Motion-stereo mechanisms sensitive to inter-ocular phase

We measured depth from interocular delay (The Pulfrich effect) using a dyna mic random-dot pattern, consisting of a spatially-random noise field, the i ndividual elements of which were sinusoidally-modulated in luminance over t ime. When an interocular phase difference in the flicker was introduced the display appeared to rotate in depth around a vertical axis like a transpar ent textured cylinder. The threshold phase lag was in the region of 5-10 de grees in different observers, which translated into a non-constant, decreas ing interocular delay (ms) as the flicker frequency was increased. We concl ude that phase, not delay, is the critical parameter in determining the det ection of depth. Threshold signal/noise ratios were measured at different d elays to determine the optimum phase difference, which was found to be in t he region 60-90 degrees. However, delays centred around 180 degrees were le ss detectable than those around zero, ruling out a quadrature input to the stereo-motion mechanisms. We show that depth-from-phase is a natural conseq uence of paired monocularly motion-direction sensitive neurones. Complex en ergy-detecting neurones are not required to explain the findings. (C) 2000 Elsevier Science Ltd. All rights reserved.