Retinal Mechanisms Underlying Pedestal Effects in Brightness Discrimination: A Model

Abstract

The dependence of the minimal discriminable intensity difference (MDD) of two flashed stimuli on the pedestal (the intensity of the dimmer one) is known to follow a nonmonotonic ‘dipper’ function. When the pedestal is zero, the MDD is by definition identical to the threshold for light detection. Very dim pedestals have little effect on the MDD, but there is a range of fairly dim pedestals, of intensities around the detection threshold, where the MDD decreases with increasing intensity. Beyond that range, the MDD is a monotonically increasing function of intensity. Functions of this type have been observed with spatially very different stimuli. We propose that dipper functions arise owing to the fact that different aspects of the primary light response of photoreceptors are relevant to near-threshold and suprathreshold visual signals. Decisions near the detection threshold are based on signals determined by the peak amplitude of photoreceptor responses, which for dim stimuli grows linearly with intensity. By contrast, visual signals for clearly suprathreshold stimuli reflect the rate of rise of early parts of the photoreceptor response, tracing only a short segment of the photoreceptor response just after the point where it has reached threshold amplitude. The latter type of signal shows a similar intensity dependence as do psychophysical brightness judgments, starting with a supralinear segment and passing via linearity into a compressive power function at high stimulus intensities (Donner, 1989 Visual Neuroscience3 39 – 51; Djupsund et al, 1996 Vision Research36 3253 – 3264). Discrimination sensitivity is in principle proportional to the derivative of this function. The transition from the ‘peak-amplitude-based’ linear signalling range around detection threshold to the ‘early-rise-based’ signalling range described by the nonlinear brightness function implies a sharp contraction of the response time window. Intensity discrimination improves, because the intensity dependence of the signal scaled by its variability becomes steeper than linear. As pedestal intensity is raised further, however, the MDD begins to increase again, basically in inverse proportion to the decreasing derivative of the brightness function.