Evidence and Counterevidence in Motion Perception

Sensoryneurons gather evidence in favor of the specific stimuli towhich they are tuned, but they could improve their sensitivity by also taking counterevidence into account. The Bours–Lankheet model for motion detection uses counterevidence that relies on a specific combination of the ON and OFF channels in the early visual system. Specifically, themodel detects pairs of flashes that occur separated in space and time. If theflashes have the same contrast polarity, they are interpreted as evidence in favorof the corresponding motion. But if they have opposite contrasts, they are interpreted as evidence against it. This mechanism provides an explanation for reverse-phi (the perceived reversal of an apparent motion stimulus due to periodic contrast-inversions) that is a conceptual departure from the standard explanations of the effect. Here, we investigate this counterevidence mechanism by measuring directional tuning curves of neurons in the primary visual and middle temporal cortex areas of awake, behaving macaques using constant-contrast and inverting-contrast moving dot stimuli. Our electrophysiological data support the Bours–Lankheet model and suggest that the counterevidence computation occurs at an early stage of neural processing not captured by the standard models. Key words:middle temporal cortex, motion perception, primary visual cortex, reverse-phi motion, single-unit recordin