Electrophysiological signatures of masked transposition priming in a same-different task: evidence with strings of letters vs. pseudoletters.

Research on masked transposed-letter priming (i.e., jugde-JUDGE triggers a faster response than jupte-JUDGE) has become a key phenomenon to reveal how the brain encodes letter position. Recent behavioural evidence suggests that the mechanism responsible for position coding in a masked priming procedure works with familiar "object" identities (e.g., letters, digits, symbols) but not with unfamiliar object identities (e.g., pseudoletters). Here we used event-related potentials (ERPs) to explore the time course of masked transposition priming of letters vs. pseudoletters in a cue-target same-different matching task. Target stimuli were preceded by a masked prime that could be: (i) identical to the target; (ii) identical to the target except for the transposition of two internal letters/pseudoletters; or (iii) identical to the target except for the substitution of two internal letters/pseudoletters. Only cue-target 'same' trials were analyzed. The priming manipulation affected the "same" trials of the letter strings between 250 ms and 450 ms: identity and transposition conditions produced less negative amplitudes than the substitution condition. Because of the onset latency of this priming effect, we suggest that masked primes affected mainly the cognitive processes related to the categorization of the trials (match versus mismatch), rather than to the initial stages of orthographic processing.

Are transposition effects specific to letters?

Recent research has consistently shown that pseudowords created by transposing two letters are perceptually similar to their corresponding base words (e.g., jugde-judge). In the framework of the overlap model (Gomez, Ratcliff, & Perea, 2008), this effect is due to a noisy process in the localization of the "objects" (e.g., letters, kana syllables). In the present study, we examine whether this effect is specific to letter strings or whether it also occurs with other "objects" (namely, digits, symbols, and pseudoletters). To that end, we conducted a series of five masked priming experiments using the same-different task. Results showed robust effects of transposition for all objects, except for pseudoletters. This is consistent with the view that locations of familiar objects (i.e., letters, numbers, and symbols) can be best understood as distributions along a dimension rather than as precise points.