Chromaticity-type (C-type) horizontal cells of the turtle retina
receive antagonistic inputs from cones of different spectral types, and
therefore their response to background illumination is expected to
reflect light adaptation of the cones and the interactions between
their antagonistic inputs. Our goal was to study the behavior of C-type
horizontal cells during background illumination and to evaluate the
role of wavelength in background adaptation. The photoresponses of
C-type horizontal cells were recorded intracellularly in the everted
eyecup preparation of the turtle Mauremys caspica during
chromatic background illuminations. The voltage range of operation was
either reduced or augmented, depending upon the wavelengths of the
background and of the light stimuli, while the sensitivity to light was
decreased by any background. The response–intensity curves were
shifted to brighter intensities and became steeper as the background
lights were made brighter regardless of wavelength. Comparing the
effects of cone iso-luminant backgrounds on the Red/Green C-type
horizontal cells indicated that background desensitization in these
cells could not solely reflect background adaptation of cones but also
depend upon response compression/expansion and changes in synaptic
transmission. This leads to wavelength dependency of background
adaptation in C-type horizontal cells, that is expressed as increased
light sensitivity (smaller threshold elevation) and improved
suprathreshold contrast detection when the wavelengths of the
background and light stimuli were chosen to exert opponent effects on
membrane potential.