Monkeys perceive the flash lag illusion.

<p>(A) Psychometric function from a single session of monkey B for a moving bar speed of 20°/s. The abscissa shows the veridical spatial offsets of the stimuli. The ordinate shows the probability of the monkey reporting that the flash lagged behind the moving bar. Four different points along the psychometric function are illustrated with cartoons of veridical stimuli (filled vertical bars) and hypothetical perceived stimuli (open vertical bars). The point of subjective equality (PSE) is the spatial offset at which the monkey reports the flash as lagging in 50% of trials (i.e. he perceived the two bars as vertically aligned). In this session the PSE was −0.31°, indicating that the flash had to be placed 0.31° ahead of the moving bar to make them perceptually aligned. Ns: average number of trials per suprathreshold spatial offset data point (spatial offset magnitude >1.5°); Np: average number of trials per subthreshold spatial offset data point (spatial offset <1.5°). Error bar: 95% bootstrap percentile-based plug-in estimate of confidence interval. (B) Psychometric function fit of the pooled responses (5–8 sessions) of monkeys B and H. For clarity, data from only two speeds (10 and 20 °/s) are shown. Ns and Np: as in A. For the speed that is not shown (14 °/s): for monkey B, Ns = 4061 and Np = 628; for monkey H, Ns = 852 and Np = 108. Error bars as in A.(C) Perceived spatial lag for monkey B (open circles) and monkey H (filled circles), as a function of the speed of the moving bar. Each data point is the PSE estimated from the psychometric function fit (shown in panel B) of responses pooled from five to eight sessions. Error bars as in A.(D) Slope of the psychometric functions measured at the threshold point (0.5 on the ordinate). Coding and error bars as in C.