During visual word recognition, phonology is accessed within 100 ms and may be mediated by a speech production code: evidence from magnetoencephalography

Abstract

Debate surrounds the precise cortical location and timing of access to phonological information during visual word recognition. Therefore, using whole-head magnetoencephalography (MEG), we investigated the spatiotemporal pattern of brain responses induced by a masked pseudohomophone priming task. Twenty healthy adults read target words that were preceded by one of three kinds of nonword prime: pseudohomophones (e.g., brein-BRAIN), where four of five letters are shared between prime and target, and the pronunciation is the same; matched orthographic controls (e.g., broin-BRAIN), where the same four of five letters are shared between prime and target but pronunciation differs; and unrelated controls (e.g., lopus-BRAIN), where neither letters nor pronunciation are shared between prime and target. All three priming conditions induced activation in the pars opercularis of the left inferior frontal gyrus (IFGpo) and the left precentral gyrus (PCG) within 100 ms of target word onset. However, for the critical comparison that reveals a processing difference specific to phonology, we found that the induced pseudohomophone priming response was significantly stronger than the orthographic priming response in left IFG/PCG at approximately 100 ms. This spatiotemporal concurrence demonstrates early phonological influences during visual word recognition and is consistent with phonological access being mediated by a speech production code.

Figure 1.

a, Three-dimensional rendered cortical representations showing significant activity above baseline for PSEUD at four frequency bands and two time windows. t-Maps are thresholded at p < 0.05 (corrected). b, Sagittal section showing border (dashed line) between left IFGpo and PCG with activation in the three priming conditions overlaid and thresholded at p < 0.05 (corrected). c, ROI analysis of the responses of left IFGpo/PCG comparing PSEUD and ORTH. Regions in this time–frequency space showing a significant difference between conditions at the group level are color coded: PSEUD > ORTH (p < 0.05) in the induced and evoked signal components is shown by the blue–aqua and yellow-orange scales, respectively. No instances of ORTH > PSEUD were identified in IFGpo/PCG or MOG.