Near-instant automatic access to visually presented words in the human neocortex: neuromagnetic evidence

Abstract

Rapid and efficient processing of external information by the brain is vital to survival in a highly dynamic environment. The key channel humans use to exchange information is language, but the neural underpinnings of its processing are still not fully understood. We investigated the spatio-temporal dynamics of neural access to word representations in the brain by scrutinising the brain's activity elicited in response to psycholinguistically, visually and phonologically matched groups of familiar words and meaningless pseudowords. Stimuli were briefly presented on the visual-field periphery to experimental participants whose attention was occupied with a non-linguistic visual feature-detection task. The neural activation elicited by these unattended orthographic stimuli was recorded using multi-channel whole-head magnetoencephalography, and the timecourse of lexically-specific neuromagnetic responses was assessed in sensor space as well as at the level of cortical sources, estimated using individual MR-based distributed source reconstruction. Our results demonstrate a neocortical signature of automatic near-instant access to word representations in the brain: activity in the perisylvian language network characterised by specific activation enhancement for familiar words, starting as early as ~70 ms after the onset of unattended word stimuli and underpinned by temporal and inferior-frontal cortices.

Figure 1

(A, top). An example of visual stimulus and stimulation sequence. The participants were instructed to track colour/location combinations of circles that were continuously present on the screen, while matched words or pseudowords were tachistoscopically flashed at unattended symmetric locations at the same time when the non-linguistic stimuli changed colour, ensuring maximal distraction from linguistic information. (B, middle). Grand average MEG responses (magnetic gradient RMS amplitudes) over the left posterior and anterior areas. Both waveforms and difference topographies of event-related responses indicated differential brain activation in response to unattended meaningful words and meaningless pseudowords, starting with the first difference in the temporal sensors already at 70–90 ms and continuing until ~250 ms post-onset. Schematic MEG helmet diagrams illustrate the selection of sensor clusters used to separate the relatively more temporal and frontal ERF dynamics. (C, bottom). Grand average word-pseudoword contrast in MEG source space (minimum-norm estimates on inflated cortical surface based on single-subject MR images) confirmed early processing of lexical contrasts in temporal and inferior-frontal neocortex. A schematic diagram based on average cortical surface illustrates the atlas-based neuroanatomical ROIs used for extracting source amplitudes.