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Abstract
Hemispheric asymmetries in semantic processing, namely the dissimilar sensitivity of the two hemispheres to different semantic representations and relations, have been shown in several behavioral and neuro-imaging studies. Among the different models that have been put forward to account for this are: the coarse-coding hypothesis (Beeman 1994) which proposes asymmetries at the semantic level, and the Dual Hemispheric Reading Model (DHRM, Peleg & Eviatar 2008) which proposes asymmetries at level of the phonology, and suggests that semantically ambiguous words will be processed differently depending on their phonological status. The goal of the current study was to examine brain activation patterns in response to semantic and phonological ambiguities in regions known to be involved in activation, retrieval and selection of lexical representations–the bilateral inferior frontal gyri (IFG). This is a mean to evaluate the coarse-coding hypothesis and the DHRM. Twenty three participants were scanned in FMRI while they performed semantic relation judgments between visually presented two words that were spaced in time, a prime and a target, and brain activations were analyzed in a whole brain and region of interest analyses. At the critical trials, primes were either Homophones (semantically but not phonologically ambiguous words) or Heterophones (semantically and phonologically ambiguous words), while targets were unambiguous words that were related either to the more salient (dominant) or less salient (subordinate) meaning of the primes. In line with the DHRM, we predicted a different pattern of hemispheric asymmetry in IFG between Homophones and Heterophones during their presentation ('prime stage') and during their integration with the targets ('target stage'). Our results show that during the 'prime stage', Heterophones exhibited hemispheric asymmetry with a strong left IFG (LIFG) pars opercularis (BA 44) activation, while Homophones showed greater activation in bilateral IFG pars orbitalis (BA 47). During the 'target stage', Heterophones' subordinate targets activated the IFG bilaterally while their dominant targets showed decreased activation compared to baseline, whereas Homophones' both target types activated the IFG bilaterally. Compared to their dominant targets, subordinate targets activated the LIFG pars orbitalis (BA 47) stronger in Heterophones than in Homophones. These results may suggest that in the absence of disambiguating context, Heterophones were processed phonologically in favor of their dominant meaning in a process that relies asymmetrically on the LIFG, while Homophones rely on a more symmetric and semantic oriented process that is performed in the IFG bilaterally and serves to retrieve both their meanings. When biased toward their subordinate meaning, both Heterophones and Homophones recruit the IFG bilaterally, however Heterophones require even further recruitment of the LIFG, apparently because a weaker phonological representation needs to be activated. Although not fully conclusive in favor of the DHRM, our results demonstrate the importance of phonological characteristics in processing semantic ambiguity.
