Effect of similarity of surround on target-letter processing

Embedding a target letter in a similar surround slowed its processing in both a distortion-detection task and an identification task. If the target was not merely similar but also identical to the surround, however, it was processed more rapidly. The present results favor a two-factor (bias, discriminability) model similar to that of Estes (1982), who also found that performance varied nonmonotonically with changes in target--surround similarity. As similarity increases, performance steadily declines because of reduced discriminability but when improves when the surround becomes identical to the target because of bias or criterion adjustment induced by the surround. Evidence is presented that the decrease in discriminability reflects feature-specific lateral inhibition (Bjork & Murray, 1977), whereas the bias factor reflects priming at encoding. As predicted by the noisy-operator theory, performance in the distortion-detection task was faster but less accurate when the target matched a letter in long-term memory than when it did not. The latter results were found when the distortion involved a missing feature but not when it involved an added inappropriate feature, which indicates that internal noise more often deleted than added features (Proctor & Rao, 1983).