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Detection symmetry and asymmetry.

Experiments were performed on the detection symmetry and asymmetry of incremental and decremental disks, as a function of both disk diameter and duration. It was found that, for a background luminance of 300 cd.m-2, thresholds of dynamic (briefly presented) foveal disks are symmetrical for all diameters, and that thresholds of quasi-statically presented disks are symmetrical for large diameters only. Threshold curves of quasi-static incremental and decremental disks are mutually shifted along the log area axis. In order to obtain a better insight into the underlying detection mechanism, additional experiments were performed with halfwave-rectified concentric cosine gratings, i.e. with either incremental or decremental phases. Threshold curves of these gratings proved to be identical, that is independent of polarity, and of the same shape as those obtained with fullwave cosine gratings, though 0.3 log unit shifted in amplitude. The discrepancy between threshold curves of quasi-static disks (asymmetry) and halfwave-rectified gratings (symmetry) is discussed in terms of a multiple-channel theory.