The Detection of Interocular Phase Differences

Abstract

When dynamic visual noise such as the ‘snow’ on a detuned television receiver is viewed with a temporal delay between the two eyes, the noise appears to rotate in depth around a vertical axis [Ross, 1974 Nature (London)248 363 – 364; Morgan and Tyler, 1995 Proceedings of the Royal Society of London, Series B262 371 – 376]. Random noise evidently contains a wide spread of spatiotemporal Fourier components, including those for horizontal motion, which may cause a Pulfrich effect when there is an interocular delay. To investigate the temporal-frequency components necessary for the effect, a display was designed containing only a single temporal frequency. Spatially broad-band grey-level noise was flickered, such that each pixel of the noise was sinusoidally modulated over time. An interocular delay was introduced as a phase lag of the flicker in one eye. This produced a rotating depth effect. The threshold for detecting the phase shift was about 5° of phase angle, irrespective of temporal frequency, except for frequencies greater than ∼20 Hz, when the depth effect was no longer seen. Tests of several models of the phase-lag detection are described, including the possibility that there are dichoptic motion detectors also tuned to stereodisparity.