is not much information about how corresponding cortical regions differ between the left and right hemispheres. As Gazzaniga explains in his Perspective, new results reveal the surprising finding of asymmetry in area 22 (which is important for language processing) of the left and right hemisphere (Galuske et al.). Clusters of neurons in area 22 of the left hemisphere are spaced farther apart and have longer axons cabling them together than neuronal clusters in area 22 of the right hemisphere.

An evolutionary perspective on hemispheric asymmetries.

The evolution of hemispheric specialization of function has obvious benefits in terms of overall processing capacity, but it may also have associated costs to each hemisphere in isolation. These costs are not apparent in an intact brain since information can be readily transferred between the hemispheres via the cortical commissures. We suggest that the evolution of language in the left hemisphere may have occurred at the expense of some visuospatial functions. Because the right hemisphere was still capable of performing those functions, the relative left-hemisphere impairment for visuospatial processing would be invisible. If the cortical commissures are severed, as in callosotomy patients, the costs of specialization may become more apparent. We report data supporting the idea that the left hemisphere may have lost visuospatial abilities that it once possessed, and suggest that this process may also result in right hemisphere deficits in abilities related to linguistic processing.

Cerebral specialization and interhemispheric communication: does the corpus callosum enable the human condition?

The surgical disconnection of the cerebral hemispheres creates an extraordinary opportunity to study basic neurological mechanisms: the organization of the sensory and motors systems, the cortical representation of the perceptual and cognitive processes, the lateralization of function, and, perhaps most importantly, how the divided brain yields clues to the nature of conscious experience. Studies of split-brain patients over the last 40 years have resulted in numerous insights into the processes of perception, attention, memory, language and reasoning abilities. When the constellation of findings is considered as a whole, one sees the cortical arena as a patchwork of specialized processes. When this is considered in the light of new studies on the lateralization of functions, it becomes reasonable to suppose that the corpus callosum has enabled the development of the many specialized systems by allowing the reworking of existing cortical areas while preserving existing functions. Thus, while language emerged in the left hemisphere at the cost of pre-existing perceptual systems, the critical features of the bilaterally present perceptual system were spared in the opposite half-brain. By having the callosum serve as the great communication link between redundant systems, a pre-existing system could be jettisoned as new functions developed in one hemisphere, while the other hemisphere could continue to perform the previous functions for both half-brains. Split-brain studies have also revealed the complex mosaic of mental processes that participate in human cognition. And yet, even though each cerebral hemisphere has its own set of capacities, with the left hemisphere specialized for language and speech and major problem-solving capacities and the right hemisphere specialized for tasks such as facial recognition and attentional monitoring, we all have the subjective experience of feeling totally integrated. Indeed, even though many of these functions have an automatic quality to them and are carried out by the brain prior to our conscious awareness of them, our subjective belief and feeling is that we are in charge of our actions. These phenomena appear to be related to our left hemisphere's interpreter, a device that allows us to construct theories about the relationship between perceived events, actions and feelings.

Insights into the functional specificity of the human corpus callosum.

Patient VP underwent complete callosotomy for the control of intractable epilepsy at the age of 27 years. Subsequent MRI, however, revealed spared callosal fibres in the rostral and splenial ends of the corpus callosum. We report a series of experiments designed to determine whether these fibres support functional transfer of information between the two cerebral hemispheres. Although we found no evidence for transfer of colour, shape or size information, there is good evidence for transfer of word information. This suggests that the spared splenial fibres in VP's corpus callosum are material-specific. The results of these experiments illustrate the remarkable degree of functional specificity within the corpus callosum