Characterizing changes in perceived speed and speed discriminability arising from motion adaptation

A A Stocker and E P Simoncelli

Published in 7th Annual Meeting, Vision Sciences Society, May 2007.

DOI: 10.1167/7.9.1005

This paper has been superseded by:
Visual motion aftereffects arise from a cascade of two isomorphic adaptation mechanisms
A A Stocker and E P Simoncelli.
Journal of Vision, vol.9(9), pp. 1--14, Aug 2009.

There is ample evidence that humans have the ability to estimate local retinal motion. These estimates are typically not veridical, but are biased by non-motion stimulus characteristics (e.g. contrast, spatial pattern) and the system's contextual state (e.g. attention, adaptation). A complete characterization of human speed perception should thus incorporate all of these effects. Here, we focus on adaptation, and characterize its influence on both the bias (i.e. shift in perceived speed) and variance (i.e. discrimination threshold) of subsequent estimates. We measured the perceived speed of a spatially broadband noise stimulus with veridical speed chosen from the range 0.5-16 deg/s in either horizontal direction, for several different adaptor speeds. Subjective responses were gathered using a 2AFC discrimination paradigm, with a simultaneous presentation of a reference and test stimulus within 3deg apertures on either side of fixation. The reference location was adapted, initially for 40s, and for an additional 5s between each trial.

We find that adaptation affects the subsequent estimation of stimulus speed over the entire range of speeds tested and across direction boundaries. The bias, relative to the unadapted percept, is repulsive yet asymmetric, with a perceived speed at the adaptor that is typically reduced. Discrimination thresholds, measured as the slope of the psychometric function gathered under each reference/test condition, typically increase around the adaptor speed. However, using signal detection theory, we can infer the change in variability and bias of the estimate of the reference speed due to adaptation and predict the discriminability that would result if both the test and reference locations were adapted. We predict a clear increase in discriminability around the adaptor, consistent with some previous literature.

We discuss the relationship of these findings to our previously proposed Bayesian model of speed perception, as well as the implications for the brain's internal representation of retinal speed.

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