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Can I Have a Quick Word? The Time Course of Visual Word Recognition as Revealed by Linear Regression Analysis of ERP Data
The Third International Conference on Cognitive Science, Volume 1, p58
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Visual word recognition is a complex process which is affected by a large number of psycholinguistic variables, such as word length, word frequency etc. The corresponding brain processes are very rapid, i.e. they occur within a few hundreds of milliseconds after word presentation. In the studies reported here, we used multiple linear regression analysis of EEG data in order to reveal the time course of electrophysiological activation related to several important psycholinguistic variables. We submitted electroencephalographic (EEG) responses recorded in a visual lexical decision task to linear regression analysis, yielding event-related regression coefficients (ERRCs) instead of the traditional event-related potential (ERP) measure. In a first step, 10 psycholinguistic features of each of 300 stimulus words were subjected to a principal component analysis, which yielded four orthogonal variables likely to reflect separable processes in visual word recognition: Word length, Letter n-gram frequency, Lexical frequency and Semantic coherence of a word’s morphological family. The binary variable Lexicality (words vs. pseudowords) was also investigated using a subtraction analysis, and first differences between words and pseudowords in the ERP appeared at 160 ms after word onset. However, regression analysis of EEG data documented a much earlier effect of both Word Length and Letter n-gram Frequency around 90 ms. Lexical frequency showed its earliest effect slightly later, at 110 ms, and Semantic coherence significantly correlated with neurophysiological measures around 160 ms, i.e. simultaneously with the lexicality effect. Source estimates (distributed minimum norm estimation) indicated parieto-temporo-occipital generators for the factors Length, Letter ngram frequency and Word frequency, but widespread activation with foci in left anterior temporal lobe and inferior frontal cortex related to Semantic coherence. At later stages (>200 ms), all variables exhibited simultaneous EEG correlates. In a separate analysis, we attempted to disentangle the earliest ERP effects of Word Length and another psycholinguistic variable, Neighbourhood Size (N). These variables are strongly negatively correlated. However, because Word Length should mainly affect processing of the surface form of the word, while N is commonly assumed to affect lexical processing, it is of great interest to determine their differential contribution to the brain response. Our results demonstrate that N and Word Length have distinct neurophysiological manifestations around 100 ms after written word onset. Furthermore, the effect of N distinguished between word and pseudoword processing. The overall results indicate that information about surface form and meaning of a lexical item is first accessed at different times in different brain systems and then processed simultaneously, thus supporting cascaded interactive processing models. We highlight the value of ERRCs in combination with source analysis for the investigation of early electrophysiological correlates of word processing. June 20–25, 2008, Moscow, Russia