Stimulus magnification equates identification and discrimination of biological motion across the visual field

Abstract

There is conflicting evidence about whether stimulus magnification is sufficient to equate the discriminability of point-light walkers across the visual field. We measured the accuracy with which observers could report the directions of point-light walkers moving +/-4 degrees from the line of sight, and the accuracy with which they could identify five different point-light walkers. In both cases accuracy was measured over a sevenfold range of sizes at eccentricities from 0 degrees to 16 degrees in the right visual field. In most cases observers (N=6) achieved 100% accuracy at the largest stimulus sizes (20 degrees height) at all eccentricities. In both tasks the psychometric functions at each eccentricity were shifted versions of each other on a log-size axis. Therefore, by dividing stimulus size at each eccentricity (E) by an appropriate F=1+E/E(2) (where E(2) represents the eccentricity at which stimulus size must double to achieve equivalent-to-foveal performance) all data could be fit with a single function. The average E(2) value was .91 (SEM=.19, N=6) in the walker-direction discrimination task and 1.34 (SEM=.21, N=6) in the walker identification task. We conclude that size scaling is sufficient to equate discrimination and identification of point-light walkers across the visual field.

Perception of Biological Motion in Central and Peripheral Visual Fields

“…The experimental setup of our study was similar to that used by Ikeda et al (2005) and Gurnsey et al (2008;2010), meaning that the size of the point-light walkers and the chosen eccentricities for demonstrating the stimulus in near the periphery, were similar.…”

“…Object size and chosen eccentricities were similar to parameters used in Ikeda et al (2005) and Gurnsey et al (2008). Point size was scaled proportionally to the object size.…”

“…Ikeda et al (2005) described the eccentric perception of biological motion as "unscalably poor", showing that when the stimulus ass embedded in motion noise the discrimination of a point light walker from its scrambled version was more effective in the central visual field and stimulus magnification did not compensate for reduced performance in the peripheral visual field. On the other hand, Gurnsey et al (2008) come to completely opposite results by analysing the facing direction of a biological object and demonstrated that increase in stimulus size equalised the performance in both central and peripheral visual fields. Although both of these studies used a similar experimental setup, their results and main conclusions seem to be entirely opposite.…”

“…Analogously to the previously mentioned studies that analysed visual acuity, face perception, etc., studies analysing the sensitivity to biological motion also use stimulus magnification (Ikeda et al, 2005;Gurnsey et al, 2008;2010). However, the results of these studies are quite ambiguous.…”

“…Although both of these studies used a similar experimental setup, their results and main conclusions seem to be entirely opposite. Gurnsey et al (2008) argued that the differences might have been related to the fact that discriminating an object from motion noise (used by Ikeda et al, 2005) requires global perception of the whole body, and determining facing direction (without additional motion noise) is more related to local analysis of motion trajectories of the feet. Hence, the functional differences between these results were not due to differences in perception of biological motion but rather due to the reduced performance in element grouping or figureground discrimination in the peripheral visual field.…”