Afterimage duration reflects how deeply invisible stimuli were suppressed
56.508, Tuesday, 20-May, 2:45 pm - 6:45 pm, Pavilion
Motomi Shimizu1, Eiji Kimura2; 1Graduate School of Humanities and Social Sciences, Chiba University, Japan, 2Dept of Psychology, Faculty of Letters, Chiba University, Japan
[Background] A monocularly-presented high-contrast dynamic pattern such as a counterphase-flickering grating can produce strong interocular suppression and render invisible another stimulus presented to the other eye, even when the stimulus is of high contrast. Under these situations, psychophysical quantification of the interocular suppression is difficult because the stimulus cannot be seen. This study demonstrates that the duration of afterimage of the invisible stimulus can be used to quantify the strength of suppression. [Methods] The suppressing stimulus was a counterphase-flickering Gabor patch (1.5 cpd, s=0.22, ~100% contrast). Flickering frequency was 5 Hz. During exclusive dominance of the suppressing stimulus, a static Gabor patch (adaptor, with the same spatial properties and contrast) was presented to the other eye for 3 sec. After the offset of the two stimuli, observers indicated afterimage duration by holding down a button as long as the afterimage was visible. The relative orientation of the adaptor to the suppressing stimulus was varied from 0 to 90°. Only the trials were used to measure afterimage duration on which observers did not see the adaptor at all throughout the adaptation period. [Results & Discussion] Even when the adaptor was completely suppressed from awareness, afterimage duration varied as a function of the relative orientation. The duration was almost zero when the relative orientation was 0°, and increased gradually with relative orientation. Interestingly, when the relative orientation was 90°, the duration was nearly the same as that under the no-suppression condition where the adaptor was clearly visible throughout the adaptation period. Clearly, afterimage duration did not depend on adaptor visibility, but reflected the strength of interocular suppression which exhibited orientation selectivity. These findings suggest that afterimage formation at least partly involves an early cortical process and that afterimage duration can be a useful psychophysical measure of strong interocular suppression.