Alterations of visual cortical connections in cats following early removal of retinal input

The major afferent pathways to the visual cortex were studied in cats enucleated at postnatal days 0, 15, 30 and 60 using the retrograde tracer wheat germ agglutin conjugated horseradish peroxidase (WGA-HRP). Following enucleation at days 0 and 15, the lateral gyrus was shrunken and cortical thickness was decreased, but the thickness of layer I was increased relative to normal adult cats. The laminar portion of the lateral geniculate complex occupied only half its normal volume, but the pathway from the intralaminar nuclei to visual cortex was doubled in volume. The ipsilateral and contralateral claustrum and ipsilateral medial septal nucleus also provided increased input to visual cortex. The changes in these projection patterns were not as dramatic in the cats enucleated at days 30 and 60. The callosal pathway between areas 17 and 18 was also significantly altered in cats enucleated on the day of birth and at day 15. Callosal cells were found in the infragranular layers throughout area 18. The number of supragranular callosal cells in the medial half of area 18 and in area 17 was reduced to approximately one third the normal adult number, but the mediolateral extent of this zone was not reduced. In the infragranular layers, the callosal cell number was approximately twice that in adult cats, and they occupied at least twice their normal mediolateral extent. Following bilateral enucleation at days 30 and 60, the cell numbers and mediolateral extent of the callosal cell zones approximated those in normal adult cats. Taken together, these results indicate that early deafferantiation of a cortical area can alter the thalamic, extrathalamic and callosal connections of that area.