Abstract
Visual processing is segregated into ON and OFF channels as early as in the retina, and the superficial (output) layers of the primary visual cortex are dominated by neurons preferring dark stimuli. However, it is not clear how the timing of neural processing differs between “darks” and “brights” in general, especially in light of psychophysical evidence; it is also equally not clear how subcortical visual pathways that are critical for active orienting represent stimuli of positive (luminance increments) and negative (luminance decrements) contrast polarity. Here, we recorded from all visually-responsive neuron types in the superior colliculus (SC) of two monkeys. We presented a disc (0.51 deg radius) within the response fields (RF’s) of neurons, and we varied, across trials, stimulus Weber contrast (5%, 10%, 20%, 50%, or 100%) relative to the gray background. Moreover, the stimuli could have either positive or negative contrast polarity. As expected, within a given contrast polarity (e.g. darks alone), neurons responded earlier and more strongly to higher contrast stimuli. However, there was a diversity of preferences for darks and brights across the population: some neurons exhibited noticeably higher contrast sensitivity for darks, others clearly preferred brights, and yet other neurons were equally sensitive to both darks and brights. Most intriguingly, regardless of individual neural sensitivity, practically all neurons responded significantly earlier to dark than to bright stimuli. This resulted in a dissociation (Chen et al., 2018) between neural sensitivity and visual response onset latency: a neuron could exhibit a weaker response to a 100% contrast dark stimulus than to a 100% contrast bright stimulus but still have an earlier response to the dark stimulus. Our results highlight an additional candidate neural pathway for explaining behavioral differences between processing darks and brights, and they demonstrate the importance of temporal aspects in the visual neural code.