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A neurobiologically-grounded model of syntactic processes
In: A.Glenberg, M.DeVega, A.Graesser (eds.) Proceedings of the Garachico Workshop on Symbols, Embodiment and Meaning, Universidad de La Laguna, Tenerife, 2005
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Among the multitude of questions related to grounding of human language function in the brain one of the most intriguing is that of the embodiment of syntax and grammar. Grammar is one of the main intrinsic properties of human language. The very presence of a grammatical system distinguishes our ability of verbal communication from all signalling systems used by animals. Grammar, which dictates the patterns of formation of sentences or phrases in a language, can be seen as a mechanism establishing order among morphemes and words. Serial syntactic order, along with hierarchical and heterarchical relationships between constituents, is a property of many languages, although to a different degree of rigidity. Although a number of studies could reveal brain correlates of language units, phonemes, morphemes and words, one of the most burning questions remains how serial order of these units is processed in the human brain. In animals, neurophysiological studies have revealed neuronal sequence detectors specifically responding to the serial order of elementary events [1, 4, 11] e.g. in visual system. A sequence detector is a neuronal element, a neurone or larger neuronal ensemble, which specifically responds to a defined sequence of elementary sensory events [5], but not to the same events appearing in a different order. We suggest that sequence detectors similar to those for elementary visual events may process information about the serial order of words in sentences [8, 9]. A grammatical sequence detector would be connected to two word category representations, A and B, so that, whenever a stimulus word from category A is followed by a member of a category B, the sequence detector would become active. The neural representations of words regularly following each other in grammatical sentences would therefore be connected through a sequence detector, but not so the representations of words or morphemes that would form an ungrammatical and uncommon sequence. Knoblauch with colleagues has recently successfully argued, using neural model simulations, that sequence detectors can develop and be generalised by auto-associative cell-assembly networks [6, see also this volume]. However, the question still remains whether existence of such sequence detectors can be supported by empirical, physiological data from the human brain. Critical predictions from this minimal neurobiological grammar model are the following. Since the sequence detector would connect the neuronal representations of words from categories A and B, any presentation of an A-member would therefore prime B-members through connections via the sequence detector. This syntactically related priming effect should become visible in the brain responses to grammatical and ungrammatical strings. Language-related priming effects are known to reduce negative-going components of the event-related potential (e.g., N2 and N4 [2, 3]), and, therefore, a specific prediction can be made on the behaviour of the response that might reflect the expected syntactically-related priming effect. If a word appears in a grammatically correct sequence, its memory trace is primed, and, therefore, the neural response it elicits should be smaller than in an out-of-context condition. In contrast, the memory trace of a word placed in an ungrammatical and unusual context would not be primed and should, therefore, elicit a pronounced response with activation pattern similar to that elicited by the word standing alone outside of any syntactic context. A second prediction is that, after the activation of the word representation, the sequence detector should become active as well. This subsequent activation of a sequence detector should be specific to grammatically correct word chains, that is, it should not occur if the stimulus word sequence is ungrammatical. To test these predictions and to address the cerebral processing of grammar, we used whole-head high-density electro- and magnetoencephalography to record the brain's responses elicited by grammatically correct and incorrect auditory stimuli in the absence of directed attention to the stimulation. Using the two methods, we evaluated mismatch negativity (MMN), a brain response earlier proposed as an index of pre-attentive automatic speech processing and of long-term memory traces for language elements in the brain [7, 12, 13]. First, two studies were performed [10, 14]: (i) English grammatical and ungrammatical phrases (we come/*we comes) were physically identical up to a divergence point where the grammatical string terminated but the ungrammatical one continued in final -s. In condition 1, the grammatical string was presented as a deviant stimulus against the background of ungrammatical strings and in condition 2, the reverse was used. 64-channel EEG recordings revealed differential responses to the two strings starting already 100-150ms after the divergence point. The ungrammatical deviant elicited an early left-lateralised MMN maximal at fronto-central sites, and later another fronto-central negativity (~300ms). Instead, the grammatical string produced a less pronounced MMN with fronto-central distribution. This was followed (at ~200ms) by a more negative-going ERP than in the ungrammatical context, this difference being maximal at centro-parietal loci. As a control for acoustic/phonetic differences, the responses to "we come" and "*we comes" were compared with those to just "come" and "comes" presented outside of context. Significant Context x Affixation interactions confirmed the conclusion that modulations of the early left-anterior negativity, the sharp positivity, and the late negativity indicate grammatical processing. Minimum-norm current estimates (MCE) of the grammaticality effect revealed a dynamic source in the left frontal cortex. Critically, the left-anterior source was equally strong in the ungrammatical and out-of-context conditions but reduced in the syntactic condition, consistently with the model predicting syntactic priming and activity reduction in grammatical context only. [For details, see 10.] (ii) The stimuli were minimal Finnish phrases differing only in one phoneme (also affix), which rendered them as either grammatical or ungrammatical. Acoustic and lexical differences were controlled for by using an orthogonal design in which the phoneme's effect on grammaticality was inverted (mä tuon/*mä tuot vs. *sä tuon/sä tuot). 306-channel MEG demonstrated that occasional syntactically incorrect stimuli elicited larger MMNs (at ~200ms) than correct phrases. Source analysis (single-dipole models and MCE) indicated grammaticality-dependent differential activation of the left superior-temporal cortex suggesting that this brain structure may play an important role in such automatic grammar processing. [For details, see 14.] MMN modulation by grammaticality under non-attend conditions suggests that early syntax processing in the human brain may take place outside the focus of attention. This was found in two unrelated languages with two methods using different stimuli. No P6-like effect was found in either study. We explain the enhanced MMN response to grammatical violations by an unprimed activation of morpheme-related neuronal ensembles, whereas morphemes that match grammatically would be linked through neuronal sequence detectors that provide syntactic priming for correctly placed morphemes. The similar responses to ungrammatical and out-of-context placement of words are consistent with the critical prediction of the sequence-detector model. The possible reflection of the sequence detector activation could be instantiated as the relatively increased negativity in response to grammatical phrases at ~200ms in (i). The structures of the left perisylvian cortex appear to play an important role in carrying out such automatic grammar processing. To further address the issue of automaticity, an additional EEG study was performed, in which the amount of active distraction from the syntactic and asyntactic phrases was systematically varied. We found that up to ~140ms in time, the brain responses to grammatical and ungrammatical strings were not affected by the attention modulation, which played a role only at later time intervals. 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