The time and space of lexicality: a neuromagnetic view
Illuminating the neural mechanisms subserving lexico-semantic processing is requisite to further understanding the neurophysiological basis of the dyslexias. Yet, despite numerous functional neuroimaging experiments, the location and temporal behavior of brain regions mediating word-level language processing remain an area of debate. Such investigations typically utilize the word/pseudoword contrast within hemodynamic measurements, and report several left hemisphere regions that respond more strongly to pseudowords but fail to replicate neural areas unique to real word processing. The present experiment addressed this problem from a different perspective. Mainly, we hypothesized that the time course, but not the neuroanatomy, would show within-subject across-condition disparities. For that purpose, we applied dipole-modeling techniques to high-density magnetoencephalographic recordings of healthy subjects, and utilized excellent spatiotemporal accuracy to demonstrate significant across-condition differences in the time domain, along with indistinguishable neural correlates within-subject. In all participants, both words and pseudowords elicited activity in left perisylvian language areas, with words consistently activating these regions ~100 ms earlier than pseudowords. Considerable functional heterogeneity was also observed, and this might underlie the inconsistencies among previous studies. We conclude that the neural distinction in word/pseudoword processing is not in spatial localization, but is better conceptualized as a dynamic difference in processing time.