Pulfrich phenomena are coded effectively by a joint motion-disparity process.

Pulfrich phenomena are a class of depth illusions generated by an interocular time delay. This may be demonstrated with continuously moving stimuli, stroboscopic displays undergoing apparent motion, or dynamic noise patterns. Previous studies suggest that neurons jointly tuned to motion and disparity may be responsible for the phenomena. Model cells with such joint coding can explain all Pulfrich phenomena in a unified way (N. Qian & R. A. Andersen, 1997). However, the joint-coding idea has been challenged by recent models (J. C. Read & B. G. Cumming, 2005a, 2005c) that focus on the S shaped functions of perceived disparity in stroboscopic Pulfrich effect (M. J. Morgan, 1979). Here we demonstrate fundamental problems with the recent models in terms of causality, physiological plausibility, and definitions for joint and separate coding, and we compare the two coding schemes under physiologically plausible assumptions. We show that joint coding of disparity and either unidirectional or bidirectional motion selectivity can account for the S curves, but unidirectional selectivity is required to explain direction-depth contingency in Pulfrich effects. In contrast, separate coding can explain neither the S curves nor the direction-depth contingency. We conclude that Pulfrich phenomena are logically accounted for by joint encoding of unidirectional-motion and disparity.