Perceptual Coherence : Hearing and Seeing

258 Perceptual Coherence
flashed light would be minimal. However, the multiplicative + subtractive
adaptation processes reduce the response rate to the background light to
such a degree that the dynamic range increases by as much as fivefold compared
to the range if there were no adaptation processes.
To summarize, background illumination uses up most of the ability of
neurons to signal increments in intensity. Over time periods of roughly
200–300 ms, multiplicative and subtractive mechanisms reduce the firing
rate to the background so that a larger part of the response range can be
used to signal changes in intensity. The multiplicative mechanism affects
both the background and target, while the subtractive mechanism affects
only the background, at least within a limited time span.
A real example of the effects of multiplicative and subtractive mechanisms
can be found in recordings of ganglion cells in the mudpuppy
(Makous, 1997, taken from Werblin, 1974). In this example, the surround
appeared to have a subtractive effect on the response to a flickering stimulus
confined to the center mechanism of the ganglion cell. Experimentally,
the flicker was continuous, and the surround was alternately on and off.
Figure 6.8A shows that the alternation of the surround over time reduces
the overall rate of response but, most important, the oscillation in output
voltage generated by the flicker does not change. The membrane potential
is plotted in figure 6.8B in terms of the intensity of the surround. Again, the
important point is that the response function to the flicker with the surround
on has the same slope and is merely shifted to the right. I would argue that
this effect is mainly due to subtractive adaptation because the response to
the flicker does not change at the different background intensities.
Figure 6.8. The surround acts as a subtractive gain control in bipolar cells. The
gain control maintains the response to the flicker modulation in spite of the increase
in background intensity. From “Control of Retinal Sensitivity. II. Lateral Interactions
at the Outer Plexiform Layer,” by F. S. Werblin, 1974, Journal of General
Physiology, 63, 62–87. Copyright 1974 by the Journal of General Physiology.
Reprinted with permission.