The perceived motion of stimuli moving with, or opposite to, the direction of eye motion

Our eyes are constantly moving, even when fixating on an object. Despite this motion, we perceive the world as stable. In 2013, using an adaptive optics scanning laser ophthalmoscope (AOSLO), Arathorn et al. found that the direction of a moving stimulus relative to these eye movements affects the magnitude of motion perceived. Specifically, stimuli moving on the retina in a direction correlated with eye movements were perceived as moving; while stimuli moving in the opposite direction of the eye were perceived as stable. Interestingly, this perception of stability persisted even in conditions when the magnitude of motion was greater than the eye motion. Here, we further studied this perception of stability through a psychophysical experiment using the AOSLO. Subjects compared the perceived motion of two images on the retina: one moving in a random walk trajectory (with a controllable magnitude) and the other moving contingent to the retina, either moving correlated with eye motion or in the opposite direction. After presentation, the subject reported which image appeared to be moving more and this response informed the random-walk stimulus in the subsequent trial. At the end of the experiment, the motion of the two stimuli were perceptually equal. We quantified the motion of the random-walk stimulus by calculating a diffusion constant at this point of subjective equality. We tested in the fovea and at two degrees in the periphery. We found that, although perceived motion of images moving opposite to the retinal motion was suppressed, there was less suppression in the fovea compared to the periphery. We found that, for conditions when images were not moving in the opposite direction to eye motion, the perceived motion was equivalent to the actual motion within the raster. Our results suggest that the mechanisms underlying motion perception may differ between the fovea and periphery.