Independent processing of suprathreshold spatial gratings as a function of their separation in spatial frequency

Experiments were done to examine the minimum separation with respect to spatial frequency required for two superimposed gratings to be processed independently in the visual system. The contrasts of all stimuli were well above the detection threshold. The lack of independent processing was measured by the observers' failure to see both gratings as separate sinusoids; changes in the apparent contrast or spatial frequency of one grating when the other was present; and changes in the ability to discriminate small differences in the contrast or spatial frequency of one grating when the second grating was present. All measures point to a lack of independent processing when the gratings differ by less than 2 octaves. When they differ by more than 2 octaves, they are seen as separate, apparent spatial frequency is not altered systematically, and discrimination performance essentially is unaffected. However, the apparent contrast of the high-frequency grating varies, depending on whether its bars are seen against the bright bars or against the dark bars of the low-frequency grating.