Independence versus interference in the perceptual processing of letters

Several experiments are reported that help resolve the discrepancy between studies indicating that letters are perceived independently and studies showing perceptual interference between letters. Two variables were examined which, based on the existing literature, appeared to be important in distinguishing between the perceptual independence and perceptual interference studies. In Experiment 1, the amount of spatial separation between letters was varied. Surprisingly, the letters were perceived independently, regardless of the amount of separation or degree of similarity between them. Thus, it was concluded that the amount of interletter separation is not sufficient to account for the difference between studies demonstrating independence and those demonstrating interference. The results of Experiments 2 and 3 indicated that the type of perceptual limitation that is used to control response accuracy determines whether perceptual independence or interference is obtained. Specifically, perceptual interference was obtained only under masking conditions, whereas perceptual independence was obtained only under energy-limited (no-mask) conditions. Experiment 4 extended these results, indicating that perceptual interference is obtained under prestimulus as well as poststimulus masking conditions. Several interpretations of the perceptual interference data are considered, and it is concluded that the data can best be accounted for in terms of a model based on the principle of feature-specific inhibition.