AN INTRACELLULAR STUDY OF SPACE AND TIME REPRESENTATION IN PRIMARY VISUAL CORTICAL RECEPTIVE-FIELDS

In contrast with previous knowledge based on extracellular recordings, the recent development of intracellular techniques in vivo (sharp elec trode or 'blind patch') ideally allows experimenters to analyze and di ssect the contribution of feedforward and lateral connectivity in the functional expression of a synaptic 'integration field'. We will prese nt recent data which demonstrate that the visual receptive field of co rtical neurons described at the level of subthreshold synaptic events extends over much larger regions of the visual field than previously t hought, and that the capacity of cells to amplify subthreshold respons es depends on the immediate past history of their membrane potential. Our data suggest that visual cortical receptive fields should not be c onsidered as a fixed entity bur more as a dynamic field of integration and association. Two types of dynamics can be argued for: 1) the spat ial structure of the minimal discharge field (defined by suprathreshol d activation of the cell) can be profoundly reorganized at least durin g development and most probably during selective phases of learning un der the control of activity-dependent mechanisms. Adaptive changes in visual responses are thought to reflect long-lasting potentiation and/ or depression of synaptic efficacies conveying ON- and OFF-center info rmation; and 2) during sensory processing, reconfiguration of synaptic weights may be achieved on a much faster time-scale and linked to non -linear properties of the postsynaptic membrane as well as that of rec ruited networks. Association of information available in the central p arr of the receptive field CRF) and of input coming from the reputedly 'unresponsive' regions surrounding it, or arising simultaneously from different parts of the visual Field, might be suppressive in certain cases and capable of boosting hidden responses in other cases, dependi ng on the global stimulus configuration.