Abstract

Spatial frequency detection (Dt) and discrimination (Ds) thresholds were measured by a 2 × 2 AFC procedure. The Dt/Ds ratio was shown (1) to be independent of stimulus duration (from 10 to 1000 ms), (2) but to depend on the stimulus onset asynchrony (SOA) between the test stimulus and a suprathreshold compound mask (composed of the two frequencies to be discriminated) with a maximum ratio for SOAs of ~25 ms (backward masking). These findings are true independent of the objective spatial frequency difference between the stimuli to be discriminated. Nevertheless, (3) when the SOA is fixed at 25 ms and the contrast of the mask is increased, the Dt/Ds ratio shows a correlative increase only if it is significantly greater than 1 (i.e., perfect discrimination) in the absence of the mask. The three sets of results can be accounted for in terms of a serial processing of Dt and Ds where (i) the time constant of the Ds-stage is shorter than or equal to the time constant of the Dt-stage; (ii) the transfer of information from one stage to another requires ~25 ms, and (iii) the transducer of the DS-stage is nonlinear with respect to the transducer of the Dt-stage.

© 1985 Optical Society of America