Depth-Discrimination Thresholds for Stationary and Oscillating Targets at Various Levels of Retinal Illuminance*

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Abstract
Equidistance settings were obtained for stationary and oscillating targets presented in “real depth” at scotopic and photopic levels of retinal illuminance. The data have been analyzed in terms of the effects of retinal illuminance and target velocity on both the constant and variable errors of the settings. The results show that both stimulus variables have a systematic effect on the stereoscopic threshold: The variability of the settings progressively increases as either the retinal illuminance is decreased or the target velocity is increased. Their effects on the constant errors are less systematic: The localization error is smallest for stationary and slowly oscillating targets, particularly at high levels of retinal illuminance. The curves relating stereoscopic threshold angle and level of retinal illuminance for both stationary and oscillating targets exhibit the typical discontinuity predicted by the duplicity theory of vision. The discontinuities occur at progressively higher values of retinal-illuminance level as target velocity is increased, in quantitative agreement with expectations based on the Bunsen–Roscoe–Bloch law. The curves are progressively displaced upward as target velocity is increased. The results are discussed in relation to data obtained in earlier experiments on stereoscopic acuity and on the Pulfrich stereophenomenon.