Abstract
A 560 nm amplitude-modulated flickering stimulus undergoes a brightness change at low to medium intensities and desaturates at high intensities. This change in appearance is consistent with a distortion product produced by a nonlinear stage, which can be used to dissect the visual system into pre-nonlinearity and post-nonlinearity stages whose frequency responses can be measured separately. Despite previous investigations in distortion of temporally varying visual stimuli, the location of the nonlinearity and the frequency and intensity responses of the individual pre- and post-nonlinearity stages are still not understood for a 560 nm stimulus, which is the subject of this study. A Maxwellian-view system was used to generate the visual stimulus. The input-output function of the nonlinearity was measured at different frequencies by matching the distortion product with the change in appearance of a sinusoidal stimulus of equal wavelength and intensity. Changes in the frequency responses were measured at four intensities between 8.56 and 10.41 log10 quanta s-1 deg-2. The results of the experiments show that the peak frequency response for the pre-nonlinearity stage is at 7.5-25 Hz and the upper frequency limit is at 35-45 Hz, depending on intensity. At large amplitude-modulations, there is greater inter-subjective variability and a plateau is reached in the matching data. The post-nonlinearity stage is low-pass and most sensitive at medium intensity levels. The study provides new data for the early and late frequency responses of the visual system including how they differ with systematic variation in intensity and frequency of amplitude-modulated stimuli. The results are consistent with retinal cell physiology data, which indicates an early nonlinearity. Different models are considered as possible explanations to the underlying nonlinearity including an expansive nonlinearity and asymmetric slew rate. The findings of this study are consistent with the Bezold-Brucke, Brucke-Bartley and Broca-Sulzer effects, and have wide possible applications