A Bayesian Model of Perceived Head-Centered Velocity during Smooth Pursuit Eye Movement

Abstract

SummaryDuring smooth pursuit eye movement, observers often misperceive velocity. Pursued stimuli appear slower (Aubert-Fleishl phenomenon [1, 2]), stationary objects appear to move (Filehne illusion [3]), the perceived direction of moving objects is distorted (trajectory misperception [4]), and self-motion veers away from its true path (e.g., the slalom illusion [5]). Each illusion demonstrates that eye speed is underestimated with respect to image speed, a finding that has been taken as evidence of early sensory signals that differ in accuracy [4, 6–11]. Here we present an alternative Bayesian account, based on the idea that perceptual estimates are increasingly influenced by prior expectations as signals become more uncertain [12–15]. We show that the speeds of pursued stimuli are more difficult to discriminate than fixated stimuli. Observers are therefore less certain about motion signals encoding the speed of pursued stimuli, a finding we use to quantify the Aubert-Fleischl phenomenon based on the assumption that the prior for motion is centered on zero [16–20]. In doing so, we reveal an important property currently overlooked by Bayesian models of motion perception. Two Bayes estimates are needed at a relatively early stage in processing, one for pursued targets and one for image motion.

Accounting for direction and speed of eye motion in planning visually guided manual tracking

“…Indeed, if the efference copy of the eye velocity was slightly biased toward a smaller value, this would nicely explain our results. This has also been widely speculated in the motion perception literature (see, for example, Freeman et al 2010;Gur 2005, 2012).…”

“…In the motion perception literature, many studies have investigated motion perception during smooth pursuit. Studies tend to show that the eye velocity signal is underestimated compared with the retinal motion because the reference frame transformation is not completely performed by the neural network underlying motion perception, which leads to visual illusions like the Filehne and Aubert-Fleischl illusions (see, for example, Freeman et al 2010;Furman and Gur 2012). But the effect of smooth pursuit on motion perception depends on the observer's task: some experiments reveal an impairment of visual motion perception during smooth pursuit, while other studies show an improvement of the perception performance during smooth pursuit (Bennett et al 2010;.…”

“…Recently, a Bayesian framework has been used to explain the partial compensation for the eye velocity observed in the motion perception literature and that can lead to several visual illusions (Freeman et al 2010). The key idea is that the undercompensation is due to a difference in precision (in other words, the variability) between the retinal and eye velocities rather than a difference in accuracy.…”

“…In other words, the results can be explained with sensory estimates that are both unbiased but differently affected by noise. Before being combined, the retinal motion and the eye motion are first separately estimated with a Bayes estimator that combines the likelihood and the prior distribution by assuming that the likelihood is unbiased but with an uncertainty growing with the velocity (see Freeman et al 2010) and by assuming a Gaussian prior with a mean of 0°/s (preference for slow motion, see Weiss et al 2002). The prior distribution has, for example, been estimated in Stocker and Simoncelli (2006).…”

Bayesian Models of Individual Differences

“…Low-contrast stimuli therefore appear to move more slowly, because reducing the contrast makes the sensory evidence less reliable, which allows the prior to dominate the estimate of speed (Hürlimann, Kiper, & Carandini, 2002; Sotiropoulos, Seitz, & Seriès, 2014; Stocker & Simoncelli, 2006, although see Hassan & Hammett, 2015). Similarly, in the case of the Aubert-Fleischl phenomenon, pursuit eye movements lower the reliability of sensory evidence, which causes motion thresholds to increase along with a corresponding slowing of perceived speed (Freeman, Champion, & Warren, 2010). …”