The effect of background luminance on visual responses to strong flashes: perceived brightness and the early rise of photoreceptor responses.

The threshold intensity for large-long incremental stimuli rises proportionally to adapting background luminance IB (Weber adaptation), but the intensity required to evoke a criterion high-brightness sensation rises much less steeply. We propose that this difference originates in the very first stage of visual processing, in the phototransduction and adaptation properties of the retinal photoreceptor cells. A physiological model previously found to account for visual latency and brightness as functions of stimulus intensity in the dark-adapted state [Donner, K. (1989). Visual Neuroscience, 3, 39-51] is extended to cover different states of adaptation. It is assumed that the neural coding of high intensities is based on the rate of rise (quasi-derivative) of the photoreceptor response just after it reaches a small threshold amplitude. The shallow background adaptation functions for high-brightness criteria emerge as a consequence of the relative constancy of the leading edge of large responses under backgrounds, a phenomenon that can be formally described by compensating changes in photoreceptor sensitivity and time scale. We first test the model on supra-threshold responses in the frog retina, where the discharge rate of ganglion cells (a possible neural code for brightness) and the primary rod hyperpolarizations can be recorded under identical conditions. The two are related as predicted over at least 3 log units of background intensity. We then show that published data on the background adaptation of human foveal high brightness judgments conform to the same model, assuming that human cones accelerate as IB-b with b = 0.14-0.15.

A digital feedback controller application for studying photoreceptor adaptation by 'voltage clamp by light'.

We present a new digital feedback application for the study of the sensitivity characteristics of photoreceptors. The amplitude of the recorded membrane voltage of a cell is steered by changing the incoming light intensity with a motor-driven circular, linear neutral-density wedge (CFW). The voltage response is sampled and fed to a software position controller of the CFW. The controller determines the position of the wedge according to the desired (command) value of the response. The light intensity changes during steady-state represent the sensitivity change, the time-course of adaptation.

Changes in retinal time scale under background light: observations on rods and ganglion cells in the frog retina.

The kinetics of rod responses to flashes and steps of light was studied as a function of background intensity (IB) at the photoreceptor and ganglion cell levels in the frog retina. Responses of the rod photoreceptors were recorded intracellularly in the eyecup and as ERG mass potentials across the isolated, aspartate-superfused retina. The kinetics of the retinally transmitted signal was derived from the latencies of ganglion cell spike discharges recorded extracellularly in the eyecup. In all states of adaptation the linear-range rod response to dim flashes could be modelled as the impulse response of a chain of low-pass filters with the same number of stages: 4 (ERG) or 4-6 (intracellular). Dark-adapted time-to-peak (tp, mean +/- SD) at 12 degrees C was 2.4 +/- 0.6 sec (ERG) or 1.7 +/- 0.4 sec (intracellular). Under background light, the time scale shortened as a power function of background intensity, I-bB with b = 0.19 +/- 0.03 (ERG) or 0.14 +/- 0.04 (intracellular). The latency-derived time scale of the rod-driven signal at the ganglion cell agreed well with that of the photoreceptor responses. The apparent underlying impulse response had tp = 2.0 +/- 0.7 sec in darkness and accelerated as I-bB with b = 0.17 +/- 0.03. The photoreceptor-to-ganglion-cell transmission delay shortened by 30% between darkness and a background delivering ca 10(4) photoisomerizations per rod per second. Data from the literature suggest that all vertebrate photoreceptors may accelerate according to similar power functions of adapting intensity, with exponents in the range 0.1-0.2. It is noteworthy that the time scale of human (foveal) vision in experiments on flicker sensitivity and temporal summation shortens as a power function of mean luminance with b approximately 0.15.

A digital feedback controller application: a light response control system to reveal photoreceptor adaptation.

We present a new digital feedback application for the study of sensitivity characteristics of biological photoreceptors. The voltage response of a cell is controlled by feeding back to the stimulating LED--via the controller--the recorded membrane voltage. The light intensity changes represent the change in the sensitivity of the photoreceptors, revealing the time course of adaptation.