Area 18 corticotectal cells: response properties and identification of sustaining geniculate inputs.

1. We examined the response properties and geniculate inputs of 35 antidromically identified corticotectal (CT) cells within area 18 of the paralyzed, anesthetized cat. Twenty-three were either standard complex or hypercomplex, 11 were special complex, and 1 was simple. 2. The response properties of CT cells in area 18 were in general quite similar to those examined in a previous study of area 17 CT cells, including similar proportions of standard and special complex CT cells, virtually identical length-response functions, and similar orientation and direction tuning. 3. Area 18 CT cells are rapidly conducting. They are considerably faster than area 17 CT cells. 4. We investigated the composition of thalamic inputs to CT cells by reversibly inactivating a portion of layer A and/or the C layers of the dorsal lateral geniculate nucleus with injections of cobaltous chloride. Blocking layer A strongly attenuated the visual responsiveness of about half of the cells tested. Blocking the C layers alone generally had only moderate effects, but simultaneous blockade of layer A and the C layers demonstrated a substantial C-layer input to many cells. Unlike area 17 in which there is a strong correlation between CT cell class and dependence on layer A, no single receptive-field parameter nor set of parameters was correlated with dependence on layer A. However, cells least affected by simultaneous blockade of layer A and the C layers were special complex, suggesting that, as in area 17, area 18 special complex CT cells integrate more geniculate inputs than standard complex CT cells. 5. We propose that the similarities of response properties of area 17 and area 18 CT cells results from their participation in similar interlaminar columnar circuits and that differences in the patterns of geniculate control reflect differences in the global patterns of geniculate inputs to these two areas.