Contrast Gain Control in the Cat's Visual System
Izumi Ohzawa, Gary Sclar, and Ralph D. Freeman (1985a)
Contrast gain control in the cat's visual system.
J. Neurophysiol. 54: 651-667.
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We have examined the idea that the adaptation of cortical neurons to
local contrast levels in a visual stimulus is functionally advantageous.
Specifically, cortical cells may have large differential contrast
sensitivity as a result of adjustments that center a limited response range
around a mean level of contrast. To evaluate this notion, we measured
contrast-response functions of cells in striate cortex while systematically
adapting them to different contrast levels of stimulus gratings. For the
majority of cortical neurons tested, the results of this basic experiment
show that contrast-response functions shift laterally along a log-contrast
axis so that response functions match mean contrast levels in the stimulus.
This implies a contrast-dependent change in the gain of the cell's
contrast-response relationship. We define this process as contrast gain
control. The degree to which this contrast adjustment occurs varies
considerably from cell to cell. There are no obvious differences regarding
cell type (simple vs. complex) or laminar distribution. Contrast gain
control is almost certainly a cortical function, since lateral geniculate
cells and fibers exhibit only minimal effects. Tests presented in the
accompanying paper (37) provide additional evidence on the cortical origin
of the process. In another series of experiments, the effect of contrast
adaptation on physiological estimates of contrast sensitivity was
evaluated. Sustained adaptation to contrast levels as low as 3% was capable
of nearly doubling the thresholds of most of the cells tested. Adaptation
may therefore be an important factor in determinations of the contrast
sensitivity of cortical neurons. We tested the spatial extent of the
mechanisms responsible for these gain-control effects by attempting to
adapt cells using both a large grating and a grating patch limited to that
portion of a cell's receptive field from which excitatory discharges could
be elicited directly (the central discharge region). Adaptation was found
to be an exclusive property of the central region. This held even in the
case of hypercomplex cells, which received strong influences from
surrounding regions of the visual field. Finally, we measured the time
course of contrast adaptation. We found the process to be rather slow, with
a mean time constant of approximately 6 s. Once again, there was
considerable variability in this value from cell to cell.