Perceptual Coherence : Hearing and Seeing

per Second
Figure 6.10. Continued
Gain Control and External and Internal Noise 263
C
chapter 5) that did stimulate the cell and measured the response to the
drifting grating. As the contrast of the counterphase grating was increased,
the response to the drifting test grating was shifted to the right,
suggesting a multiplicative contrast-gain mechanism. These results are
shown in figure 6.10.
Independence of Contrast and Illumination
Walraven, Enroth-Cugell, Hood, MacLeod, and Schnapf (1990) summarized
data from cells in the cat’s optic nerve, demonstrating that the
response to counterphase gratings at different levels of contrast is independent
of the mean illumination. Counterphase gratings are ideal stimuli for
these experiments because it is possible to independently vary the average
luminance as well as the contrast between the light and dark bands (essentially,
the mean and variance). The response rate at each level of contrast
(ranging from 1 to 100%) was nearly identical over a 100-fold range in
retinal illumination. Thus, the contrast gain control achieves the goal of
tuning only to contrast, not to illumination.
As discussed toward the beginning of this chapter, the results of experiments
delaying the onset of a uniform test disk relative to the onset of the
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