Relative laterality of the N170 to single letter stimuli is predicted by a concurrent neural index of implicit processing of letter names

Abstract

While previous research reports a consistently left-lateralized N170 to whole words relative to control stimuli, much less is known about the nature of single letter processing. Yet single letter processing is of both theoretical and practical interest, as letters form an initial unit of literacy learning for alphabetic scripts and may be particularly useful in the study of literacy development. In the present study, adult fluent readers completed an implicit processing one-back task while event-related brain potentials (ERPs) were recorded. Separate blocks included single letter or matched false-font stimuli. Results indicated that single letters elicited a bilateral (rather than left-lateralized) enhancement of the N170 relative to the false font stimuli. Although participants did not make overt rhyming judgments, letters preceded by a rhyming as compared to non-rhyming letter (e.g., e-b versus e-h) also tended to elicit an N450 rhyme effect, as previously reported in explicit letter rhyme tasks. Moreover, individuals with a larger N450 rhyme effect showed greater relative left-lateralization of the response to single letters. Taken together, these findings suggest that early neural specialization for orthographic stimuli extends to the case of single letters and, further, that automatic mappings between visual symbols and phonological codes can account for at least some portion of the relative left-lateralization of early neurophysiological responses to printed text. These findings help resolve discrepancies in the existing literature concerning relative laterality of early neural responses to single letters and provide critical baseline data for future developmental neuroimaging studies.