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Abstract

In natural settings, our eyes tend to track interesting or dangerous objects moving towards us. This complicates the relationship between an object’s motion and the binocular retinal projection of its motion. To perceive the motion correctly, the brain must take account of eye movements. This could be done using retinal cues (the retinal slip of static scene structures) or extra-retinal cues (e.g. copies of motor commands). Previous studies have found that extra-retinal cues provide a very poor cue to eye rotation for objects moving in depth (Erkelens Collewijn, Vision Research, 1985, p.583-588; Regan et al., Invest Opthal Vis Sci, 1986, p.584-597) thus suggesting that extra-retinal cues are not used to perceive motion-in-depth. Here, we re-evaluate this issue using conditions designed to isolate extra-retinal cues. We measured motion-in-depth discrimination thresholds when the eyes were moving or stationary. To get the eyes moving without the subject seeing any motion-in-depth we presented the target (a small square) in a plane that contained a large structured background and then varied the simulated depth position of the plane over time without changing retinal size. Motion-in-depth is imperceptible under these conditions (due to cue conflict) whilst the large scene promotes accurate pursuit. After bringing the eyes into motion, the background disappeared and the target continued moving; observers then indicated whether the target approached or receded. Observers were able to make fine discriminations of motion direction whether the eyes were initially stationary or moving. This suggests that, in contrast to previous reports, observers can use extra-retinal cues to make accurate motion-in-depth discriminations. Thus, extra-retinal signals may routinely contribute to our perception of objects moving towards us.