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To understand the influence of the ascending path linking area 17 to area 18 of visual cortices, experiments were carried out in which a small neuronal population of area 17 was inactivated with GABA, while unitary responses were recorded in area 18. In the latter, cells are identified as belonging to the simple or complex family according to their firing pattern evoked in response to sine-wave gratings scrolling through 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 obtained in both areas and the difference in optimal orientation between areas 17 and 18 was recorded. In area 18, cells were classified as simple or complex. The strategy was to study the reaction of neurons in area 18 prior to, during and after area 17 depression. In most simple cells, 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 was in the cross-range (60 degrees). In contrast to simple cells, GABA depression enhanced the responses in the majority of complex cells with like orientations in both areas. When the difference between recording sites was in the cross-range, then area 17 depression produced weaker evoked firing. A tangential penetration of the injecting pipette, allowing injection of different orientation sites while testing the same unit in area 18, revealed that the latter could react with an enhancement or a decline of the responses as the injecting pipette shifted from iso (or cross) to cross (or iso) disparity in optimal orientations between areas 17 and 18. These results suggest that the path connecting area 17 to area 18 may be functionally discriminated on the basis of the orientation domain and cell types. In addition, our data suggest that the ascending visual streams are required to generate orientation specificity in area 18.