Spatial stimulus-response compatibility in callosotomy patients and subjects with callosal agenesis

Interhemispheric Integration after Callosotomy: A Meta-Analysis of Poffenberger and Redundant-Target Paradigms

The central role of the corpus callosum in integrating perception and cognition across the cerebral hemispheres makes it highly desirable for clinical and basic research to have a repertoire of experimental paradigms assessing callosal functioning. Here, the objective was to assess the validity of two such paradigms (Poffenberger, redundant-target paradigms) by conducting single-step meta-analyses on individual case data of callosotomy patients. Studies were identified by systematic literature search (source: Pubmed and WebOfKnowledge, date: 07.03.2022) and all studies were included that reported callosotomy case data for either paradigm. Twenty-two studies (38 unique cases) provided 116 observations of the crossed-uncrossed difference (CUD) for the Poffenberger paradigm, while ten studies (22 cases, 103 observations) provided bilateral redundancy gain (bRG) measures. Using linear-mixed models with "individual" and "experiment" as random-effects variable, the mean CUD was estimated at 60.6 ms (CI95%: 45.3; 75.9) for commissurotomy, 43.5 ms (26.7; 60.2) for complete callosotomy, and 8.8 ms (1.1; 16.6) for partial anterior-medial callosotomy patients. The estimates of commissurotomy/callosotomy patients differed significantly from patients with partial callosotomy and healthy controls. The mean bRGmin (minimum unilateral reference) was estimated at 42.8 ms (27.1;58.4) for patients with complete and 30.8 ms (16.8; 44.7) for patients with partial callosotomy, both differing significantly from controls. One limitation was that different formulas for bRG were used, making it necessary to split the sample and reducing test power of some analyses. Nevertheless, the present findings suggest that both paradigms assess interhemispheric callosal integration, confirming their construct validity, but likely test distinct callosal functions.

Attention and awareness: Representation of visuomotor space in split-brain patients

Each cerebral hemisphere primarily controls and receives sensory input with regard to the contralateral hand. In the disconnected brain (split-brain), when the hands are uncrossed, direct visual access to each hand is available to the controlling (contralateral) hemisphere. However, when a hand crosses the midline, visual and tactile information regarding the hand are presented to different hemispheres. It is unknown how a contralateral hemisphere codes the position and orientation of a visually inaccessible hand in the disconnected brain. The present work addresses this issue. We ask how each hemisphere represents "its" hand across hand positions that span the midline in the absence of cortical input from the contralateral hemisphere. In other words, when a hand is placed across the midline and is visually inaccessible, is it represented by the controlling hemisphere: (1) in accordance with its new position with respect to the body (e.g., a left hand "becomes" a right effector when it crosses the midline), (2) with left/right position information unaltered (e.g., the left hand is represented as "left" regardless of its location), or (3) stripped of its location information altogether? The relationship between hand position and the spatial codes assigned to potential responses (an index of hand representation) was investigated in two split-brain patients using direct (Experiment 1) and orthogonal (Experiment 2) S-R compatibility paradigms. S-R compatibility effects in split-brain patients were consistent with those displayed by typical individuals. These findings suggest that position-based compatibility effects do not rely on cross-cortical connections. Rather, each hemisphere can accurately represent the full visuomotor space, a process that appears to be subserved by subcortical connections between the hemispheres.

The interhemispheric transcallosal-transversal approach to the lesions of the anterior and middle third ventricle: Surgical validity and neuropsychological evaluation of the outcome

Based on the observation of the course of callosal fibres and of their artero-venous support as appearing in a microanatomic study, the Authors propose a variant of standard callosotomy procedure by the introduction of the transverse section of callosal fibres. This technique would allow the surgeon to spare a larger number of callosal fibres by the combined effect of a lower direct mechanical traction on fibres and a lower impact on artero-venous microcircle. The neuropsychological outcome of the patients who underwent this kind of procedure was evaluated. Fourteen patients affected by occupying-space lesions involving the anterior and middle third ventricle were included in the study. Ten patients underwent transverse callosotomy, four subjects received standard sagittal callosotomy. A control group was also included in the study. All patients underwent a pre-operative and six months post-operative neuropsychological evaluation focused on performance at cognitive and attentional tasks. No disturbances in executive function were observed in either group. Patients receiving transverse callosotomy performed as well as control group subjects in attentive tasks, which is not the case of patients undergoing sagittal callosotomy who show a marked deficit in selective attention for left side visual field. The observed more favourable neuropsychological outcome supports transverse callosotomy as a valid alternative method to standard longitudinal callosotomy in third ventricle surgery.

JC, Vargas FR. Clinical, neuroimaging and cytogenetic findings in 20 patients with corpus callosum dysgenesis

Twenty children with corpus callosum agenesis or hypoplasia were evaluated under a standardized investigation protocol. Psychomotor retardation, seizures, and craniofacial anomalies were the most prominent findings. There were three cases of chromosomal anomalies, all of them representing trisomy of chromosome 8.

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.

Spatial stimulus-response compatibility and coding of tactile motor events: influence of distance between stimulated and responding body parts, spatial complexity of the task and sex of subject

We studied spatial stimulus response compatibility in the somatosensory modality by instructing 16 men and 16 women to press a key using the left or the right thumb in response to a nonnoxious electric stimulation delivered either to the left or to the right little finger or, in different blocks, to the left or to the right malleolar region. The task was performed in compatible (stimulus and key-response on the same side of the corporeal midline) and in incompatible conditions (stimulus and key-response on opposite sides of the corporeal midline). In Exp. 1 subjects were tested while keeping their limbs in anatomic position; in Exp. 2 subjects performed the task while keeping the left upper and lower limbs on the right side and the right limbs on the left side of the bodily midline (crossed position). The compatibility effect was observed in both experiments and was higher for stimuli delivered to the little finger than to the malleolar region. This suggests that the cost of inhibiting compatible responses is maximal when stimulated and responding body parts are contiguous. Moreover, in the spatially most demanding task (Exp. 2) men outperformed women for both speed and accuracy suggesting a sex related specialisation in the spatial processing of somatosensory information.

Proprioception and stimulus-response compatibility

Sixteen subjects pressed a left or right key in response to lateralized visual stimuli, in uncrossed (left index finger on left key, right finger on right key) and crossed conditions (left finger on right key and vice versa), with varying finger separations. Visual, tactile, or "efference copy" cues about relative finger positions were unavailable. Subjects had to press the key on the same side as (compatible group) or opposite side to the stimulus (incompatible group). Separate proprioceptive judgements of the relative finger positions were obtained. Findings of an overall reaction time (RT) advantage for compatible instructions and for uncrossed hands were replicated. With decreasing finger separation the RT advantage for compatible instructions decreased, and the probability of responding with either hand increased. The compatibility effect disappeared completely at the 6-cm crossed position, not at the position that was hardest to judge proprioceptively. This suggests that two forms of neural activation are summed: automatic activation of the anatomically same-side limb, and an integrated, rule-based activation. The results further demonstrate that independent proprioceptive cues from each limb, unassociated with skin contact between the limbs, can mediate the determination of relative position for response selection in stimulus-response compatibility tasks.