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The response dynamics of turtle photoreceptors (cones) were studied by the cross-correlation method using a white-noise-modulated light stimulus . Incremental responses were characterized by the kernels. Whitenoise-evoked responses with a peak-to-peak excursion of >5 mV were linear, with mean square errors of^-8%, a degree of linearity comparable to the horizontal cell responses. Both a spot (0 .17 mm diam) and a large field of light produced almost identical kernels. The amplitudes of receptor kernels obtained at various mean irradiances fitted approximately the Weber-Fechner relationship and the mean levels controlled both the amplitude and the response dynamics ; kernels were slow and monophasic at low mean irradiance and were fast and biphasic at high mean irradiance . This is a parametric change and is a piecewise linearization. Horizontal cell kernels evoked by the small spot of light were monophasic and slower than the receptor kernels produced by the same stimulus . Larger spots of light or a steady annular illumination transformed the slow horizontal cell kernel into a fast kernel similar to those of the receptors. The slowing down of the kernel waveform was modeled by a simple low-pass circuit and the presumed feedback from horizontal cells onto cones did not appear to play a major role .

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