Non-linearity of visual signals in relation to shape-sensitive adaptation responses

Abstract

We have studied binocularly transferred effects arising from visual adaptation to sinusoidal grating stimuli, using laser interference methods to produce both test and adaptation gratings. It is shown that if a uniform background field is superimposed on a given adaptation grating, the binocularly transferred effect is reduced. Numerical data relating the level of the uniform background field to the change in contrast threshold illumination level of the test grating are shown to be consistent with known non-linear visual responses. The distortion of a sinusoidal grating input stimulus resulting from the action of the non-linearity is examined and the associated spatial frequency spectrum computed by numerical methods. However, direct measurement of adaptation effects arising from a sinusoidal grating possessing large modulation amplitude fails to reveal the computed non-linear responses and this is discussed in relation to proposed organizations of physiological responses to spatially structured stimuli. Finally, it is shown that the adaptation response system is quasi-linear, the logarithmic increase in test contrast threshold illumination level being linearly related to the logarithm of the modulation depth of the adapting sinusoidal grating. Thus, frequency response methods can be applied to input-output relations expressed in terms of these two variables.