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
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