Visual interhemispheric transfer to areas 17 and 18 in cats with convergent strabismus

Abstract

Commissural connections between primary visual cortical maps of the two hemispheres are essential to unify the split representation of the visual field. In normal adult cats, callosal connections are essentially restricted to the border between areas A17 and A18, where the central vertical meridian is projected. In contrast, early convergent strabismus leads to an expanded callosal-receiving zone, as repeatedly indicated by anatomical experiments. We investigated here the functional correlates of this widespread distribution of callosal terminals by analysing transcallosal visual responses in five anaesthetized and paralysed 4-10-month-old cats whose right eye had been surgically deviated on postnatal day 6. After acute section of the optic chiasm, single-unit activity was recorded from A17 and A18 of the right hemisphere while the left eye was visually stimulated. A total of 108/406 units were transcallosally activated. While they were more frequent at the 17/18 border (46% of the units recorded within this region), numerous transcallosally activated units were located throughout A17 (16%), A18 (27%) or within the white matter (17%). In all regions, transcallosally driven units displayed functional deficits usually associated with strabismus, such as decreased binocularity and ability to respond to fast-moving stimuli, and increased receptive field size. Many units also displayed reduced orientation selectivity and increased position disparity. In addition, transcallosal receptive fields were manifestly located within the hemifield ipsilateral to the explored cortex, with almost no contact with the central vertical meridian. Comparison with data from normal adults revealed that strabismus induced a considerable expansion of the callosal receiving zone, both in terms of the cortical region and of the extent of the visual field involved in interhemispheric transfer, with implications in the integration of visual information across the hemispheres.

Interhemispheric connections between primary visual areas: beyond the midline rule

“…Therefore, callosal neurons within an ocular dominance column also display their own orientation selectivity and the recent findings do not contradict electrophysiological evidence of matched orientation between thalamic and callosal inputs both in normal and strabismic cats (15).…”

“…In 1997, Schmidt et al (14) used a combination of deoxyglucose, optical recording and retrograde tracing in strabismic cats and concluded that callosal projections were preferentially established between columns of the same orientation, as is the case for intrahemispheric long-range projections. However, although converging thalamic and callosal inputs still appear to be matched for orientation in strabismic cats (15), it is known that the overall distribution of callosal projections and terminals is profoundly modified (16). Therefore, the relation of the pattern of callosal connections to the columnar systems of the visual cortex in normal adults cannot be inferred from data obtained in animals with altered visual experience.…”

“…34,35). d) As is the case for long-range horizontal connections, callosal connections are selected during development according to experience-dependent rules (6,15,36). Accordingly, their architecture is presumed to reflect frequently occurring constellations of features (37,38).…”

One hundred million years of interhemispheric communication: the history of the corpus callosum

“…Reports indicate that damage to the posterior corpus callosum produces inability to judge depth in patients with damage of the optic chiasm, and section of the corpus callosum may reduce the proportions of binocular cells (27)(28)(29). In addition, experimentally induced early strabismus leads to an expanded callosal receiving zone in areas 17 and 18; in these conditions callosal cells also show decreased binocularity, decreased ability to respond to fastmoving stimuli, a small receptive field size, and poor orientation selectivity (19). Midline visual deficits (30) and impairment of interhemispheric depth comparisons when using head movements to determine relative depth (31) have been reported in acallosal humans.…”

“…It is possible that the corpus callosum is involved in more subtle mechanisms of depth perception than stereopsis (which is specifically defined as binocular disparity), such as relative motion or parallax, that is, using the differences in relative motion of near and far objects to judge depth (31). In this context, it has been suggested that visual callosal fibers participate in predicting trajectories of moving objects across the midline, and in the generation of binding mechanisms in the central visual field (19,20).…”