Abstract
Abstract: :
Purpose: To analyze and compare the transfer of contrast signals in the outer retina for sinusoidal modulation vs. steps of contrast. Methods: Intracellular recordings were obtained from cones and cone–driven bipolar cells in the light–adapted retina of the tiger salamander (Ambystoma tigrinum). Sinusoidal modulation of contrast (3Hz) and contrast steps (0.5sec) were applied at the center of the receptive field. Responses were analyzed by a Fast Fourier Transform (FFT) and by computer averaging. Results:The 3 Hz–modulated response of both classes of bipolar cells was initially linear and rose steeply, thus showing high contrast gain. Thereafter, marked saturation occurred at higher contrasts. At high contrasts, the second harmonic ranged up to 17% of the fundamental, and to satisfy linearity, the fundamental amplitude would have to be 400% greater than observed. On the other hand, the responses of cones and horizontal cells were virtually linear over the entire response range. To reveal a linear response range for bipolar cells for contrast steps, exceedingly small contrasts were applied and computer–averaged responses were analyzed. A small but unequivocal linear range was found. The stimulus range of linearity for steps and 3 Hz sinusoids was similar when expressed in units of %ΔL/L, where ΔL is the change from the background luminance, L. Conclusions:1) Cones and horizontal cells are highly linear over virtually the entire range for sinusoidal modulation but are nonlinear for contrast steps of high contrast. 2) In response to sinusoidal modulation of contrast, both classes of bipolar cells are similar, showing high contrast gain, linearity for small modulations (≤ 5%) and marked nonlinearity at higher contrasts. Thus, contrary to previous views, the response of the bipolar cells to contrast modulation can not be simply viewed as a linear transform of the light input. 3) Our results for sinusoidal modulation show evidence for a large increase (8x) of contrast gain from cones to bipolar cells. However, the increase for contrast steps is almost twice as large (15x). These and other observations suggest that the contrast response of bipolar cells is modulated by a mechanism for light adaptation acting between the cone and bipolar photovoltage.
Keywords: bipolar cells • horizontal cells • photoreceptors