Limits of peripheral direction discrimination of point-light walkers.

Many previous studies have used noise tolerance to quantify sensitivity to point-light walkers heading +/-90 degrees from straight-ahead. Here we measured the smallest deviations from straight-ahead that observers could detect (azimuth thresholds) in the absence of noise. Thresholds were measured at a range of stimulus sizes and eccentricities for (1) upright and (2) inverted walkers, (3) intact walkers, those without feet and those with only feet, and (4) in the presence and absence of a second, attention-absorbing task. At large stimulus sizes azimuth thresholds were very small (between 1 and 2 degrees ) except in the case of inverted walkers. Size scaling generally compensated for eccentricity dependent sensitivity loss, however in the case of inverted walkers the data were quite noisy. At large sizes walkers without feet elicited higher thresholds than those with only feet, suggesting a special role for the feet even when walkers are not viewed side-on. Unlike others, we found no evidence that competing tasks affected performance. We argue that the value of our modified direction-discrimination task lies in its focus on the limits of discrimination within the domain of interest, rather than the amount of noise needed to impair discrimination of widely separated stimulus values.