Connectivity in Cortical Networks During Word Reading

Background: The neural processes underlying word-reading remain much of a mystery.  In particular, the flow of information within and between language networks during word reading has not been adequately explored. The present study investigated local spectral power changes and functional and effective (causal) connectivity at each stage of word reading. Methods:  EEG was used to record brain activity from healthy volunteers (n = 15), with no reported learning or reading disabilities, during a reading task.  The task required participants to view a sequence of three letters, followed by a three-letter word.  Participants were instructed to respond as to whether or not the word matched the letter sequence.  Independent component analysis yielded many sources of activation previously identified as being crucial to word-reading.  A combination of event-related spectral perturbation and phase synchrony analyses was performed on these independent components. Additionally, analyses of Granger causality were conducted to investigate the possible causal information flow between sites of interest. Results: Analyses show local theta power increases prior to long-distance theta synchronization between several of pairs of cortical sites, reflecting previously-established paths of activation involved in reading text.  Additionally, causal connectivity was established between object recognition sites and semantic processing sites, suggesting a feed-forward flow of information. Conclusions: These results highlight the interplay between local and long-distance neural dynamics involved at each stage of processing during reading. Additionally, these measures of functional and causal connectivity may be used as a benchmark for comparison with clinical populations (e.g. individuals with certain kinds of dyslexia), such that disturbances in connectivity may provide insight as to underlying neurological problems.