The stability of compromised interhemispheric processing in callosal dysgenesis and partial commissurotomy

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.

Brain plasticity following corpus callosum agenesis or loss: a review of the Probst bundles

The corpus callosum is the largest axonal tract in the human brain, connecting the left and right cortical hemipheres. This structure is affected in myriad human neurodevelopmental disorders, and can be entirely absent as a result of congenital or surgical causes. The age when callosal loss occurs, for example via surgical section in cases of refractory epilepsy, correlates with resulting brain morphology and neuropsychological outcomes, whereby an earlier loss generally produces relatively improved interhemispheric connectivity compared to a loss in adulthood (known as the "Sperry's paradox"). However, the mechanisms behind these age-dependent differences remain unclear. Perhaps the best documented and most striking of the plastic changes that occur due to developmental, but not adult, callosal loss is the formation of large, bilateral, longitudinal ectopic tracts termed Probst bundles. Despite over 100 years of research into these ectopic tracts, which are the largest and best described stereotypical ectopic brain tracts in humans, much remains unclear about them. Here, we review the anatomy of the Probst bundles, along with evidence for their faciliatory or detrimental function, the required conditions for their formation, patterns of etiology, and mechanisms of development. We provide hypotheses for many of the remaining mysteries of the Probst bundles, including their possible relationship to preserved interhemispheric communication following corpus callosum absence. Future research into naturally occurring plastic tracts such as Probst bundles will help to inform the general rules governing axon plasticity and disorders of brain miswiring.

Intra and inter hemispheric dynamics revealed by reaction time in the Dimond paradigm: a quantitative review of the literature

In stimulus matching tasks requiring discrimination of two unilaterally or bilaterally presented stimuli (Dimond paradigm), a well established intrahemispheric processing bottleneck model predicts that an increase in task difficulty as measured by reaction time should provide an advantage to bilateral stimulations. The purpose of the current investigation was to review the entire relevant literature on the Dimond paradigm and identify the experimental variables which reliably yield such effects. Forty nine experimental effects compatible with the "intrahemispheric processing bottleneck" model and 26 contrary effects were found. Manipulation of the complexity of the stimulus matching criterion significantly produced intrahemispheric bottleneck effects. This effect was also significantly greater when non-target stimuli required heavier processing. These two findings support the intrahemispheric bottleneck model: computationally complex tasks seem to overload a hemisphere׳s processing capacity, an effect seen in the unilateral presentation conditions. However, manipulating the similarity of target stimuli produced contrary effects. Contrary effects were also obtained more readily when two physical matching tasks were compared. These two latter effects may best be explained as low level visual-perceptual limitations of interhemispheric transfer or integration.

Processing speed delays contribute to executive function deficits in individuals with agenesis of the corpus callosum

Corpus callosum malformation and dysfunction are increasingly recognized causes of cognitive and behavioral disability. Individuals with agenesis of the corpus callosum (AgCC) offer unique insights regarding the cognitive skills that depend specifically upon callosal connectivity. We examined the impact of AgCC on cognitive inhibition, flexibility, and processing speed using the Color-Word Interference Test (CWIT) and Trail Making Test (TMT) from the Delis-Kaplan Executive Function System. We compared 36 individuals with AgCC and IQs within the normal range to 56 matched controls. The AgCC cohort was impaired on timed measures of inhibition and flexibility; however, group differences on CWIT Inhibition, CWIT Inhibition/Switching and TMT Number-Letter Switching appear to be largely explained by slow performance in basic operations such as color naming and letter sequencing. On CWIT Inhibition/Switching, the AgCC group was found to commit significantly more errors which suggests that slow performance is not secondary to a cautious strategy. Therefore, while individuals with agenesis of the corpus callosum show real deficits on tasks of executive function, this impairment appears to be primarily a consequence of slow cognitive processing. Additional studies are needed to investigate the impact of AgCC on other aspects of higher order cortical function.

Physical, motor, sensory and developmental features associated with agenesis of the corpus callosum

BACKGROUND

The study objective was to develop a profile of characteristics and diagnostic indicators of agenesis of the corpus callosum (ACC) using a large sample of individuals with ACC and their siblings. Very few previous studies have been able to access large populations in order to develop a comprehensive profile.

METHODS

Caregivers of 720 individuals with ACC and 219 siblings, the largest sample studied to date, provided surveys with data on diagnoses, physical characteristics, developmental patterns and physical functioning.

RESULTS

Compared with siblings, individuals with ACC exhibited a pattern of delayed motor development, difficulty with balance and bimanual movements, large head size, poor muscle tone, poor depth perception, reduced pain perception, sleeping difficulties and an increased proportion of left and mixed handedness.

CONCLUSIONS

These results extend previous descriptions but are also consistent with published reports that used small samples and single case studies. The data provide a profile that has implications for early detection and intervention of individuals with ACC as well as for highlighting future research directions to extend knowledge about ACC.

Sensory and motor interhemispheric integration after section of different portions of the anterior corpus callosum in nonepileptic patients

OBJECTIVE

We evaluated somatosensory and motor interhemispheric integration in four patients who underwent transection of different portions of the anterior corpus callosum (CC) for removal of an intraventricular cyst. The study goal was to relate their performances to the topographical organization of the CC.

METHODS

Experimental tasks included bimanual coordination, tactile cross-localization, and intermanual and interfield comparisons of somesthetic information. Response accuracy and response times were measured. In addition, interhemispheric transmission times were obtained in the somesthetic modality.

RESULTS

Section of the middle portion of the genu caused a deficit in motor coordination, which was absent in patients with more posteriorly located lesions, whereas section of more rostral portions of the genu seemed to interfere with motor planning. The most posterior section in our sample, including the anterior portion of the body of the corpus, abolished interhemispheric transfer of simple somesthetic information (perception of touch) but not tactile discrimination (intermanual comparisons of shapes). We speculate that more complex somesthetic information is transferred through the caudal region of the body of the CC, which was spared in all patients. Thus, it seems that section of different portions of the anterior CC (genu and anterior body) produces specific deficits in interhemispheric integration in the motor and somesthetic modalities. These deficits are consistent with the anteroposterior topography of anterior callosal fibers.

CONCLUSION

The specific disconnections deficits observed in this study may provide the surgeon with information regarding the consequences of anterior callosotomy and allow for remedial measures to be implemented if required.