Suppressive regions in the visual receptive fields of single cells of the pigeon's optic tectum.

Single unit extracellular recordings were obtained from non-directional and from directionally selective cells in the pigeon's optic tectum. Non-directional cells were classified according to their excitatory dynamic profiles into three classes. Class A: cells showing spatially superimposed light and dark regions in their fields. Class B: cells showing spatially separated light and dark regions in their fields. Class C: cells which responded exclusively to one sign of contrast. Most of the cells in our sample showed a suppressive region outside the excitatory area from where moving stimuli were able to reduce the units' "spontaneous" background firing. Suppressive regions were found in directionally selective as well as in non-directional cells, regardless of their dynamic excitatory profiles, with the exception of Class C cells. Evidence of spatial overlapping between the excitatory and the suppressive region was obtained with both moving and flashed stimuli. Furthermore, suppressive antagonistic effects were observed in Class B cells.