Perceptual Coherence : Hearing and Seeing

Gain Control and External and Internal Noise 249
The firing rates of individual neurons rates go from their minimum to
maximum firing rates within a narrow band of intensities around 10 2 and
therefore cannot track that variation. The auditory and visual systems have
evolved similar strategies to cope with this problem in two ways.
1. Both the auditory and visual systems are composed of two classes of
receptors. The auditory system contains low-threshold neurons that
reach their maximum firing rate at relatively low intensities and less
numerous high-threshold neurons that respond to the highest intensities.
In the same manner, the visual system contains rods that respond
to the lowest three to four logarithmic units, and less numerous
higher-sensitivity cones that respond to the higher intensities.
2. Both the auditory and visual systems have built-in mechanical and
neural gain control mechanisms. The gain controls adjust the sensitivity
of cells so that they do not saturate except at the highest intensities.
As the background level changes over a range of 10 6 to 10 7 (six
or seven logarithm units), the steady-state response rate changes very
little, and the number of spikes necessary to signal a change in intensity
is relatively constant. The gain control increases the amplification
of the incoming energy when the level is low, so that the signal
will be more intense than the internal noise, and decreases the amplification
when the level is high, so that the signal intensity does not
exceed the sensory firing capacity, to preclude “saturation clipping.”
3. Both (1) and (2) deal with the adaptation to mean intensity. But the
visual system also has evolved a type of gain control based on the
contrast of the illumination, independent of the mean illumination.
The fundamental problem for both looking and listening is to partial out
overall changes in intensity (i.e., brightness and loudness) from changes that
signify the properties of objects: surface reflectance and contrast and the auditory
frequency spectrum. What is invariant in vision across different brightness
levels is the percentage of reflected light and the ratio of the reflectance
between different surfaces (i.e., contrast). What is invariant in audition across
different loudness levels is the ratio of the intensities of the different frequency
partials (i.e., the contrast among the partials, although this does
change, as is discussed in chapter 8). This implies that it is the average illumination
and average sound pressure that should be controlled in order for
the two perceptual systems to isolate contrast in its most general sense.
Adaptation and Gain Control in the Visual System
In the visual system, there are two kinds of gain control. At the periphery,
there is gain control for the overall intensity of the light. At the cortical