Probed-sinewave paradigm: a test of models of light-adaptation dynamics.

Studies of light adaptation have, in general, employed either aperiodic or periodic stimuli. In earlier work, models originally developed to predict the results from one tradition failed to predict results from the other but the models from the two traditions could be merged to predict phenomena from both. To further test these merged models, a paradigm combining both types of stimuli was used. The threshold for a brief flash (the probe) was measured at various phases on a background that was varied sinusoidally in time. The probe threshold depends upon the phase at which it is presented for all background frequencies used, 0-16 Hz. These threshold variations are not well described by a sinewave; the peak threshold is > 180 deg out of phase with the trough threshold. Further, the positions of the peaks and troughs shift fairly abruptly at background modulations of 4-8 Hz. The difference between the peak and trough thresholds varies as a function of temporal frequency in a manner approximating the temporal contrast sensitivity function. The dc level (mean threshold) does not. The peak-trough difference dominates at low frequencies of background modulation, while the dc level dominates for higher frequencies. Existing models of light adaptation do not predict the key features of the data.