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Brian White, Douglas Munoz; Independent Influence of Luminance and Color on Saccade Initiation During Target Selection In the Superior Colliculus. Journal of Vision 2010;10(7):1320. doi: https://doi.org/10.1167/10.7.1320.
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One of the main functions of the superior colliculus (SC) is to orient visuospatial attention towards behaviorally relevant stimuli on the basis of visual features such as color. However a longstanding view has held that visual activity in the SC arises exclusively from achromatic pathways. Recently we (White et al., 2009) reported evidence that the SC also receives a significant contribution from chromatic pathways, supported by the fact that the arrival time for color was significantly delayed relative to luminance. Here we explored consequences of this shift in visual arrival time on target selection processes in the SC. Monkeys were trained to perform a color-oddball search task under three conditions: 1) Isoluminance-easy (“pink” target amongst “yellow” distractors, isoluminant with the background), 2) Luminance-easy (luminance pedestal added to all items), and 3) Isoluminant-difficult (distractor chromaticity shifted closer to target chromaticity). These conditions allowed us to formulate predictions based on three parameters, visual response onset latency (ROL), neural discrimination time (DT), and saccadic reaction time (SRT). We predicted longer SRTs in the isoluminance-easy relative to the luminance-easy condition because of the delay in ROL for the former. In contrast, we predicted longer SRTs in the isoluminance-difficult relative to the isoluminance-easy condition because of reduced target-distractor discriminability for the former. Results showed that SRTs were prolonged in the isoluminance-easy relative to the luminance-easy condition due to the delay in ROL, while DT remained relatively fixed. In contrast, SRTs were prolonged in the isoluminance-difficult relative to the isoluminance-easy condition due to prolonged DT, while ROL remained fixed. The results support the idea that during search for a color singleton, task irrelevant luminance signals associated with the stimuli can directly trigger the accumulation of SC neuronal activity towards response threshold independent and in advance of the arrival of the task-relevant color component of the stimuli.
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