Spatio-temporal dynamics of synaptic integration in cat visual cortical receptive fields

Two major constraints in connectivity decide of the spatial extent of visual cortical receptive fields, both during development and adult functioning. 1) Visual projections extrinsic to visual cortex are organized orderly to form a point to point mapping of the retina onto the cortical mantle. The retinal origin and the spatial weighting of the stream of retino-geniculo-cortical synapses feeding any mammalian area 17 cell is such that at the adult age the average size of the minimal discharge field (MDF) of first-order cortical neurons, where presentation of a stimulus can elicit suprathreshold firing, is of the order of 1 degrees of solid angle (for the central visual field representation, review in Orban, 1984). 2) A second type of connectivity consists in long distance horizontal cortico-cortical connections and feedback cortico-geniculate projections which violate the retinotopy rules expressed by feedforward inputs both at the cortical and thalamic levels (review in Gilbert, 1992; Salin and Bullier, 1995). The functional role of these diffuse non-topographic ''lateral'' pathways is less known: it is thought to exert a synchronizing action within and between each successive stage of integration, thus favoring the binding of local visual operations occurring simultaneously in different parts of the visual field (review in Singer and Gray, 1995).