Abstract
The temporal properties of visual processing vary with the stimulus being processed and its projected retinal location. Here, we present data with sub-millisecond resolution showing that retinal eccentricity strongly modulates how quickly retinal images are processed. We show that changes in luminance (and blur) cause large changes in the speed of visual processing and, crucially, that these effects are modulated — up to a factor of ten — by where on the retina the signals are processed. To compare temporal processing in the retinal periphery to that in the fovea, we developed a novel stimulus that leverages the Pulfrich effect, a classic illusion caused by processing delays between the eyes. The stimulus was a dichoptically presented set of eight white circular beads, arranged in a circle, which rotated clockwise or counter-clockwise around the circle’s center at a constant angular velocity. Luminance (or blur) differences between the eyes induced interocular processing delays. These delays made the rotating beaded circle appear slanted either top-back or bottom-back with respect to the screen. The task was to fixate the circle’s center and report which type of slant was perceived. We probed processing at five eccentricities ranging from 0.5deg to 6.0deg by changing the circle’s radius. The speed of movement (1 to 12deg/sec) and the size of each circular bead (10 to 120arcmin) scaled with the radius. Seven-level psychometric functions were collected, with onscreen delay as the independent variable. The data shows that substantially larger delays occur near the fovea than in the periphery for a given interocular image difference. We discuss why eccentricity-dependent processing implies the existence of a temporal binding mechanism, and detail the resultant perceptual consequences when the temporal binding problem is incorrectly resolved. The results highlight the severe computational challenge of obtaining stable percepts of the environment with a temporally-variant retina.