Authors:

Stamatkis, E., MARSLEN-WILSON, W.D., Tyler, L.K., & Fletcher, P

Reference:

Year of publication:

2004

CBU number:

5784

Abstract:

The ways in which language processes and representations are instantiated in the brain constitute a key area of research in cognitive neuroscience. The distinction between regular and irregular verb inflections provide an important input to this research since evidence from behavioural dissociations in patients with lesions suggests that processing regular forms involves a frontal neuronal circuitry and irregulars engage more temporal regions (Marslen-Wilson & Tyler, 1998; Tyler et al, 2002). This general distinction has been supported in a recent neuroimaging study using efMRI, in which we contrasted regular (stayed/stay) and irregular (teach/taught) past tense forms with “pseudo” past tense forms matched on phonological complexity (jade/jay, peach/port). All words were matched on number of syllables, familiarity, lemma and wordform frequencies. There were 56 word-pairs in each condition. 18 Subjects performed a timed same-different judgement task on spoken word-pairs for regulars/irregulars and pseudo regulars/irregulars. We showed that a fronto-temporal network, linking anterior cingulate (ACC), left inferior frontal cortex (LIFG) and bilateral superior temporal gyrus (STG), is preferentially activated for regular past tense forms (Tyler et al., submitted). We propose that this reflects the additional processing demands posed by regular inflected forms, requiring modulation of temporal lobe lexical access processes by morpho-phonological parsing functions supported by the LIFG. We carried out a complementary re-analysis of the data, in order to investigate this system in terms of inter-regional covariances, which are taken as an index of functional connectivity. We identified cortical regions in which activity was predicted by LIFG and ACC, and critically, by the interaction between these two regions. Furthermore, we determined the extent to which these inter-regional correlations were influenced differentially by the four experimental conditions. Using this approach, we expressed brain activity in terms of inter-regional correlations and of modulations of these correlations by other brain regions and by the experimental conditions.We found that a functional connection between LIFG and left fusiform gyrus (LFG) BA20 (p=0.029 corrected cluster level, Fig. 1a) was significantly stronger for processing real regulars than pseudo regulars. A similar result was found for connectivity between LIFG and RSTG BA42 (p=0.046 corrected cluster level, Fig. 1b). This finding was not replicated in the case of real irregulars vs. pseudo irregulars (or in other pair-wise comparisons). Additionally, we found that functional connectivity between LIFG and left MTG (Fig. 1c) is positively modulated by activity in ACC and that this effect is significantly modulated by experimental condition (greater for regular than irregular items). This latter finding constitutes a three-way interaction that may be explicable in terms of three integrated effects: first, a positive influence of LIFG on temporal activity; second a modulatory influence of ACC activity upon this fronto-temporal connectivity; third a condition-specificity of this modulatory effect such that the cingulate influence upon fronto-temporal connectivity is greater when processing regular items. These findings point to a complex network underlying the processing of spoken words differing in morpho-phonological complexity.