Motion-stereo mechanisms sensitive to inter-ocular phase

Abstract

We measured depth from interocular delay (The Pulfrich effect) using a dynamic random-dot pattern, consisting of a spatially-random noise field, the individual elements of which were sinusoidally-modulated in luminance over time. When an interocular phase difference in the flicker was introduced the display appeared to rotate in depth around a vertical axis like a transparent textured cylinder. The threshold phase lag was in the region of 5–10° in different observers, which translated into a non-constant, decreasing interocular delay (ms) as the flicker frequency was increased. We conclude that phase, not delay, is the critical parameter in determining the detection of depth. Threshold signal/noise ratios were measured at different delays to determine the optimum phase difference, which was found to be in the region 60–90°. However, delays centred around 180° were less detectable than those around zero, ruling out a quadrature input to the stereo-motion mechanisms. We show that depth-from-phase is a natural consequence of paired monocularly motion-direction sensitive neurones. Complex energy-detecting neurones are not required to explain the findings.