Authors:

BOZIC, M., Tyler, L.K., & MARSLEN-WILSON, W.D.

Reference:

Year of publication:

2008

CBU number:

7014

Abstract:

There is abundant evidence that bilateral temporal and left frontal systems play a key role in speech comprehension. This evidence also suggests a differentiation between the functional role of bilateral fronto-temporal systems involved in basic lexical access processes, and a more specialised left-lateralised system, which supports combinatorial processes invoked by morpho-phonologically and syntactically complex lexical items (Tyler & Marslen-Wilson, 2007). It remains unclear, however, what specific types of lexical complexity trigger the shift from the bilateral to the left-lateralised system. We present two fMRI experiments designed to selectively engage processes due to different types of lexical complexity: morpho-phonological complexity, where the presence of a potential grammatical affix triggers stem-affix decomposition, and more general processing complexity due to the presence of an embedded stem, which can trigger cohort-based competition between lexical candidates. Their effects were compared in inflected and derived words, since the properties of these two types of complex words present different challenges for the processing system. The first experiment employed inflected words, where morpho-phonological complexity is regular and predictable. In a set of single spoken words we manipulated the presence of potential embedded stems and inflectional affixes, and contrasted them against a baseline that has the complex auditory properties of speech but does not trigger a speech percept (envelope-shaped ‘musical rain’, Uppenkamp et al, 2006). The two types of complexity showed differential effects: the presence of potential inflectional affixes activated the left inferior frontal gyrus (IFG), while words with embedded stems activated both right and left IFG regions. The second experiment asked whether similar left-lateralised decomposition and bilateral competition processes hold for derivationally complex words, where the relationship between stem and affix is strongly lexicalised and less semantically predictable. We manipulated the presence of embedded stems and derivational suffixes with varying degrees of productivity, forming a gradient in the extent to which derived words are expected to trigger competition and decomposition processes. Words were compared against the same ‘musical rain’ baseline. The results show that stem competition generated by derivational complexity engages analysis processes bilaterally in the fronto-temporal language regions, but with no hemispheric dissociation comparable to that observed for inflections. This is arguably because the presence of derivational affixes does not trigger decompositional processes in the same way as inflectional affixes. We discuss the results within the framework of a neuro-cognitive theory of the mental representation of inflectionally and derivationally complex words. Methods: data were acquired on 3T Trio Siemens scanner, using a fast sparse imaging protocol (TR/TA= 3.4/2s). Twelve participants took part in Experiment 1, and eighteen took part in Experiment 2. In both experiments, test items (40 per condition) were randomly intermixed with filler items, auditory baseline and silence events, and participants performed a gap-detection task (Mattys & Clark 2002). Data were analysed in SPM5.