Interhemispheric Connections between the Primary Visual Cortical Areas via the Anterior Commissure in Human Callosal Agenesis

Abstract

Aim: In humans, images in the median plane of the head either fall on both nasal hemi-retinas or on both temporal hemi-retinas. Interhemispheric connections allow cortical cells to have receptive fields on opposite sides. The major interhemispheric connection, the corpus callosum, is implicated in central stereopsis and disparity detection in front of the fixation plane. Yet individuals with agenesis of the corpus callosum may show normal stereopsis and disparity vergence. We set out to study a possible interhemispheric connection between primary visual cortical areas via the anterior commissure to explain this inconsistency because of the major role of these cortical areas in elaborating 3D visual perception.Methods: MRI, DTI and tractography of the brain of a 53-year old man with complete callosal agenesis and normal binocular single vision was undertaken. Tractography seed points were placed in both the right and the left V1 and V2. Nine individuals with both an intact corpus callosum and normal binocularity served as controls.Results: Interhemispheric tracts through the anterior commissure linking both V1 and V2 visual cortical areas bilaterally were indeed shown in the subject with callosal agenesis. All other individuals showed interhemispheric visual connections through the corpus callosum only.Conclusion: Callosal agenesis may result in anomalous interhemispheric connections of the primary visual areas via the anterior commissure. It is proposed here that these connections form as alternative to the normal callosal pathway and may participate in binocularity.

Differential timing of a conserved transcriptional network underlies divergent cortical projection routes across mammalian brain evolution

“…humans. For example, in line with the result of Satb2 knockout mouse displaying rerouting through the anterior commissure, in some cases of human callosal agenesis interhemispheric connections are retained through the anterior commissure, resembling the preeutherian connectome (20,21). This potentially atavistic phenomenon of long-range axonal plasticity may underlie the improved functional outcomes of individuals with developmental callosal agenesis compared to those who have had their corpus callosum surgically severed in adulthood to treat drug-resistant epilepsy.…”

Effect of corpus callosum agenesis on the language network in children and adolescents

“…In the complete absence of CC, white matter reorganization may compensate for the lack of callosal connections and allow some kind of interhemispheric transfer. In fact, an enlarged anterior commissure has shown to be favourable for the interhemisperic integration of visual stimuli in patients with ACC ( van Meer et al 2016 ; Barr and Corballis 2002 ). In addition, the sigmoid bundle has been identified as an asymmetric, heterotopic commissural tract.…”

Interlimb Transfer of Reach Adaptation Does Not Require an Intact Corpus Callosum: Evidence from Patients with Callosal Lesions and Agenesis

“…While bilateral motor and premotor transcallosal connections were disrupted in patients MM and MS, it is possible that splenial connections could mediate transcallosal mechanisms of interlimb transfer between bilateral posterior parietal, temporal or visual cortex areas. In line with this, in an agenesis patient like patient AM, visual areas normally connected via the splenium, were shown instead to be connected via the anterior commissure (Van Meer et al, 2016). Further work, for instance on other patients with rare stroke types affecting specifically the corpus callosum, and in particular the splenium, would thus be necessary to test this hypothesis.…”