Showing papers by "Michael S. Gazzaniga published in 2001"

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

Abstract: 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.

Representation of Visuotactile Space in the Split Brain

Abstract: 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 representtion ofvisuotactile 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.

Chapter 22 Speculations on the neural basis of islands of blindsight

Abstract: 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.

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

Abstract: 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.

Visual Implicit Memory in the Left Hemisphere: Evidence From Patients With Callosotomies and Right Occipital Lobe Lesions

Abstract: 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 mem- ory in patients with extensive right occipital lobe lesions, using lexi- cal-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 com- plete 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 im- plicit memory, and that the isolated left hemisphere can support nor- mal levels of visual priming in a variety of tasks In order to determine how the brain utilizes information about past events, it is necessary to identify the brain regions that are involved in different memory processes The examination of patients with local- ized brain damage has played a critical role in identifying the neuro- logical substrates of several memory processes (eg, Gabrieli, 1998; Squire, 1988) For example, patients with damage to the medial tem- poral lobes exhibit pronounced deficits in explicit tests of memory, such as free recall, that require conscious recollection of previous epi- sodes (eg, Cohen & Squire, 1980; Scoville & Milner, 1957) This in- dicates that the medial temporal lobes play a necessary role in this form of memory These same patients perform normally on other tasks, such as tests of visual implicit memory, in which a person's ability to identify or make judgments about visually presented items is primed, or enhanced, by previous visual encounters with those items (eg, Graf & Mandler, 1984; Warrington & Weiskrantz, 1974) The preserved priming observed in these patients indicates that the medial temporal lobes are not necessary for normal visual implicit memory The precise neuroanatomical substrates of visual implicit memory are unknown However, studies of brain-damaged patients and neu- roimaging studies of healthy subjects show that the occipital lobes are involved in this form of memory, and several of these studies suggest that the right hemisphere may be particularly important For example, a number of patients with occipital lobe lesions have been shown to exhibit deficits on visual implicit memory tasks (Carlesimo, Fadda, Sabbadini, & Caltagirone, 1994; Curran, Schachter, & Galluccio,

MRI assessment of spared fibers following callosotomy: A second look

Abstract: The development of callosotomy (or “split-brain”) surgery for the relief of intractable epilepsy offered the first opportunity to assess the roles of the cortical and subcortical connections in transferring information between the hemispheres Some of the early research on interhemispheric transfer in these patients produced confusing and conflicting results, partly due to the lack of availability of noninvasive methods to verify that the entire corpus callosum had been resected The advent of MRI scanning revealed that some patients had residual callosal fibers that had been inadvertently spared during surgery1 In 1985 our research group reported such a case2 We described MRI assessment of callosotomy in three patients In one of these, MRI revealed areas of bright signal in the splenium and rostrum of the corpus callosum, which were assumed to reflect spared callosal fibers Like most callosotomy patients, our patient fails most tests of interhemispheric information transfer2,3⇓ Several studies have shown, however, that she is sometimes able to integrate information between the two hemispheres when the stimuli are visually presented words4-6⇓⇓ We have speculated that …

Linking neuroimaging and cognitive neuropsychiatry

Abstract: Introduction. The continuing goal of cognitive neuropsychiatry will be to shed light on the nature of psychiatric disorders. Methods. We reviewed recently the yield of a variety of experimental approaches, including cognitive studies, of schizophrenia, one of psychiatry's most difficult challenges. Results. We discussed the constraints of contemporary experimental methods which limit our ability to develop an integrated view of the pathological processes that underlie schizophrenia. Conclusions. An integrated model of the underlying nature of major psychiatric disorders, such as schizophrenia, will have to await, among other developments, the union of cognitive experimental methods with imaging techniques with the requisite temporal and spatial resolution, and the capacity to detect changes in brain form and function that reflect accommodations to the disorders.