Academic journal article Cognitive, Affective and Behavioral Neuroscience

Imaging the Past: Neural Activation in Frontal and Temporal Regions during Regular and Irregular Past-Tense Processing

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Imaging the Past: Neural Activation in Frontal and Temporal Regions during Regular and Irregular Past-Tense Processing

Article excerpt

This article presents fMRI evidence bearing on dual-mechanism versus connectionist theories of inflectional morphology. Ten participants were scanned at 4 Tesla as they covertly generated the past tenses of real and nonce (nonword) verbs presented auditorily. Regular past tenses (e.g., walked, wugged) and irregular past tenses (e.g., took, slept) produced similar patterns of activation in the posterior temporal lobe in both hemispheres. In contrast, there was greater activation in left and right inferior frontal gyrus for regular past tenses than for irregular past tenses. Similar previous results have been taken as evidence for the dual-mechanism theory of the past tense (Pinker & Ullman, 2002). However, additional analyses indicated that irregulars that were phonologically similar to regulars (e.g., slept, fled, sold) produced the same level of activation as did regulars, and significantly more activation than did irregulars that were not phonologically similar to regulars (e.g., took, gave). Thus, activation patterns were predicted by phonological characteristics of the past tense rather than by the rule-governed versus exception distinction that is central to the dual-mechanism framework. The results are consistent with a constraint satisfaction model in which phonological, semantic, and other probabilistic constraints jointly determine the past tense, with different degrees of involvement for different verbs.

Language is often construed in terms of a mental grammar incorporating symbolic rules, of which the English past tense (bake-baked, walk-walked) is a well-studied example (Bybee & Slobin, 1982; Halle & Mohanan, 1985; Pinker, 1999). However, in English, as in many other languages, there are irregular forms that violate the rules (e.g., took, gave). Several theories have addressed how rule-governed forms and exceptions are learned, represented, and used. Traditional linguistic approaches make a strong distinction between rules of grammar and the lexicon. Thus, regular forms such as walked are thought to be generated by a rule, with exceptions such as took stored in and retrieved from a mental dictionary or lexicon. This account reflects a long-standing view within linguistic theory that the lexicon includes all and only information that cannot be derived by more general grammatical mechanisms (Spencer, 1991).

In 1986, Rumelhart and McClelland proposed an alternative view: that all past tenses are generated by the same mechanism, a connectionist network. There were three principal motivations for this approach. First, they noted that most irregular past tenses are not arbitrary; rather, they tend to show many partial regularities. As is widely noted, there are pools of phonologically similar irregular past tenses, such as ring-rang, sing-sang, springsprang or sleep-slept, keep-kept, weep-wept (Halle & Mohanan, 1985; Pinker, 1991). These reflect subpatterns within the putatively irregular forms. However, there are also similarities between many regular and irregular pasttense forms. For example, the regular past tense (e.g., hake-baked) retains the onset and coda of the present tense (/b/ and /k/, respectively). The same occurs in forming irregular past tenses such as took, sat, and ran, which retain the onsets and codas of take, sit, and run, respectively. Similarly, the codas of irregular past tenses such as hit, said, and fled are the same as regular past-tense endings (e.g., baked, baled). The partial systematicity of irregulars is not captured by systems that treat fully regular forms and exceptions as unrelated. The second motivation for this approach was that connectionist networks employing distributed representations can learn and represent both fully regular forms and exceptions-and, importantly, the differing degrees of overlap between the two (for early illustrations of this point, see Daugherty & Seidenberg, 1992; MacWhinney & Leinbach, 1991). This approach contradicted the claim that regular and irregular forms require separate mechanisms (Pinker, 1991). …

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