How plastic is the "slow speeds prior"?

Grigorios Sotiropoulos, Aaron Seitz, Peggy Series

Research output: Contribution to conferencePosterpeer-review


Summary In recent years, perception has been viewed as a form of Bayesian inference, where noisy or ambiguous sensory evidence (likelihood) is combined with expectations (prior) about the world. It has been suggested that our visual system employs a prior that objects are static or move slowly rather than quickly [1-2]. This prior was postulated because it could elegantly explain a number of perceptual biases observed in situations of uncertainty. Interestingly, those biases affect not only the perception of speed but also the direction of motion. For example, the direction of a line whose endpoints are hidden (as in the "aperture problem") or poorly visible (e.g. at low contrast or for short presentations) is more often perceived as being perpendicular to the line than it really is -- an illusion consistent with expecting that the line moves more slowly than it really does. Is this prior plastic? Two groups reported the direction of a field of parallel lines that moved either perpendicularly or obliquely to their orientation, as we varied stimulus contrast and duration. One group was exposed, during a "training" phase, to high speeds (8 deg/s), the other to low speeds (4 deg/s). Initial perception of motion direction was accurate for both groups at high contrast and biased towards perpendicular judgements at low contrast. For the low-speed group, the illusion was unaltered across sessions. For the high-speed group, the initial bias gradually diminished until the illusion reversed and direction was most often perceived as being oblique. This suggests that systematic exposure to high-speed stimuli can change the prior towards favouring higher speeds. Additionally, we show that our results are well described by the Bayesian model of [2], when extended to incorporate a speed prior with a nonzero mean. In the best-fit model, the mean increases approximately linearly across experimental sessions. Additional Detail The model of [2] suggests that motion perception can be described as an optimal estimation of object velocities under the assumption of local measurement noise and an a priori preference for slower velocities. The measurements are temporal and spatial intensity gradients. In our extension, the prior, initially centered near zero, shifts away from zero from session to session due to exposure, with a mean of 0 deg/s at the start of the first experimental session and 6.2 deg/s by the end of the last session for the high-speed group. The prior of the low-speed group started at 0 deg/s and showed little change - achieving only 0.63 deg/s by the end of the last session. Along with these, we will also discuss current experiments that allow us, under certain assumptions, to reconstruct the changing prior distribution from psychophysical measurements [1]. Subjects are exposed to drifting gratings of either high or low speeds and subsequently try to match the speeds of two gratings of different contrast via a staircase procedure. Our hypothesis is that the illusion of the lower-contrast grating moving slower than the higher-contrast grating of the same actual speed will diminish or reverse. In this paradigm, both the mean and the variance of the prior will be determined (in the extended model of [2], the prior variance is not a separate parameter; it always appears as a likelihood/prior ratio).

[1] Stocker, A.A., and Simoncelli, E.P. (2006). Noise characteristics and prior expectations in human visual speed perception. Nat. Neurosci. 9, 578-585.
[2] Weiss, Y., Simoncelli, E.P., and Adelson, E.H. (2002). Motion illusions as optimal percepts. Nat. Neurosci. 5, 598-604.
[3] Sotiropoulos, G., Seitz, A.R., and Seriès, P. (2011). Changing expectations about speed alters perceived motion direction. Curr. Biol. 21, R883-884.
Original languageEnglish
Publication statusPublished - 2012
Event9th Annual Annual Computational and Systems Neurscience Meeting (COSYNE 2012) - Salt Lake City, United States
Duration: 23 Feb 201226 Feb 2012


Conference9th Annual Annual Computational and Systems Neurscience Meeting (COSYNE 2012)
Country/TerritoryUnited States
CitySalt Lake City


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