Abstract

The ability to identify the direction of apparent motion in a sequence of two short light pulses of different amplitudes at separate spatial locations was studied. The product of pulse amplitudes is a very poor predictor of such performance when one of the two signals is much higher in amplitude than the other: above a certain amplitude the probability of correct identification becomes virtually independent of the amplitude of the larger pulse. There was no noticeable difference in performance between low–high and high–low contrast sequences. Both the direction identification and the simple contrast-detection probabilities can be represented by the same psychometric function of the luminance increment ΔL, provided that ΔL is normalized by the nth power of the background luminance level, Lb. These results suggest that the general Reichardt-type scheme of movement encoding should be modified in the manner proposed for the fly’s visual system [ J. Opt. Soc. Am. A 6, 116 ( 1989)]: (1) the mean luminance is subtracted from the input signal before the signal is subjected to a nonlinear compression and (2) saturation characteristics are inserted into both branches of the two mirror-symmetric motion-detection subunits before multiplication of the input signals. The identical metric of the contrast response suggests that movement discrimination and luminance detection are two different special-purpose computations performed on the output of the same encoding network.

© 1995 Optical Society of America

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