Parallel independent encoding of orientation, spatial frequency, and contrast.

Subjects were presented with a set of 216 test gratings in random order. Each had a different combination of orientation, spatial frequency, and contrast. For each test grating, subjects were instructed to judge whether or not orientation was clockwise of the mean of the stimulus set, whether or not spatial frequency was higher than the mean of the stimulus set, and whether or not contrast was higher than the mean of the stimulus set. Each of the three sets of button presses was analyzed with respect to each of the three parameters, giving nine psychometric functions from one response set. It is concluded that, for gratings of high visibility, changes of orientation, spatial frequency, and contrast are encoded independently and in parallel, at least for small changes in these three visual parameters. In another experiment only one of the three parameters was varied at a time. Neither orientation-discrimination threshold, nor spatial-frequency-discrimination threshold, nor contrast-discrimination threshold was appreciably, if at all, lower than when all three parameters were varied simultaneously. It is concluded that interactions between the processing of small changes in orientation, spatial frequency, and contrast are negligible when all three are processed simultaneously. It is proposed that trial-to-trial variations of orientation, spatial frequency, and contrast are unconfounded by opponent processing within a population of neurons, each of which confounds the three variables.