INFLUENCES OF AREA-17 ON NEURONAL-ACTIVITY OF SIMPLE AND COMPLEX CELLS OF AREA-18 IN CATS

To understand the influence of the ascending path linking area 17 to a rea 18 of visual cortices, experiments were carried out. in which a sm all neuronal population of area 17 was inactivated with GABA, while un itary responses were recorded in area Is. In the latter, cells are ide ntified as belonging to the simple or complex family according to thei r firing pattern evoked in response to sine-wave gratings scrolling th rough the receptive fields. Anesthetized cats were prepared for single -cell recordings. In area 17, a GABA-containing pipette was placed in superficial layers in order to inactivate reversibly a small neuronal population. Prior to blockade, the orientation tuning curves were obta ined in both areas and the difference in optimal orientation between a reas 17 and 18 was recorded. In area 18, cells were classified as simp le or complex. The strategy was to study the reaction of neurons in ar ea Is prior to, during and after area 17 depression. In most simple ce lls, whenever the difference in orientation was in the iso-range, that is when the difference in optimal orientations of the injected site ( in area 17) and of the neuron in area 18 was less than 30 degrees, the GABA application produced a decline of the evoked discharges, whereas GABA injection augmented the evoked firing rate when the difference w as in the cross-range (>60 degrees). In contrast to simple cells, GABA depression enhanced the responses in the majority of complex cells wi th like orientations in both areas. When the difference between record ing sites was in the cross-range, then area 17 depression produced wea ker evoked firing. A tangential penetration of the injecting pipette, allowing injection of different orientation sites while testing the sa me unit in area 18, revealed that the latter could react with an enhan cement or a decline of the responses as the injecting pipette shifted from iso (or cross) to cross (or iso) disparity in optimal orientation s between areas 17 and 18. These results suggest that the path connect ing area 17 to area 18 may be functionally discriminated on the basis of the orientation domain and cell types. In addition, our data sugges t that the ascending visual streams are required to generate orientati on specificity in area 18. (C) 1998 IBRO. Published by Elsevier Scienc e Ltd.