All Pulfrich-like illusions can be explained without joint encoding of motion and disparity

In the Pulfrich effect, an interocular time delay results in the perception of depth. Two modified versions, the stroboscopic Pulfrich effect and dynamic visual noise with a delay, are generally explained by postulating an early stage of space/time-inseparable filtering, encoding motion and disparity jointly. However, most disparity sensors in monkey V1 do not show joint motion/disparity encoding, and we recently showed that depth perception in the stroboscopic Pulfrich effect is equally compatible with space/time-separable filtering. Here, we demonstrate that this filtering can be implemented with a population of physiologically plausible energy model units. Similar results are obtained whether the neurons are pure disparity sensors (like most V1 neurons) or joint motion/disparity sensors (like MT). We also demonstrate that the dynamic noise stimulus produces correlations between the activity in pure disparity sensors, and in a separate population of pure motion sensors. These correlations are sufficient to explain the percept. Thus, joint encoding of motion and disparity is not required to explain depth perception in Pulfrich-like stimuli: a brain which encoded motion and disparity in entirely separate neuronal pathways could still experience all of these illusions.