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Biomedical subjects

M S Gazzaniga

Publications and source records attributed to M S Gazzaniga.

At least 19 recordsLinked to original sources
PubMed1992

Residual vision in a scotoma: implications for blindsight.

Blindsight, the ability of some blind patients to describe attributes of stimuli they have no conscious awareness of seeing, has been attributed to a secondary (retinotectal) visual pathway. However, it has also been proposed that blindsight could be due to residual function within the primary (geniculostriate) visual pathway. Data have now been obtained that support the second alternative. With an image stabilizer ensuring the accurate retinal placement of stimuli, dense visual field mapping was carried out with a hemianopic patient. This perimetry revealed, embedded in the patient's scotoma, an isolated 1-degree island of residual vision that was not disclosed by conventional perimetric methods. Stimuli presented to this island could be detected and discriminated, although the subject reported he did not see them. The existence of this island of vision implies a corresponding island of functioning cortex within the patient's lesion. Other instances of blindsight may be mediated by similar islands of functioning cortex.

Eye Movements↗
PubMed1992

Bidirectional control of saccadic eye movements by the disconnected cerebral hemispheres.

The present investigation demonstrates that callosotomy patient J.W. can generate either leftward or rightward saccades in response to color cues presented unilaterally. When asked to name the colors, performance was at chance for left visual field presentations, demonstrating a disability in interhemispheric transfer of chromatic information. The successful control of saccadic direction based on discriminative color cues that appear confined to a single hemisphere may suggest a capacity for bidirectional control of saccadic eye movements in the disconnected cerebral hemispheres.

Brain↗
PubMed1992

Hemispheric differences in mnemonic processing: the effects of left hemisphere interpretation.

Several studies suggest that the abilities to make inferences and interpret events are stronger in the left hemisphere than the right hemisphere (Gazzaniga, M. S., The Social Brain. Basic Books, New York, 1985). Given that inference and interpretation are important aspects of normal memory functioning (Bartlett, F. C., Remembering: A Study in Experimental and Social Psychology, Cambridge University Press, 1932), one would expect this hemispheric difference to extend to mnemonic processing. Two split brain subjects were shown a series of pictures representing a common scene. Their memory for these pictures was later tested with a lateralized Yes-No recognition test where the distractor pictures were either consistent or inconsistent with the scene. The left hemisphere performed below chance on consistent distractor pictures whereas the right hemisphere was above chance on these pictures and performed at the same level of accuracy as the pictures originally presented. These results suggest that recognition performance in the left hemisphere was more strongly influenced by the expectations for actions common to a scene than the right hemisphere and provide evidence that the left hemisphere superiority in interpretation and inference effect memory performance.

Attention↗
PubMed1992

Reading with a limited lexicon in the right hemisphere of a callosotomy patient.

The generality of the observation that there is a lexicon present in the right hemisphere of callosotomy patients has been the subject of some dispute. In the series operated on at Dartmouth Hitchcock Medical Center, only two patients have been shown to have a right hemisphere lexicon. This paper reports the existence of a visual and an auditory lexicon in a new patient D.R. and discusses its significance in understanding the role of the right hemisphere in normal and dysfunctional language.

Corpus Callosum↗
PubMed1989

Independent hemispheric attentional systems mediate visual search in split-brain patients.

The primate visual system is adept at identifying objects embedded within complex displays that contain a variety of potentially distracting elements. Theories of visual perception postulate that this ability depends on spatial selective attention, a mechanism analogous to a spotlight or zoom lens, which concentrates high-level processing resources on restricted portions of the visual field. Previous studies in which attention was pre-cued to specific locations in the visual field have shown that the spotlight has a single, unified focus, even in the disconnected hemispheres of patients who have undergone surgical transection of the corpus callosum. Here we demonstrate that an independent focus of attention is deployed by each of the surgically separated hemispheres in a visual search task, such that bilateral stimulus arrays can be scanned at a faster rate by 'split-brain' subjects than by normal control subjects. The attentional system used for visual search therefore seems to be functionally and anatomically distinct from the system that mediates voluntary orienting of attention.

Attention↗
PubMed1989

Organization of the human brain.

Examination of structure-function correlates in the human brain reveals that there is a high degree of functional specificity in the information transmitted over neural systems. It also appears that the human brain has a modular organization consisting of identifiable component processes that participate in the generation of a cognitive state. The effects of isolating entire modular systems or of disconnecting the component parts can be observed. The features of a left hemisphere specialized capacity to interpret the actions of modules are discussed in terms of human consciousness.

Brain↗
PubMed1989

Magnetic resonance imaging morphology of the corpus callosum in monozygotic twins.

Recent reports describe wide variations in the size and shape of the human corpus callosum. To investigate genetic influences on this variability, magnetic resonance images from 5 pairs of monozygotic twins and 10 unrelated control subjects were analyzed. Measurements of size and shape revealed greater similarity in twin pairs than in randomly paired controls. The results are consistent with the view that the anatomy of the corpus callosum, while clearly influenced by nongenetic factors, is under considerable genetic control.

Adult↗
PubMed1989

Evidence of foveal splitting in a commissurotomy patient.

A bilateral projection of the central fovea, produced by naso-temporal overlap at the retinal vertical meridian, has been proposed as the anatomical basis of macular sparing. To evaluate this claim, a commissurotomy patient was required to compare target figures presented 1 degrees or less from the retinal midline with reference figures presented 2.5 degrees from the midline in the same or opposite visual field. The subject judged whether the target and reference were the same or different. It was found that the subject's fovea was effectively split with respect to the cerebral hemispheres; targets in the same visual field as the reference were readily matched with the reference, but accuracy dropped to chance for targets in the opposite field. Ways of reconciling this data with anatomical evidence of nasotemporal overlap and reports of macular sparing are discussed.

Adult↗
PubMed1989

Pain perception in a man with total corpus callosum transection.

While classical and current theories of pain emphasize the critical role of central neural pathways that represent the contralateral body surface and cross within the spinal cord, the role of neural input representing the ipsilateral body surface is uncertain. In the present experiments with a complete corpus callosum-sectioned patient, both tactile and low intensity noxious stimuli (43-47 degrees C) ipsilateral to the responding cerebral hemisphere were poorly perceived and/or rated low on verbal and visual analogue scales (VAS). Surprisingly, however, high intensity noxious thermal stimuli (49-51 degrees C) were rated on verbal or visual analogue scales as very intense and unpleasant, thereby reflecting both sensory-discriminative and motivational-affective dimensions of pain. Thus, the pathways and mechanisms subserving this ipsilateral input have high thresholds for activation, but once activated are sufficient to evoke all of the critical dimensions of the experience of pain.

Adult↗
PubMed1989

Human callosal function: MRI-verified neuropsychological functions.

A commissurotomy patient, with MRI-revealed sparing of some rostral and splenial fibers of the corpus callosum, judged whether pairs of words rhymed. We presented one word in each pair to her left visual field and the other to her right visual field. The 2 words in each pair either sounded and looked alike (R + L +), sounded alike but looked different (R + L -), sounded different but looked alike (R - L +), or both sounded and looked different (R - L -). Although in previous studies the patient has demonstrated little or no ability to transfer information between her brain hemispheres, she was able to perform the rhyming judgment significantly better than chance when the words both looked and sounded alike. However, her accuracy did not differ from chance in the other 3 conditions, or when she was asked to indicate if 2 letters presented to her opposing visual fields were the same or different. A second commissurotomy patient, with an MRI-verified full callosal section, performed at chance in all conditions, and normal control subjects were significantly better than chance in all conditions but R + L -. We discuss the results in terms of the specificity of the information carried by groups of callosal fibers.

Adult↗
PubMed1988

Processing of semantic anomaly by right and left hemispheres of commissurotomy patients. Evidence from event-related brain potentials.

The ability of 5 commissurotomized patients to appreciate semantic anomalies presented to their right and left hemispheres was tested using both electrophysiological and behavioural measures. In all cases, the patients heard sentence fragments that were completed either by semantically congruous or incongruous words briefly flashed to the left visual field, right visual field or to both fields simultaneously. A dissociation between behavioural and event-related brain potential (ERP) measures was observed. All 5 patients were able to indicate by a pointing response with greater than chance accuracy whether the terminal word of a sentence made sense (i.e., appropriate for the context) or was nonsensical. This was true regardless of the hemisphere receiving the terminal word. Likewise, all the patients responded to right visual field anomalies with a cerebral potential (N400) that was typically elicited by such words in control subjects. In contrast, only those 2 patients who developed an overt speech capability under the control of the right hemisphere produced N400 waves in response to left visual field anomalies. These findings were interpreted as suggesting possible relationships within language generation and semantic priming.

Adolescent↗
PubMed1987

No sex-related differences in human corpus callosum based on magnetic resonance imagery.

Recent studies have reported sex-related differences in the morphology of the human corpus callosum in cadaver brains. To further investigate these reports, sagittal magnetic resonance image scans from 40 male and 40 female subjects were used to compare callosal morphology. Relative callosal measurements were calculated by morphometric analysis. Significant sex-related differences were not found for callosal areas, maximal callosal width, or callosal curvature. These results indicate the need for further study before claims of sex-related differences can be accepted.

Corpus Callosum↗
PubMed1987

Perceptual and attentional processes following callosal section in humans.

Studies are reviewed that examine how perceptual and attentional systems operate in the cortically disconnected human. The data indicate that even though both simple and complex perceptual information associated with the cognitive activities of each disconnected half brain show virtually no interactions, the attentional system remains largely integrated in the split-brain patient. It also appears that the human brain is subject to a set of finite resources it can allocate to cognitive activities. These resources are not changed following cortical disconnection. Taken together, studies to date support the view that the attentional system is an independently functioning and integrated entity, following brain bisection, that participates in both perceptual and cognitive activities of each hemisphere.

Attention↗
PubMed1987

Disturbances in conceptual space involving language and speech.

We report on a patient, with a CT-verified low density lesion in the right parietal area, who exhibited not only deficits in left conceptual space, but also in reading, writing, and the production of speech. He presented with a left homonymous hemianopia, tactile inattention, and dysphasia that quickly resolved, leaving a left-sided inattention and less marked dysphasia. Several language tasks and language-based imagery tasks revealed poor language processing of the left portion of words which could be remedied if the patient was instructed to do things like spell the word backwards. The overall results were consistent with the idea that some aspects of language processing involve spatial mechanisms.

Brain Diseases↗
PubMed1987

Speech without conscious awareness.

Following commissurotomy, it is usually the case that information presented to the left hemisphere can be named and described, while information presented to the mute, right hemisphere cannot be spoken about. In the present study, it was discovered that under special test conditions, an MRI-verified, callosally sectioned adult could name or write about information presented to the right hemisphere. Studies revealed this was not an instance of right hemisphere expression. Rather, the right hemisphere was somehow able to program the left hemisphere for a spoken or written response. Further, the studies also showed that the left hemisphere was not consciously aware that it possessed the information transmitted to it by the right half brain.

Adult↗
PubMed1986

Proportional anterior commissure area in humans and monkeys.

Behavioral testing following division of the corpus callosum has shown that the anterior commissure can transfer information related to visual tasks interhemispherically in nonhuman primates but not in humans. We have found that the ratio of anterior commissure cross-sectional area to total neocortical commissural area is significantly greater for rhesus monkeys than for humans. This difference persists when the proportions are weighted for these structures' known differences in axon density. We hypothesize that this interspecies dissimilarity reflects a difference in the distribution of interhemispheric communication and may explain the behavioral differences observed after callosal section.

Adult↗
PubMed1985

Enhanced dual task performance following corpus commissurotomy in humans.

A commissurotomy patient and two neurologically intact control observers were required to encode spatial patterns presented concurrently to the two hemifields. Under conditions of maximal perceptual load, in which different patterns appeared in the two fields, the commissurotomy patient encoded more information than the control observers. Based on these findings, it is concluded that competition for common internal processing mechanisms interferes with overall processing efficiency.

Adult↗