From our stead-state flicker data, Kelly's (1971) model correctly predicts the transient thresholds for rectangular pulses of variable duration when the (flickering or flashed) stimulus is a 4 cycle/deg grating, but the same prediction fails for a uniform (8 degrees) field. However, if we augment the model with a "hard" nonlinearity, we can fit both types of transient thresholds as well as the steady-state thresholds. The most plausible embodiment of this essential nonlinearity is an asymmetric recifier, which seems to represent the behavior of retinal ganglion cells. Unlike the symmetric models of Roufs and Rashbass, this asymmetry also correctly predicts that the decrement thresholds for some stimuli are smaller than the corresponding increment thresholds.

In the literature on visual contrast thresholds for sine wave gratings, little attention has been paid to the psychophysical methods used to obtain these spatial-frequency response curves. Here we report a comparison of such data obtained by two quite different psychophysical methods, but otherwise under identical conditions, using five Ss. Both experiments were run by computer: (1) In the method of adjustments, the computer program merely controls the order of the stimuli and records S's contrast settings. (2) In the forced-choice staircase (FCS) technique, the program determines how often S can discriminate the sinusoidal grating from a uniform field, informs S of his accuracy, controls the stimulus contrast on the basis of S's preceding responses, and brackets his threshold by a series of successive approximations. Method 2 eliminates criterion effects that occur in Method 1, and hence tends to minimize individual differences. However, the FCS technique requires an order of magnitude more observing time to obtain equally smooth contrast sensitivity curves. FCS also increases the overall sensitivity of some Ss by as much as five times, but it does not significantly change the shape of the contrast sensitivity curve; both methods show strong effects of lateral inhibition at low spatial frequencies.

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