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Neural basis of morphological processing in visual word recognition

Final Activity Report Summary - N.B.M.P.V.W.R. (Neural basis of morphological processing in visual word recognition)

The ability to speak, read and understand language is one of the most complex and exclusively human of our cognitive abilities. It is made possible by the coordinated operation of multiple brain regions with diverse cognitive functions. Central to language comprehension and production is the mental lexicon, i.e. a store of the spoken and written forms of words and their associated meanings. Understanding the function of the mental lexicon has been an important focus for research in linguistics, psychology, and more recently, cognitive neurosciences.

Using brain imaging methods we assessed the functional and anatomical organisation of brain systems involved in recognising simple words, like dark, and complex words, like darkness, as well as in testing single-route and dual-route accounts in which these items are processed by similar or distinct neural mechanisms. Both experiments contrasted brain responses to words and pseudowords varying in their typicality and internal structure. The first functional magnetic resonance imaging (fMRI) experiment contrasted monomorphemic words and pseudowords such as time and tave. These stimuli varied in terms of length, frequency of use, orthographic typicality and semantic imageability. The results pointed towards an involvement of the left fusiform gyrus, sensitive to the frequency of use of the words and the orthographic typicality of the pseudowords; the left precentral gyrus, associated with the retrieval of phonological information and also the left inferior frontal gyrus, linked to the retrieval of semantic information.

A second fMRI experiment was designed to extend these results with polymorphemic words and pseudowords. The critical comparisons were between complex words that had a transparent meaning, such as talker, or not, such as corner (a corner is not someone who 'corns') and between transparent pseudowords, such as blondify, or opaque pseudowords such as seekify, among which only the former made sense and could be processed fully for meaning. The results showed that regions of the left frontal lobe usually associated with higher-level language processes, e.g. inferior frontal gyrus, and thought responsible for syntactic and semantic unification and interpretation were equivalently engaged for real words and transparent pseudowords alike.

Altogether these results provided important neuroscientific backing to a cognitive theory by which the same processing stages were involved in recognising words and pseudowords, whether they were simple or complex. They supported a single-route model of lexical processing that did not initially distinguish between words and pseudowords or between (pseudo-) words with transparent and opaque meaning.