Monkey superior colliculus: properties of single cells and their afferent inputs.

1. Receptive-field (RF) properties of 212 single cells in the superior colliculus of paralyzed macaque monkeys were studied with microelectrodes. Units were divided into superficial (0-1 mm) and deep (1-2.5 mm) layers. Orthodromic action potentials were evoked in these cells by shocking optic chiasm. 2. The vast majority of superficial cells responded to stationary or moving stimuli with transient bursts of activity and were nondirectionally selective. Moving stimuli were most effective and three main cell groups, based on response patterns to leading and trailing stimulus edges, were identified. 3. All cells had chromatically nomopponent RFs, as judged by their spectral response functions in the presence of neutral and chromatic backgrounds and on their lack of response to moving, equal-luminance chromatic borders. 4. With the exception of some very short and very long values, orthodromic latencies were unimodally distributed with a mean of 7.8 ms. The prime determinant of a cell's latency was its depth below the collicular surface rather than a specific RF feature. 5. Cells with shorter latencies (located in superficial layers) were able to reliably signal high-velocity stimulus movement; those with longer latencies (located in deeper layers) reliably signaled low-velocity motion only. 6. Results support the hypothesis that response latency is related to differences in RF organization between layers.