A split-brain case study on the hemispheric lateralization of inhibitory control.

Understanding the neurobiological mechanisms underlying inhibitory control is crucial given its role in various disease states and substance abuse/misuse. Neuroimaging research examining inhibitory control has yielded conflicting results on the relative importance of the left and right hemisphere during successful inhibition of a motor response. In the current study, a split-brain patient was examined in order to assess the independent inhibitory capabilities of each hemisphere. The patient's right hemisphere exhibited superior inhibitory ability compared to his left hemisphere on three inhibitory control tasks. Although inferior to the right, the left hemisphere inhibited motor responses on inhibitory trials in all three tasks. The results from this study support the dominance of the right hemisphere in inhibitory control.

The calculating hemispheres: studies of a split-brain patient.

The purpose of the study was to investigate simple calculation in the two cerebral hemispheres of a split-brain patient. In a series of four experiments, the left hemisphere was superior to the right in simple calculation, confirming the previously reported left hemisphere specialization for calculation. In two different recognition paradigms, right hemisphere performance was at chance for all arithmetic operations, with the exception of subtraction in a two-alternative forced choice paradigm (performance was at chance when the lure differed from the correct answer by a magnitude of 1 but above chance when the magnitude difference was 4). In a recall paradigm, the right hemisphere performed above chance for both addition and subtraction, but performed at chance levels for multiplication and division. The error patterns in that experiment suggested that for subtraction and addition, the right hemisphere does have some capacity for approximating the solution even when it is unable to generate the exact solution. Furthermore, right hemisphere accuracy in addition and subtraction was higher for problems with small operands than with large operands. An additional experiment assessed approximate and exact addition in the two hemispheres for problems with small and large operands. The left hemisphere was equally accurate in both tasks but the right hemisphere was more accurate in approximate addition than in exact addition. In exact addition, right hemisphere accuracy was higher for problems with small operands than large, but the opposite pattern was found for approximate addition.

A dissociation between the representation of tool-use skills and hand dominance: insights from left- and right-handed callosotomy patients.

The overwhelming majority of evidence indicates that the left cerebral hemisphere of right-handed humans is dominant both for manual control and the representation of acquired skills, including tool use. It is, however, unclear whether these functions involve common or dissociable mechanisms. Here we demonstrate that the disconnected left hemispheres of both right- and left-handed split-brain patients are specialized for representing acquired tool-use skills. When required to pantomime actions associated with familiar tools (Experiment 2), both patients show a right-hand (left hemisphere) advantage in response to tool names, pictures, and actual objects. Accuracy decreases as stimuli become increasingly symbolic when using the left hand (right hemisphere). Tested in isolation with lateralized pictures (Experiment 3), each patient's left hemisphere demonstrates a significant advantage over the right hemisphere for pantomiming tool-use actions with the contralateral hand. The fact that this asymmetry occurs even in a left-handed patient suggests that the left hemisphere specialization for representing praxis skills can be dissociated from mechanisms involved in hand dominance located in the right hemisphere. This effect is not attributable to differences at the conceptual level, as the left and right hemispheres are equally and highly competent at associating tools with observed pantomimes (Experiment 4).

Numerical processing in the two hemispheres: studies of a split-brain patient.

Neuroimaging and lesion studies have provided insights into the neural mechanisms underlying numerical processing, yet the roles of the right and left hemispheres have not been systematically investigated within a single study. To address this issue, we investigated subitizing and magnitude comparison abilities in a split-brain patient. The first experiment examined the two hemispheres' abilities to enumerate briefly presented sets of one to four stimuli. Both hemispheres were equally able to perform this task. The second and third experiments examined the hemispheres' abilities to make magnitude judgments about two simultaneously presented stimuli that were either identically coded (i.e., two Arabic numerals, two number words, or two arrays of dots) or differently coded (e.g., an Arabic numeral and a number word). Although the left hemisphere was more accurate than the right when the task involved number words, both hemispheres were able to make comparisons between numerical representations regardless of stimuli coding. In addition, both hemispheres exhibited a distance effect. The results are discussed in the context of Dehaene's triple-code model.

Temporal discrimination in the split brain.

Divided visual field studies of neurologically normal adults indicate that the left hemisphere is superior to the right in making temporal judgments. Some neuroimaging and neuropsychological studies, however, have suggested a role for the right hemisphere in temporal processing. We tested the divided hemispheres of a split-brain patient in two tasks requiring temporal judgments about visually presented stimuli. In one task, the patient judged whether two circles presented to one visual field appeared for the same or different durations. In the second task, the patient judged whether the temporal gaps in two circles occurred simultaneously or sequentially. In both tasks, the performance of the right hemisphere was superior to that of the left. This suggests that the right hemisphere plays an important role in making temporal judgments about visually presented stimuli.

Mike or me? Self-recognition in a split-brain patient.

A split-brain patient (epileptic individual whose corpus callosum had been severed to minimize the spread of seizure activity) was asked to recognize morphed facial stimuli--presented separately to each hemisphere--as either himself or a familiar other. Both hemispheres were capable of face recognition, but the left hemisphere showed a recognition bias for self and the right hemisphere a bias for familiar others. These findings suggest a possible dissociation between self-recognition and more generalized face processing within the human brain.

An investigation of the line motion effect in a callosotomy patient.

When a line is flashed instantaneously between two markers it can appear to propagate from one marker to the other. This illusion is known as the line motion effect. We investigated this effect in the two hemispheres of a callosotomy ("split-brain") patient. We found that both hemispheres perceived the line motion effect, and that flashing one of the markers biased the direction of motion away from that marker regardless of which hemisphere received the stimulus. In contrast, matching the width of the line to the width of one of the markers biased the direction of motion away from the marker only when it appeared in the left visual hemifield. This suggests that multiple mechanisms can contribute to the line motion effect, and that some of these mechanisms rely on different neural structures.

Hemispheric asymmetries for simple visual judgments in the split brain.

While it is commonly noted that the right cerebral hemisphere is specialized for visuospatial processing, the scope and nature of this specialization remain somewhat ill defined. Our previous research with callosotomy ('split-brain') patients has suggested that the asymmetry may be limited to conditions that have an explicit spatial component. To investigate this we compared the performance of the divided hemispheres of two callosotomy patients on four simple visual-matching tasks. These tasks were orientation discrimination, vernier offset discrimination, size discrimination, and luminance discrimination. In each task, two stimuli were presented briefly to one visual hemifield and the patient was asked to discriminate whether they were the same or different. The first three tasks (orientation, vernier, and size) were all spatial in nature and were performed better by the right hemisphere. The luminance discrimination task, which is non-spatial, was performed equivalently by the two hemispheres. These results support the view that the fundamental difference in visual function between the hemispheres is in the ability to perform spatial discriminations.