Nonlinearities of Near-Threshold Contrast Transduction

The existence of analytic threshold nonlinearities was probed with a variety of local and extended stimuli. Incremental thresholds were measured by the 2AFC procedure for same-sign and opposite-sign stationary pedestals. In contrast to the dipper effect for same-sign pedestals, subthreshold bumper effects were observed of threshold elevation (up to a factor of 4 in some conditions). The results for local difference of Gaussians (DoG) and 10 cycles deg⁻¹ Gabors were consistent with accurate hard-threshold behaviour. The results for negative DoG (whose increment corresponds to the darkening of the central spot) and 2 cycles deg⁻¹ Gabor profiles revealed a quadratic nonlinearity for one observer and linear transduction for the other. These results repudiate the uncertainty explanation for the steep psychometric function near threshold, and suggest that there are two different hard-wired nonlinearities: one for luminance increments and another for luminance decrements. According to our analysis, in low-spatial-frequency gratings, a contrast change is detected within those bars that become darker; in high-frequency gratings, transient changes are detected in the bars that become brighter.