Abstract
The primate superior colliculus (SC) receives substantial cortical visual inputs, including from the primary visual cortex (V1). However, there is currently no detailed neurophysiological comparison of visual response properties in the SC and V1. Here, we recorded from 107 V1 neurons and 70 SC neurons in one male rhesus macaque monkey. Of these, 107 V1 and 52 SC neurons were recorded simultaneously. The V1 neurons were all from the lower visual field representation (~6-10 deg eccentricity); some SC neurons (18) were from the upper visual field representation, but the rest were matched in visual response field (RF) location with the V1 recordings. Behaviorally, the monkey fixated, and we presented gabor gratings of various spatial frequencies and orientations for 300 ms over a gray background. The gratings were sized to approximately fill the SC visual RF’s, which we mapped (along with the V1 RF’s) using an earlier behavioral task. RF mapping was performed using small white spots appearing over a gray background during fixation. V1 neurons had significantly smaller RF sizes than SC neurons. V1 neurons also responded to stimulus onsets consistently earlier than simultaneously recorded SC neurons, and V1 neurons exhibited offset responses (at stimulus removal at the end of a trial) more robustly than SC neurons. SC neurons, on the other hand, had higher baseline activity. In terms of feature tuning, SC orientation tuning widths were significantly bigger than in V1, suggesting weaker orientation selectivity. Spatial frequency tuning widths were more similar, but SC neurons preferred lower spatial frequencies in general. Interestingly, coarse-to-fine processing, evidenced by the temporal profile of responses to different spatial frequencies, was substantially more prominent in the SC. These results demonstrate that the SC does not merely inherit V1 response properties, but is functionally specialized, perhaps to best exploit its proximity to the motor periphery.