Perceived speed and uncertainty : can a Bayesian model of speed perception account for the effects of contrast?

Bayesian models of motion perception propose that human perception of velocity is influenced both by signals from the stimulus and by a prior preference for stationarity. When velocity signals from the stimulus show little uncertainty, the prior has little effect, but as uncertainty is increased the prior exerts a greater influence, causing a more substantial reduction in the magnitude of perceived velocity. This model can successfully account for several illusions of perceived direction (Weiss et al, 2002 Nature Neuroscience5598-604) and has been extended to model the well known contrast-induced speed misperception—the phenomenon that low contrast stimuli appear slower than otherwise identical high contrast stimuli (Hürlimann et al Vision Research422253-2257). This model relies upon the assumption that relative to their high contrast equivalents, low contrast stimuli are accompanied by greater uncertainty. We tested this assumption by assessing the relationship between perceived speed and uncertainty over a range of spatial and temporal frequencies (2AFC speed discrimination: SFs 0.25-8.00 c deg s⁻¹; TFs 2-16Hz; speeds 0.25-64.00deg s⁻¹). While perceived speed was indeed reduced at low contrast, we find no evidence of the model's predicted negative relationship between PSEs and JNDs, calling into question its ability to account for contrast induced speed misperceptions.1 page(s