A parametric account of contrast adaptation on contrast perception

The effect of contrast adaptation on perceived contrast is assessed by contrast matching spatially adjacent sinusoidal gratings of similar spatial frequency, but different contrast and orientation. The main empirical question asked is why a high contrast orthogonal adaptor appears to amplify contrast signals through an increase in the slope of the contrast matching function but does not affect the threshold contrast at which a grating is detected. To explain this effect of adaptation, the Naka- Rushton receptor equation is employed as a description of the visual system's contrast response function. It is reported that the effects of adaptation may be described by three isotropic components, namely, signal amplification, division and addition, and one orientation specific component of subtraction. By collating the predictions made by the Naka-Rushton receptor equation with existing psychophysical data, it is shown that the magnitude of the isotropic components of adaptation increase with the contrast of the adapting signal. The orientation specific effect, however, is shown to saturate at relatively low adapting contrast levels. This saturation appears to be inconsistent with the commonly held view that the orientation specific effect represents a functional strategy used by the visual system to combat the problem of neural saturation in response firing rates.