Neural correlates of visual function in agenesis of the corpus callosum

Abstract

Neuroplasticity is the capacity of neurons and networks in the brain to reorganise. The changes could be due to experience and learning or in response to injuries and irregularities during development. Studying the relationship between changes to brain structure and behaviour may help facilitate better life outcomes for people with brain disorders and to alleviate the burdens of neurological conditions.In a typically developed human brain, the two cerebral hemispheres are connected by the corpus callosum, the largest white matter tract, and other smaller commissures. The congenital absence of the corpus callosum does not seem to have the same effects on behaviour as those of surgical callosal section in adults. Prior studies have shown that individuals with dysgenesis of the corpus callosum (DCC) are able to match visual stimuli presented bilaterally in separate hemifields. These results indicate effective communication between the left and right visual cortices even in the absence of the usual cortico-cortical connectivity. However, the stimuli previously used comprised images such as shapes, colours, and letters that have readily available nominal identifiers. Thus, higher-order processes (e.g. language) could be recruited in the transfer of information across the midline.Here, I studied bilateral integration of early visual processes in DCC individuals using Gabor patches as stimuli. The stimuli matched the receptive field properties in the early visual cortical areas and addressed the confounds in past studies. A total of 52 control adults without known malformations of the corpus callosum and 10 DCC participants with either complete or partial dysgenesis were recruited for this study. In the behavioural tests, I measured the participants’ ability to perceive and respond to visual stimuli. Pairs of images were presented either unilaterally to one hemifield or bilaterally to the two hemifields. Control participants showed no difference in either response accuracy or latencies between unilateral and bilateral presentation conditions. There were also no significant correlations between age and accuracy or response times. In contrast, there were accuracy deficits in most DCC participants for conditions that required communication between the two hemispheres. Some DCC participants also showed accuracy deficits in conditions that did not require interhemispheric communication. There was one DCC participant whose performance was consistently within the normal range as measured in controls. As the participants performed the tasks, I recorded their eye movements. While controls were able to maintain fixation effectively, DCC participants were found to have a bias to look to the right of the designated fixation point.To examine the neural correlates associated with these behaviours, I also looked at the functional organisation and connectivity of the left and right early visual cortices using neuroimaging techniques. I used magnetic resonance imaging (MRI) to acquire structural and functional images. I also used diffusion weighted images of the DCC cohort that were available through the institute’s database to determine interhemispheric connectivity. I used phase-delay retinotopic mapping that allowed me to visualise the visual field representation in the functional primary visual cortex (V1). DCC participants had a retinotopically organised representation of the visual field that was distorted relative to the expected anatomical landmarks. However, the anomalies could not be attributed solely to DCC due to the presence of neurological comorbidities. I then used the location of V1 as an inclusion mask in whole-brain tractography. I found no direct connection between left and right V1 in individuals without a posterior portion of the corpus callosum.Overall, these results indicate that there is a deficit in DCC participants concerning low-level visual feature integration that has not previously been reported, and to which they have learned to adapt via saccadic eye movements. These eye movements thus displaced the visual scene towards the left hemifield and could be a learned strategy that lessens the interhemispheric communication burden. The bias to process images in the left hemifield suggests a right hemisphere dominance consistent with previously documented asymmetries. These results may help us design better interventions to improve lifestyle for individuals with DCC.