Perceptual transition dynamics of a multi-stable visual motion stimulus I: experiments

Abstract

When an obliquely moving sine wave grating is presented inside a square aperture, observers are faced with a conflict of local motion signals resulting from the problem. Motion signals along the grating contours on average have a diagonal direction (D) while those near the horizontal (H) and vertical (V) edges lie along these cardinal directions. We explored the temporal evolution of this perception, which shifts regularly between the cardinal directions H and V as the dominant neural population serving conscious direction perception changes over time. We use visual psychophysics with eye movement recordings to extract and characterise the temporal signatures of this phenomenon and our companion theoretical work (II) builds a mathematical description of these dynamics. As with other motion stimuli exhibiting the aperture problem, ocular following trajectories are shown to reflect perceived direction. We look at how stimulus input parameters relating to the motion signal strength influence the frequency of reported transitions (H-V or V-H) and estimate a minimum period between switches of about 5s for slow moving stimuli (4-6deg/sec) with relatively low contrast. We design a task to capture the passive ocular following patterns over this extended presentation period of the order of 20s using frequent small amplitude saccades back to central fixation. The saccades avoid placing the fovea in close proximity of one of the edges and we find little evidence that they induce perceptual shifts. We describe ocular following patterns across observers, identifying alternative perceptual transition types which emerge in the companion theoretical treatment (II) and show varying extents of representation of D. The theoretical and experimental work converges to suggests an underlying dynamic adaptation of input strength dependent representations of the cardinal directions, H and V, which are mutually inhibitory and serve both eye movements and global direction perception.