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Motion perception is arguably the best understood visual sub-modality, largely because of longstanding research focus using a range of converging methodological approaches. This symposium will present recent advances in this active field of research. Talks will report results from psychophysics, neurophysiology, computational modeling, and neuroimaging.

Nervous systems adapt to the prevailing sensory environment, and the consequences of this adaptation can be observed both in the responses of single neurons and in perception. Given the variety of time-scales underlying events in the natural world, determining the temporal characteristics of adaptation is important to understanding how perception adjusts to its sensory environment. Previous work has shown that neural adaptation can occur on a timescale of milliseconds, but perceptual adaptation has typically been studied over relatively long timescales, typically on the order of seconds. This disparity raises important questions: Can perceptual adaptation be observed at brief, functionally relevant timescales? And if so, how do its properties relate to the rapid adaptation seen in cortical neurons? We address these questions in the context of visual motion processing, a perceptual modality characterized by rapid temporal dynamics. We demonstrate objectively that 25 ms of motion adaptation is sufficient to generate a motion-after-effect (MAE), an illusory sensation of movement experienced when a moving stimulus is replaced by a stationary pattern. This rapid adaptation occurs regardless of whether or not the adapting motion is perceived. In neurophysiological recordings from cortical area MT, we find that brief motion adaptation evokes direction-selective responses to subsequently presented stationary stimuli. A simple model shows that these neural responses can explain consequences of rapid perceptual adaptation. Overall we show that the MAE is not merely an intriguing perceptual illusion, but rather a reflection of rapid neural and perceptual processes that can occur essentially every time we experience motion.

All stationary stimuli of fixed duration have motion energy and the amount of motion energy increases with decreasing duration. Consequently, perception of motion direction could be biased if the readout mechanisms are unbalanced. Previous physiological study showed prefered direction of MT neurons in peripheral tend to be oriented away from fovea(Albright, 1989). Given the broadening of motion energy in brief stimuli, such effect should increase as the stimulus duration decreases. Here, we tested this hypothesis by presenting vertical gratings (0.5c/deg, raised cosine spatial envelope, radius = 5deg, 98% contrast) with different speeds(2,4,8 16deg/sec) and direction(moving towards fovea or moving away from fovea). And Stimuli were presented in a temporal Gaussian envelope with durations ranging between 5 and 500ms. Observers' task was to identify perceived motion direction (guessing when unsure). Results showed that as predicted, the observers were biased to perceive these stimuli as moving away from fovea. In summary, briefly presented stationary stimuli are perceived as moving in centrifugal direction when presented in visual periphery. One possible explanation for this illusion is that these stimuli, by virtue of their broad temporal frequency spectrum, stimulate centrifugally biased motion mechanisms in area MT.