Our first experiment measured contrast discrimination thresholds for vertical drifting stimuli. As expected, the relation between threshold increment contrast and pedestal contrast (
Figure 2A) shows the familiar “dipper” shape, with increment contrast thresholds being lowest when the pedestal contrast is around 2%, the contrast needed for detection (
Nachmias & Sansbury, 1974). Also as expected, these data are well fitted by the predictions of a simple model based on a detector with saturating neural response (
Legge & Foley, 1980). This neural response (
Figure 2B) is estimated from contrast discrimination thresholds by assuming that the subject is at threshold when the neural response increases by a given amount. Increment contrast thresholds are thus lowest where the neural response function is steepest.
Consider now the estimated neural response to test stimuli drifting at different rates (
Figure 3A). In line with previous studies (e.g.,
Robson, 1966;
Kelly, 1979;
Watson, 1986;
Georgeson, 1987), in both subjects the estimated neural response is strong for drift rates of 2.7 Hz and 13 Hz, substantially weaker for drift rates of 27 Hz and 38 Hz, and negligible for a drift rate of 54 Hz.
Our second experiment measured the degree of masking caused by a horizontal mask. Masking had powerful effects (
Figure 2C): except for a limited range of pedestal test contrasts, the mask substantially increased the increment test contrast required for discrimination. The mask impaired detection at the lowest test contrasts and impaired discrimination for a broad range of test contrasts (
Legge & Foley, 1980;
Ross et al., 1993;
Foley, 1994).
The effect of the mask on the estimated neural response to the test stimulus (
Figure 2D) is largely a rightward shift: the mask increased the test contrast needed to obtain a given neural response. Because the scale in the abscissa is logarithmic, a rightward shift indicates a divisive effect. Just as with physiological suppression, masking divides the effective contrast seen by the neural mechanism (
Heeger, 1992;
Foley, 1994;
Watson & Solomon, 1997).
We now ask our main question: Do masks drifting rapidly cause the same masking as masks drifting slowly? The answer is affirmative for masks drifting as fast as 27–38 Hz. This effect can be seen by comparing the response to the test without a mask with those measured in the presence of masks drifting at different rates (
Figure 3B). In both subjects, the response suppression caused by masks drifting at 13 and 27 Hz was strong, similar to that caused by stimuli at 2.7 Hz. Masks drifting at 54 Hz, instead, caused essentially no masking.
This behavior can be quantified by plotting suppression strength as a function of mask drift rate (
Figure 4B). We measure suppression strength by the reduction in effective test contrast (see “Methods”). The latter is the degree to which the mask shifts the estimated neural responses to the right in a logarithmic contrast axis (
Figure 2D). A value of 2 means that the mask has divided by 2 the test contrast seen by the neural mechanism. Equivalently, it means that the mask has doubled the test contrast needed to obtain a given neural response. Plotting reduction in effective test contrast versus mask drift rate (
Figure 4B) confirms the qualitative impression that whether its drift rate is 2.7, 13, 27, or 38 Hz, a drifting mask causes a substantial amount of suppression.
By comparison, we have seen that the neural responses elicited by the mask are reduced above 13 Hz. This dependence can be observed by plotting the estimated neural responses at 30% contrast (the mask contrast) as a function of drift rate (
Figure 4A). The slower stimuli (2.7 Hz and 13 Hz) generate an approximately equally strong response, whereas the 27-Hz and 38-Hz stimuli elicit a response that is about half as strong.
The tuning curves for responses and for suppression are rather different. If one rescales the tuning curve of the responses to account for suppression with the slowest stimuli (
Figure 4B, dashed lines), this curve underestimates suppression caused by a 27-Hz mask. If one instead rescales to fit suppression caused by a 27-Hz mask, one overestimates suppression caused by slower masks (not shown). The curve fitted to the responses cannot fit the suppression data because neural responses to a pattern drifting at 27 Hz are half as strong as those elicited by slower patterns (
Figure 4A), whereas suppression caused by such a fast pattern is as strong as (or stronger than) that elicited by slower patterns (
Figure 4B). Finally, the fitted curves allow a rough estimate of the high frequency cutoff drift rates, where the curves reach half of their maximal value. This cutoff drift rate lies around 32 Hz for the neural response (33 Hz for L.M. and 32 Hz for S.G.), and is higher, around 43 Hz, for suppression (41 Hz for L.M., 46 Hz for S.G.).
A similar conclusion can be drawn if one measures masking by the decrease in estimated neural response to the test. In addition to shifting rightward the curves relating response to test contrast, masking slightly alters the shape of these curves (
Figure 3B). Therefore, one might want to measure suppression in additional ways, for example, by measuring the vertical shift rather than the horizontal shift. The results of such a measurement are illustrated in
Figure 4C, where we plot the estimated neural response to a 30% contrast, 2.7 Hz test in the presence of a mask, as a function of mask drift rate. Except for the 54 Hz mask, all masks reduced the responses. As for the reduction in effective contrast, the curve relating response and frequency clearly underestimates the reduction in response caused by fast masks.
To summarize, there is a substantial difference between the neural responses elicited by drifting stimuli and the masking caused by these stimuli. Fast gratings cause strong masking while eliciting weak cortical responses.