Visual processing of looming and time to contact throughout the visual field

We measured discrimination threshold for time to contact with a simulated approaching object at 20 locations between 0 and 32 deg eccentricity in the left, right, upper, and lower visual fields. We also measured discrimination threshold for rate of expansion at the same 20 locations. At 0 deg eccentricity, discrimination of trial-to-trial variations in time to contact was virtually unaffected by simultaneous trial-to-trial variations of both rate of expansion and starting size, discrimination of trial-to-trial variations in rate of expansion was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and starting size, and discrimination of trial-to-trial variations in starting size was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and rate of expansion. We conclude that, in foveal vision, time to contact, rate of expansion and size can be processed simultaneously, independently and in parallel. Our main finding was that this independence progressively decreased as eccentricity increased. For example, in peripheral, but not in foveal vision, variations in rate of expansion produced illusory variations in time to contact. A secondary finding was that the effect of eccentricity on discrimination threshold for the task-relevant variable (whether time to contact or rate of expansion) was considerably less than the effect of eccentricity on visual acuity and on several other aspects of visual performance. We suggest that visual processing of time to contact is developed by exposure to optic flow patterns created by self-locomotion.