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The functions of the left and right brain hemispheres in memory for presented material are tested with the possibility of a hemispheric difference that accounts for unpresented material. The hypothetical difference is tested in N400 studies involving split-brain patients. The right hemisphere accounted for more exact memory recollections than the left hemisphere. The difference indicated a more veridical right-hemisphere memory system with a vital adaptive function.

The authors examined some of the sensorimotor effects of the split-brain operation to understand how a 'dual mind' can produce unified behavior. They report psychophysical evidence of extinction to bilateral simultaneous stimulation in callosotomy patient J.W.: Although he could verbally report the occurrence of a unilateral left or right visual field target, left field report accuracy dropped by 34% when targets occurred bilaterally. Paradoxically, the same stimulus conditions produced abnormally robust redundant signal effects on simple manual and vocal reaction times, which exceeded predictions that were based on probability summation. Neural summation is often inferred from redundancy gain of this magnitude. Because this seems less likely after callosotomy, the authors suggest a model that is based on response competition between the disconnected hemispheres to account for J.W.'s redundant target effects. The dissociation between explicit report and motor performance is discussed.

Split-brain patients were studied and it was found that higher cognitive techniques such as language and strategic visuospatial attentional process are preferentially lateralized to the left cerebral hemisphere in their case. The capacity to perform guided search is not shown in the right hemisphere even if the search time in that hemisphere is superior to search time in the left. In normal subjects, on the other hand, guided searches are mediated by the callosum in the right hemisphere.

Previous studies of visuospatial attention indicated that the isolated cerebral hemispheres of split-brain patients maintain an integrated, unitary focus of attention, presumably due to subcortical attentional mechanisms. The present study examined whether a unitary attentional focus would also be observed during a visual search task in which subjects scanned stimulus arrays for a target item. In a group of four commissurotomy patients, the search rate for bilateral stimulus arrays was found to be approximately twice as fast as the search rate for unilateral arrays, indicating that the separated hemispheres were able to scan their respective hemifields independently. In contrast, the search rates for unilateral and bilateral arrays were approximately equal in a group of six normal control subjects, suggesting that the intact corpus callosum in these subjects is responsible for maintaining a unitary attentional focus during visual search.

Dans le but d'investiguer si la plasticite cerebrale rapportee dans les cas d'absence congenitale du corps calleux pouvait egalement etre observee lors de callosotomie post-natale, une serie d'etudes a ete effectuee aupres de patients epileptiques et de chatons operes a divers ages. Les patients devaient realiser des epreuves de stereognosie unilaterale et de comparaisons intermanuelles. Le plus jeune patient opere a egalement ete soumis a des taches de comparaisons intra-et interhemispheriques d'informations visuelles et tactiles. Les animaux, quant a eux, etaient testes dans une tache mesurant le transfert interhemispherique d'un apprentissage visuel. Les resultants ont demontre que la plasticite calleuse est observable meme apres la naissance tant chez l'animal que chez l'homme. De plus, les resultats suggerent que les mecanismes compensatoires utilises pour pallier a l'absence du corps calleux different selon l modalite utilisee. Des hypotheses sont emises quant a la nature des mecanismes fonctionnels et/ou physiologiques permettant de pallier a l'absence du corps calleux. A series of experiments examined the potential plasticity of the callosal system in both epileptic patients and in kittens submitted to corpus callosotomy at various ages. The patients were tested for unilateral discrimination and interhemispheric transfer of tactile information. They youngest patient was also required to perform additional inter-and intrahemispheric comparisons ov visual and tactile stimuli. The animals were tested for interhemispheric transfer of visual discriminations. The results suggest that in both animals and humans there exists a critical period before which callosal section does not discrupt interhemispheric communication. The results also indicate that the compensatory mechanisms used to achieve interhemishpheric transfer in the absence of the corpus callosum may vary according to the sensory modality involved. The possible physiological and/or functional mechanisms responsible for callosal plasticity are discussed.

A recent case report of interhemispheric correlational measures in a callosotomized patient by Corsi-Cabrera, Trias, Guevara, Haro, and Hernandez in 1995 provided evidence taken by the authors of the study to suggest that the corpus callosum may not be crucial to interhemispheric coupling. This conclusion was proposed even though (1) presurgical correlation measures necessary for evaluating coherence changes produced by surgery were not available for this subject and (2) previous studies presenting evidence inconsistent with their conclusions were not discussed. In view of these two shortcomings, the authors' conclusion concerning callosal function may be premature.

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.

Ever since the results of the first 'split brain' studies were reported in the early 1960s (Gazzaniga, Bogen, & Sperry, 1962; Sperry, 1961), laypersons and investigators alike have been fascinated [...], The ability of the two hemispheres of the brain to communicate with one another via the corpus callosum is important for a wide variety of sensory, motor, and cognitive functions, many of them communication related. Anatomical evidence suggests that aging results in structural changes in the corpus callosum and that the course over time of age-related changes in corpus callosum structure may depend on the gender of the individual. Further, it has been hypothesized that age- and gender-related changes in corpus callosum structure may result in concomitant decreased performance on tasks that are reliant on interhemispheric integrity. The purpose of this study was to investigate the effects of age and gender on auditory behavioral and visuomotor temporal indices of interhemispheric function across the life span of the normal adult. Results from 120 consistently right-handed adults from age 20 to 75 years revealed that interhemispheric integrity, as measured by dichotic listening, auditory temporal patterning, and visuomotor interhemispheric transfer time tasks, decreases relatively early in the adult life span (i.e., between the ages of 40 and 55 years) and shows no further decrease thereafter. In addition, the course over time of interhemispheric decline is different for men compared to women for some tasks. These findings suggest that decreased interhemispheric function may be a possible factor contributing to auditory and communication difficulties experienced by aging adults. In addition, results of this study hold implications for the clinical assessment of interhemispheric function in aging adults and for future research into the functional ramifications of decreased multimodality interhemispheric transfer. KEY WORDS: aging, callosum, dichotic listening, interhemispheric, auditory processing

INTRODUCTION To perceive objects in the environment as unified wholes, the visual system must often extrapolate from incomplete contour or boundary information. Under certain conditions extrapolated contours are perceived in [...], A fundamental problem in form perception is how the visual system can link together spatially separated contour fragments to form the percept of a unitary shape. Illusory contours and amodal completion are two phenomena that demonstrate this linking process. In the present study we investigate these phenomena in the divided hemispheres of two callosotomy ('split-brain') patients. The data suggest that dissociable neural mechanism are responsible for the generation of illusory contours and amodal completion. Although both cerebral hemispheres appear to be equally capable of perceiving illusory contours, amodal completion is more readily utilized by the right hemisphere. These results suggest that illusory contours may be attributable to low-level visual processes common to both hemispheres, whereas amodal completion reflects a higher-level, lateralized process.

THE preceding paper, by Louis Tinnin, challenges us to consider that there is a brain agency responsible for mental unity, volition and consciousness, which the author labels a 'governing mental system' (GMS), or 'ego,' and that the neural substrate for this GMS is Wernicke's Area. The primary evidence in support of this position comes from three sources: a review of brain and language development in normal and abnormal individuals (e.g., deaf children); studies of patients who have undergone corpus callosum section for intractable epilepsy; and some observations of memory processing during the intracarotid sodium amytal (Wada) procedure. Our comments address two issues. The first is an analysis of the adequacy of these three sources as evidence for Tinnin's position. The second is a description of an alternative model to account for the mental system we agree exists but do not yet understand.

THERE is a curious behavior observed in the human split-brain experiments in which the subject demonstrates a reflexive and obligatory ownership of the actions initiated by the silent right brain even though the speaking self is ignorant of that volition. If you add to this observation the results of another experiment, revealing the existence of a boundary to the verbal mental system in the intact brain, then -- with the knowledge of certain facts about cerebral laterality -- you may come to a startling conclusion: There exists a governing mental system that occupies the verbally dominant hemisphere and is responsible for mental unity, volition, and consciousness. I contend that this anatomical brain agency is the substrate for the ego. If this is true, then it can be seen that the ego, in its development, function, and integrity, is intimately involved in the brain dynamics of obligatory unity, cerebral dominance, and laterality.

Complementary Hemispheric Specialization in Monkeys IT IS IMPORTANT TO KNOW WHETHER nonhuman primates have complementary specialization of the cerebral hemispheres corresponding to the well-known differences described for human beings, in [...]

Aspects of a Cognitive Neuroscience of Mental Imagery PERHAPS THE MOST FUNDAMENTAL INSIGHT OF CONTEMPORARY cognitive science is the discovery that mental faculties can be decomposed into multicomponent information-processing systems. [...]