Synthetic Brain Imaging of English Past Tense Inflection

Synthetic Brain Imaging of English Past Tense Inflection Gert Westermann (gwestermann@brookes.ac.uk) Nicolas Ruh (nruh@brookes.ac.uk) Department of Psychology, Oxford Brookes University Oxford OX3 0BP, UK Abstract Brain imaging studies of English past tense inflection have found dissociations between regular and irregular verbs, but no coherent picture has emerged to explain how these dissociations arise. Here we use synthetic brain imaging on a neural network model to provide a mechanistic account of how regional dissociations between regular and irregular processing in adults can emerge in a single mechanism system with experience-dependent structural development. We show that these dissociations arise from a combination of different statistical properties of verbs relating to frequency, relationships to other verbs, and phonological complexity. The model generates predictions about the patterning of active brain regions for different verbs that can be tested in future brain imaging studies. Keywords: English past tense; verb inflection; connectionist modeling, synthetic brain imaging, neuroconstructivism Introduction The question of how verb inflections, particularly the English past tense, are represented and processed in the brain has been the subject of intense debate over the past 20 years. This is because the past tense contains regular forms (e.g., look-looked) that seem to obey a linguistic mental rule, and irregular forms (e.g., see-saw) that show properties of associative storage. One view, the Dual-Mechanism or Words-and-Rules Theory (e.g., Pinker, 1999; Pinker & Ullman, 2002) holds that the apparent processing differences between regular and irregular forms are indeed caused by qualitatively different underlying mechanisms: a rule for regulars, and associative storage for irregulars. Another view, closely linked to connectionist approaches to cognitive processing, argues that all forms are processed in a single associative system and that apparent dissociations between regulars and irregulars emerge on the basis of different statistical properties of verbs that affect their ease of processing (e.g., frequency, phonological complexity, number of similar verbs with a similar past tense (friends, e.g., sing and ring), number of similar verbs with a different past tense (enemies, e.g., sing and bring)) or on the basis of different reliance on semantic vs. phonological factors (Joanisse & Seidenberg, 1999; McClelland & Patterson, 2002; Seidenberg & Arnoldussen, 2003; Westermann, 1998). A large amount of empirical and computational work has aimed to provide evidence for either view. One methodology that has been used in several studies is brain imaging with positron emission topography (PET), functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) and evoked response potentials (ERP). These studies have found differences in brain activation patterns when participants inflect regular and irregular verbs, respectively, leading some researchers to claim that these data provide support for a dual mechanism system, with the rule component and the associative mental lexicon located in different brain regions (Dhond et al., 2003; Jaeger et al., 1996; Lavric et al., 2001). For example, in a seminal study by Jaeger et al. (1996) using PET, participants were asked to generate past tense forms of visually presented monosyllabic verb stems. Results showed that although many brain regions were activated by all verbs, production of regulars selectively activated left dorsolateral prefrontal cortex and left anterior cingulate cortex. Irregulars led to higher overall activation and involved occipital visual processing areas. These systematic differences between both verb types were interpreted by the authors as strong evidence for the dual- mechanism account of inflection. In another study, Dhond et al. (2003) asked participants to covertly generate past tense forms of visually presented verb stems while imaging their brains using MEG. Dhond et al. also found that generation of regulars and irregulars activated many areas in common, but that processing of regulars led to greater activation in left inferior prefrontal areas (Broca’s area), and processing of irregulars preferentially activated left occipitotemporal cortex as well as right dorsolateral prefrontal cortex. These results were interpreted as indicating that regulars activated rule-based grammar regions and irregulars activated areas involved in associative retrieval of forms, corresponding directly to the dual-mechanism theory. However, the results of these and other studies have been controversial. One problem is that specific methodological choices can strongly affect results. For example, because of the low temporal resolution of PET, Jaeger et al. (1996) used a block design in which all regular verbs and all irregular verbs were presented together. However, this design introduces the confound that participants could develop strategies for regular but not for irregular verbs, suggesting that differences between both verb types should be found independently of the nature of the underlying processing mechanisms (Seidenberg & Hoeffner, 1998). Several more recent imaging studies have investigated the possibility that observed activation differences between regular and irregular verbs are due to statistical factors of