Impulse-response functions for chromatic and achromatic stimuli

Thresholds were measured for detecting pairs of briefly flashed stimuli displayed successively at variable onset asynchronies. The stimuli were 1 cycle/deg vertical sinusoidal gratings, modulated either in luminance (yellow–black) or in color (red–green). The successive presentations were either of the same contrast (positive) or of opposite contrast (negative), yielding four separate summation curves: positive and negative summation for color and for luminance. Both the positive and the negative curves followed a shorter time course for luminance than for color, implying a faster response at threshold. To calculate impulse response functions from the summation data, we assumed that the neural impulse response from two successive stimuli sum linearly at threshold, that thresholds are determined by probability summation of the combined impulse response over time, and that the impulse response can be described by an exponentially damped frequency-modulated sinusoidal function with four free parameters. The predicted impulse responses for luminance and for color are quite different, being biphasic for luminance and monophasic for color. Fourier transform of these functions yielded estimates of the amplitude and the phase functions of hypothetical visual detectors: the amplitude functions predicted well the contrast sensitivity of counterphased gratings (as a function of temporal frequency) both for luminance and for chromatic stimuli.