Organization of ocular dominance in tree shrew striate cortex

Abstract

Summary Anatomical studies of tree shrew striate cortex have shown that inputs from lateral geniculate laminae representing each eye terminate in continuous, horizontal strips in layer IV rather than in vertical columns as in the macaque monkey. We examined the functional consequences of this organization in area 17 using single-unit recording techniques. As predicted by the anatomy, ocular dominance columns were absent in the tree shrew. In addition, preliminary results have shown that a well-organized vertical system for optimal stimulus orientation is present in tree shrew striate cortex. The majority of cells in all cortical layers were binocuarly driven and dominated by the contralateral eye. The strength of ipsilateral-eye influence was not equal throughout all layers; cells dominated by the ipsilateral eye were restricted to layer IV and a narrow zone immediately surrounding it. Ipsilateral-eye influence was observed outside this zone but its strength, relative to that of the contralateral eye, decreased with increasing distance from layer IV. Analysis of the ocular dominance distribution of cells within layer IV found no suborganization for eye dominance — either horizontal or vertical — within the layer. Within the central cleft of layer IV, an area known to receive only contralateral eye inputs from the lateral geniculate, the majority of cells were strongly binocular. Thus, in the tree shrew the functional result of the anatomical organization within layer IV is to combine, rather than to segregate the influence of the two eyes.