Contrast modulated maskers: Test of a late nonlinearity hypothesis

Abstract

The contrast threshold for detecting a low frequency test grating in the presence of a contrast-modulated high frequency masker is heavily dependent upon the phase relations between the gratings: test stimuli in + or −cosine phase are much more detectable than those in + or −sine phase. The present study tests a new hypothesis of these and related phenomena. It is based on the report by Derrington (1987) that recordings from cat geniculate on-center and off-center X-cells exhibit significant point nonlinearities. The major empirical finding of this study is that for highly practiced observers, sine-phase test stimuli and cosine-phase test stimuli are similar in two important respects: (1) there exist phase uncertainty effects; that is, detection thresholds for test stimuli differing in phase by 180° are elevated when they are intermixed within the same block of trials, rather than being presented in separate blocks; (2) phase identification thresholds for test stimuli differing in phase by 180° are the same as their detection thresholds. The new nonlinearity hypothesis cannot account for the results obtained with sine-phase test stimuli, though it gives a better account of the results with cosine-phase stimuli than does the early nonlinearity hypothesis which was tested and rejected by Nachmias and Rogowitz (1983).