Perceptual Coherence : Hearing and Seeing

R
Gain Control and External and Internal Noise 257
A S B
A
A
B
Figure 6.7. The effect of the multiplicative and additive gain controls on the detection
of a flashing stimulus. The basic equation is based on a generalized Naka-
Rushton equation (6.7):
R(I) = R max [m a (I − s)] n /[m a (I − s)] n + I s n
In this figure, the subtraction constant is assumed to affect both the flash and the
background. In contrast, the subtractive gain control in figure 6.6 is assumed to affect
only the background because of the presumed delay after onset for the subtractive
gain to function. In the figure, the maximum response rate R max = 300, the
background = 700, the multiplication gain constant m a = 0.143, the subtraction constant
s = 560, and the semisaturation constant I s = 100. The exponent n = 1.
Adapted from “Adaptation Mechanisms in Spatial Vision. II. Flash Thresholds and
Background Adaptation,” by P. T. Kortum and W. S. Geisler, 1995, Vision Research,
35, 1595–1609.
depicts the response rate to the flashed light if there was no background,
and the top line depicts the response rate to the flashed light in front of a
steady 700 td (troland) background light. The two solid-line curves represent
the response rate if there was multiplication or subtraction gain for a
light flashed against the 700 td background. The single dotted line depicts
the response rate if there was both multiplication and subtraction gain. If
there was no adaptation at all, the dynamic range in firing rates to the
N