CORTICOCORTICAL CONNECTIONS BETWEEN VISUAL AREA-17 AND AREA-18A OF THE RAT STUDIED IN-VITRO - SPATIAL AND TEMPORAL ORGANIZATION OF FUNCTIONAL SYNAPTIC RESPONSES

Much is known about the anatomy of corticocortical connections, yet li ttle is known concerning their physiology. In order to have access to the synaptic and temporal aspects of the activity elicited through cor ticocortical connections, we developed an in vitro approach on slices of rat visual cortex. We used extracellular recordings of field potent ials combined with electrical stimulation to localise regions of areas 17 and 18a that are connected. We found that corticocortical connecti ons between areas 17 and 18a can be preserved in 500 mu m thick slices , with a focus of activity separated from the stimulating electrode by 1.5 mm to more than 3 mm. The potentials elicited in one area after s timulation of its neighbour displayed fast events, corresponding to ac tion potentials, and slow events, corresponding to synaptic potentials . Intracellular recordings showed that the earliest synaptic responses consisted of monosynaptic excitatory potentials. Measurement of respo nse latency showed that axons involved in both feedforward and feedbac k corticocortical connections are slowly conducting (0.3-0.8 m/s). Con duction velocity for antidromically activated cells was not significan tly different for the two sets of connections. In an attempt to establ ish the spatial organisation of functional synaptic inputs, field pote ntial recordings were performed in the different cortical layers and u sed to establish current source density (CSD) graphs along the depth a xis. The CSD maps obtained were found to be somewhat variable from one case to another. It is suggested that this variability results from t he use of electrical stimulation, which activates axons that are both afferent and efferent to a given cortical area. The field potentials a re therefore likely to contain responses that correspond to the activi ty mediated by the intrinsic collaterals mixed in variable amount with responses produced by corticocortical synapses. With this restriction in mind, it is suggested that, after stimulation of the supragranular layers, the functional synaptic inputs of feedforward connections are concentrated in layer 4 and the bottom of layer 3, while those of fee dback axons involve mainly the upper part of the supragranular layers. The intrinsic collaterals of the neurones participating in corticocor tical connections seem also to provide the bulk of their inputs to the upper part of the supragranular layers. The laminar pattern of activi ty obtained after infragranular layer stimulation was comparable to th at obtained after supragranular layer stimulation, except for the addi tion of a supplementary region of activated synapses in the infragranu lar layers.