Neuroimaging and Language I

Chapter 10 Update - January 2004

Neuroimaging and language II fMRI/PET data support dual-route model of reading

Remember from Chapter 10, that one of the most influential theories of reading suggests that it relies on two different ‘routes’ which are separate and distinct. This model, the dual-route model of reading, suggests that one, indirect, route ‘translates’ written language into sounds via grapheme- phoneme correspondence rules (the graphophonological route) while another direct route helps ‘translate’ those words that do not follow grapheme-phoneme correspondence rules (the lexicosemantic route).

Using the first route, the reader can pronounce words that follow regular language and access their meaning; using the second, the reader matches the word directly with one already stored and subsequently accesses its meaning. Because some words may not follow normal spelling-to-sound rules, the model suggests that such words are stored as whole visual forms, a little like images. The more frequent the encounters with irregularly spelled words, the more direct and efficient the lexicosemantic route is. These whole word forms are stored in what has been called the orthographic lexicon. Even parts of a word may be sufficient for it to be recognised as a whole unit.

This model derived from studies of patients who, following brain injury, appeared to rely more on one route than the other. People with phonological dyslexia, for example, only appeared to have access to whole word forms (the direct route) and had difficulty in reading regularly spelled words (suggesting an impairment in the indirect route). Neuroimaging techniques have been useful to language researchers interested in the neural basis of language and a recent meta-analysis has suggested that the two routes of reading can be mapped in the healthy brain (Jobard, Crivello & Tzourio-Mazoyer, 2003).

The analysis examined 35 neuroimaging studies of real word and pseudoword reading in healthy participants. While the reviewers found that access to the visual representation of words relied on two routes, they also noted that there was no consistent brain region devoted to storing the shapes of word forms. Instead, they found that a general region appeared to be involved in the initial segmentation or classification of word-like stimuli. This region was located at the occipitotemporal junction. The phonological route was subserved by parts of the temporal lobe and also regions involved in working memory (because of the process involved in matching letters and sounds). The so-called direct route, according to this review, would recruit a pathway linking the occipitotemporal cortex with those involved in semantic processing (these regions are found in or around the temporal cortex).

Jobard, G., Crivello, F. & Tzourio-Mazoyer, N. (2003). Evaluation of the dual route theory of reading: A meta-analysis of 35 neuroimaging studies. Neuroimage, 20, 693-712.