Interocular differences in temporal integration drive anomalous Pulfrich percepts

Temporal differences in visual processing between the eyes can cause dramatic misperceptions of motion and depth. Processing delays between the eyes cause the Pulfrich effect: oscillating targets in the frontal plane are misperceived as moving along near-elliptical motion trajectories in depth. Here, we explain a previously reported but poorly understood variant of the effect. In the anomalous Pulfrich effect, the illusory motion trajectory appears oriented left- or right-side back in depth, rather than aligned with the true direction of motion. Our data strongly suggests that this apparent misalignment is due to interocular differences in neural temporal integration periods. Differences in temporal integration dampen the amplitude of effective motion in one eye relative to the other. In a dynamic analog of the ‘geometric’ effect in stereo-surface-orientation perception (Ogle, 1950), the differential motion amplitudes cause the perceived misorientation. First, four observers tracked, with a cursor, Gabor targets with different spatial frequencies. Longer temporal integration periods—hence, increased motion damping—were associated with higher spatial frequencies. Next, the observers viewed dichoptic Gabor stimuli oscillating in the frontal plane, with different carrier spatial frequencies in each eye. Observers reported perceiving more ‘left-side back’ orientations in depth when the left eye was presented the lower spatial frequency, and vice versa. Then, we measured psychometric functions with onscreen damping as the independent variable. The point of subjective equality (PSE) indicates the onscreen damping that is equal in magnitude but opposite in sign to the neural damping. All experiments indicate that neural damping is greater for higher spatial frequencies. The current findings expand the body of knowledge regarding the dependence of temporal processing on stimulus properties, while highlighting the striking perceptual consequences of interocular imbalances in temporal processing. Future work will aim to determine the computations that minimize the impact of temporal processing differences under natural conditions.