Nonlinearities of near-threshold contrast transduction.

The existence of analytic threshold nonlinearities was probed with 2AFC incremental threshold functions for both local and extended test patterns on stationary matched pedestals of the same and opposite sign. In contrast to the facilitation effect with same-sign pedestals, sensitivity with opposite-sign pedestals first deteriorated up to the mask detection level, abruptly improved and then deteriorated again. Analytic solutions for the transducer function with additive noise were derived to account for the incremental data in all conditions. The results for positive difference-of-Gaussian (DoG) stimuli (whose increment made the central spot lighter) and for 10 c deg-1 Gabor stimuli were consistent with accurate hard-threshold behavior with best-fitting d' powers from 17 to 358. The 10 c deg-1 data further implied that contrast gain control was operating throughout the subthreshold range. The results for negative DoGs (whose increment corresponds to the darkening of the central spot) and 2 c deg-1 Gabor profiles were consistent with mild nonlinearities having d' powers of 1.6-3. Significant differences between the nonlinearities for positive and negative DoGs indicate that only a small portion, if any, of the near-threshold nonlinearity could be attributed to uncertainty. Our analysis suggests that, with low spatial frequency gratings, detection was based on those bars that become darker; with high-frequency gratings, on the bars that become brighter.