Some experiments on detection symmetry in spatial vision

No consensus has been reached until now on the question whether incremental and decremental thresholds of disks, differing in diameter and duration, are equal or not. It will he shown that dynamic disk thresholds are symmetrical and that quasistatic disk thresholds are too, but only for large diameters. Quasistatic threshold curves are mutually shifted along the log area axis. Threshold curves obtained with disks should be related to thresholds of spatial sinewave gratings. In order to obtain a better insight into the underlying mechanism, further experiments have been performed with halfwave rectified radial-symmetric cosine gratings, i.e. with either incremental or decremental phases. The sensitivity curves of these gratings proved to be identical, independent of polarity, and of the same shape as those obtained with full cosine gratings, though 0.3 log units shifted in amplitude. The discrepancy between quasi static disk and single-phase grating thresholds is discussed in terms of Fourier analysis applied to a single channel as well as to multiple ones .