Are whole words perceptual units in reading?

Two experiments examined the perceptual processing of letters embedded within one- or two-syllable words and visually similar nonwords. Two-choice speeded discrimination tasks were used in which subjects were informed of both the identity and location of critical display information before stimulus presentation. Exp. 1 results indicated that one-syllable words differing by two letters were more quickly discriminated than one-syllable words differing in a single letter. Such performance gains due to the presence of redundant information suggest that one-syllable words form compelling perceptual units. In contrast, similar nonword pairs differing by two letters were discriminated no faster than two nonwords differing in a single letter. This latter pattern of results also was found for two-syllable words and similar nonwords, suggesting that neither unfamiliar nonwords nor two-syllable words form compelling perceptual units. In Exp. 2 subjects were given substantial practice on a task that forced attention to multiple letters within the stimulus displays. Results replicated the findings of Exp. 1, except that performance for both one-syllable words and matched nonwords now showed improved performance in the presence of redundant information. Taken together results of the two experiments suggest that: (a) the size of compelling perceptual units seems limited, with entire words sometimes functioning as multiunit patterns and (b) unit size is not necessarily related to the correspondence between letter order and pronounceability.