Multi-directional shifts of optokinetic responses to binocular-rivalrous motion stimuli

Previous dichoptic experiments showed that dissimilar stationary pattern stimuli resulted in the perception of binocular rivalry, whereas oppositely-directly moving grating stimuli resulted in alternating optokinetic nystagmus (OKN) and the perception of binocular motion rivalry. The present study extended these dichoptic motion experiments by introducing obliquely-oriented targets with the aim of probing further the cortical mechanisms underlying binocular processing of motion. Two-dimensional eye movements were recorded along with their subjective perceptual responses. The stimuli consisted of two tilted gratings, one moving diagonally upwards and to the right (UR, 45°) and the other diagonally upwards and to the left (UL, 135°), which were presented dichoptically to subjects under two stimulus modes. For the non-exchange mode, the OKN slow phases exhibited three types of directional shifts. Two of these directional shifts tracked the stimuli (i.e. UR or UL), whereas the third moved purely upwards (UP). Since physically there was no upward-moving target, the OKN and perceptual responses appeared to be associated with a perceptual interocular grouping of the two dichoptic stimuli in their reassembled vector-sum direction. The OKN shifts were also found to be highly correlated with the psychophysical responses of motion perception. For the rapid-exchange mode, in which the stimuli were rapidly exchanged between the two eyes, the OKN slow phases exhibited primarily two types of directional shifts, UR and UL, but no UP responses for most subjects. It also appeared that these two coherent motion percepts, UL and UR, were interocularly regrouped from the exchanged stimuli. Moreover, the lack of perceptual grouping to create an UP response in the rapid-exchange mode indicated that temporal integration of at least 200 ms was necessary for the development of a reassembled vector-sum-direction motion percept. The findings under both stimulus modes support the stimulus-feature rivalry hypothesis, in which higher cortical centers mediate interocular perceptual grouping and the associated motor response.