Vision Research 48(28):2827-2834

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

Rick Gurnsey, Gabrielle Roddy, Marouane Ouhnana and Nikolaus F. Troje

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° 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° to 16° in the right visual field. In most cases observers (N = 6) achieved 100% accuracy at the largest stimulus sizes (20° 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/E2 (where E2 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 E2 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.

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