Joint-encoding of motion and depth by visual cortical neurons: neural basis of the Pulfrich effect

Motion and stereoscopic depth are fundamental parameters of the structural analysis of visual scenes. Because they are defined by a difference in object position, either over time or across the eyes, a common neural machinery may be used for encoding these attributes. To examine this idea, we analyzed responses of binocular complex cells in the cat striate cortex to stimuli of various intra- and interocular spatial and temporal shifts. We found that most neurons exhibit space-time-oriented response profiles in both monocular and binocular domains. This indicates that these neurons encode motion and depth jointly, and it explains phenomena such as the Pulfrich effect. We also found that the relationship between neuronal tuning of motion and depth conforms to that predicted by the use of motion parallax as a depth cue. These results demonstrate a joint-encoding of motion and depth at an early cortical stage.