To assess the functional reorganization observed in blindness we analyzed the connectivity of monkeys devoided of visual inputs following early prenatal enucleation. This induces a 70% reduction of the striate cortex and the cortex that was destined to become area V1 (default extrastriate cortex -DEC), acquires features indistinguishable from normal extrastriate cortex.
Retrograde tracers injected in the DEC and area V2 shows that in the enucleate (i) no neurons were found in cortex subserving other sensory modalities (ii) An important increase in density (10-100 times that observed in normals) affects selectively the areas of the ventral stream (V4, TEO, TE, the parahippocampal cortex, area 36, parasubiculum). Some cells were found in the hippocampus (but never observed in normals) (iii) there was proportionally more labeled neurons in supragranular layers. We observed an essentially normal connectivity pattern of area V4 of the enucleate suggesting that early deafferentation does not influence the input to higher visual areas.
While the results failed to reveal a crossmodal reorganization, they do show a large expansion of inputs from the ventral pathway and the temporal pole which could have a profound influence on the function of DEC and area V2 in the operated animals.