Effect of backward masking on same-different judgments.

Abstract

Subjects judged whether two adjacent letters were identical or different. The letter pair was presented briefly, followed by a superimposed patterned mask (Experiments 1 and 2) that was intended to terminate processing early. Previous work using variation in speed stress (Krueger & Chignell, 1985) had indicated that false-same errors (i.e., ondifferent pairs) predominate in early processing (missing-feature principle), whereas false-different errors predominate in late processing (internal-noise principle). The mask did not terminate processing early, however, because it produced a large preponderance of false-different errors (9%). Also, both response time (RT) and the standard deviation of RT increased as the stimulus-onset asynchrony between letter pair and mask decreased. The results indicate that backward masking works by integration (C. W. Eriksen, e.g., 1966) rather than by interruption (Sperling, 1963), and is a graded rather than all-or-none phenomenon. Consistent with the internal-noise principle, a lateral mask (Experiment 2) produced a large preponderance of false-different errors (7%) and a large fast-same effect (50 msec).