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Ruyuan Zhang, Duje Tadin; Illusory centrifugal motion direction observed in stationary stimuli: Dependency on duration and eccentricity. Journal of Vision 2011;11(11):769. doi: 10.1167/11.11.769.
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© ARVO (1962-2015); The Authors (2016-present)
All stationary stimuli of fixed duration have motion energy. Specifically, the net motion direction in such stimuli is zero and the amount of motion energy increases with decreasing duration. Consequently, brief stationary stimuli will stimulate cortical motion detectors, and might lead to perception of illusory motion direction if the readout mechanisms are unbalanced. One such unbalancing is the centrifugal directional anisotropy in area MT, where preferred directions of neurons with peripheral receptive fields tend to be oriented away from fovea (Albright, 1989). Theoretically, this centrifugal bias might generate illusory motion direction in stationary visual stimuli. 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 stationary vertical gratings (0.5 c/deg, raised cosine spatial envelope, radius = 5 deg, 98% contrast) at 5 eccentricities along the horizontal meridian (0, 20, 40, 60, 80 deg). Stimuli were presented in a temporal Gaussian envelope with durations ranging between 5 and 500 ms. Observers' task was to identify perceived motion direction (guessing when unsure). No feedback was provided. Results showed that as the stimulus duration decreased and as eccentricity increased, observes tended to perceive consistent motion direction in these stationary stimuli. Specifically, as predicted, the observers were biased to perceive these stimuli as moving away from fovea. We also found “baseline” biases among observers for foveally presented stimuli. In a separate experiment, we quantified these results using a velocity nulling procedure. 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.
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