Similarity-related channel interactions in visual processing.

Lateral interference between letters in the visual field is a joint function of their similarity and physical separation. Data needed to evaluate hypothesis about the processes implicated in these effects were obtained from two experiments in which the task was identification of a target letter always presented in the center of a three-letter display. Variation of target-flanker similarity, the primary variable, was combined factorially with spacing of target and flanker, location of the display in the visual field, delay of patterned postmasks, and exposure duration. The effect of target-flanker similarity on target identification yielded a nonmonotonic function with a minimum at an intermediate degree of similarity. Data from same-different judgments regarding target-flanker similarity indicate that some information about similarity is available even at levels of visibility that do not permit identification of individual letters. All of the findings could be accommodated by a model assuming that only variables determining visibility--exposure duration, mask properties, location in the visual field, separation of letters--influence extraction of featural information pertaining to letter identification. In contrast, visual similarity influences performance by way of sometimes subtle effects on subjects' criteria or response biases and by effects on the encoding and retention of information regarding relative positions of characters in the visual field. The varying effects of similarity reported in the literature on letter identification all appear to be interpretable in these terms.