Effects of light adaptation on contrast processing in the outer
retina were investigated over nearly four decades of background
illumination by analyzing the intracellular responses of 111
bipolar cells, 66 horizontal cells, and 22 cone photoreceptors
in the superfused eyecup of the tiger salamander (Ambystoma
tigrinum). Light adaptation had striking and similar effects
on the average contrast responses of the hyperpolarizing (Bh)
and depolarizing (Bd) classes of bipolar cells: Over the lower
two decades of background illumination, the contrast gain increased
7-fold to reach values as high as 20–30, the dynamic range
and the half-maximum contrast decreased by about 60%, the total
voltage range increased some 40%, and contrast dominance changed
from highly positive to more balanced. At higher levels of
background, most aspects of the contrast response stabilized
and Weber's Law then held closely. In this background range,
the contrast gain of bipolar cells was amplified some 20×
relative to that of cones whereas the corresponding amplification
in horizontal cells was about 6×. Differences in the growth
of contrast gain with the intensity of the background illumination
for cones versus bipolar cells suggest that there are
at least two adaptation-dependent mechanisms regulating contrast
gain. One is evident in the cone photoresponse such that an
approximately linear relation holds between the steady-state
hyperpolarization and contrast gain. The other arises between
the voltage responses of the cones and bipolar cells. It could
be presynaptic (modulation of cone transmitter release by
horizontal cell feedback or other mechanisms) and/or postsynaptic,
that is, intrinsic to bipolar cells. Contrast gain grew with
the background intensity by a larger factor in horizontal than
in bipolar cells. This provides a basis for the widely held
view that light adaptation increases the strength of surround
antagonism in bipolar cells. On average, the effects of light
adaptation and most quantitative indices of contrast processing
were remarkably similar for Bd and Bh cells, implying that both
classes of bipolar cells, despite possible differences in
underlying mechanisms, are about equally capable of encoding
all primary aspects of contrast at all levels of light adaptation.
Modulation of cone horizontal cell activity in the teleost fish retina. I. Effects of prolonged darkness and background illumination on light responsiveness
