The effects of bottom-up target luminance and top-down spatial target predictability on saccadic reaction times.

Saccadic reaction times (SRT) are composed of the sum of multiple processes, including bottom-up sensory processing, top-down goal oriented processing, and afferent and efferent conduction delays. In order to determine the timing dependencies and potential interactions between bottom-up and top-down processes on SRTs, we trained monkeys to perform several variants of visually guided saccade tasks. Bottom-up components of SRT were manipulated by varying target luminance from near detection threshold to supra-threshold ranges (i.e., 0.001-42.5 cd/m(2)). There was a significant reduction of mean SRT with increases in target luminance up to 3.5-17.5 cd/m(2). Luminance increases above these ranges produced significant increases in SRT when the target was within 6 degrees from the fovea. Top-down components were assessed by manipulating spatial target predictability across blocks of trials using either 1, 2, 4 or 8 possible target locations. Decreasing spatial target predictability increased SRT across target luminances from 1 to 4 targets in the gap task, but then paradoxically decreased SRT again when there were 8 possible targets in both the gap and step tasks. Finally, a gap task (200 ms gap) was used to determine the dependence of target luminance on the magnitude of the gap effect. Decreasing target luminance significantly reduced the magnitude of the gap effect indicating that the gap effect is strongly influenced by bottom-up factors.