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The Mismatch Negativity (MMN) as a tool for investigating language in the brain
The neural systems underlying language function are usually explored in the context of tasks where language stimuli are presented and subjects have to focus their attention on these stimuli and perform a linguistic task. Neurobiological models of word processing and language would suggest that strongly linked networks of neurons are built up that specialize in the detection (and production) of words, or sequences of words, and that the strong neuronal connections defining these neuron ensembles lead to spreading of neuronal activity in the network. This should take place regardless of whether the brain is in an attentive or non-attentive state, and regardless of whether attention is directed toward the language input or some other input. A series of experiments has been performed to explore brain processes triggered by language stimuli when these are not the focus of the subjects' attention.
When words are being presented visually, there is, on the one hand, evidence for implicit word processing (Price, Wise, & Frackowiak, 1996), but, on the other, inattentional blindness for words (Rees, Russell, Frith, & Driver, 1999) has been reported when subjects were instructed to engage in a distracting task. Since, from both ontogenetic and a phylogenetic perspectives spoken language is more basic than written language, we used spoken words and word sequences for our investigations.
The Mismatch Negativity (MMN), a neurophysiological index of the detection of a change in the acoustic environment that can be elicited in the absence of focussed attention (NŠŠtŠnen, 2001) was recorded to spoken CV syllables. Subjects were instructed to watch a simultaneously presented silent video film and ignore the acoustic stimuli. The critical syllables followed context syllables with which they formed either words or meaningless pseudo-words. The MMN was significantly altered by the context. The same syllable elicited larger MMNs when it terminated a real Finnish word than when it was presented in pseudo-word context (PulvermŸller, Kujala et al., 2001). Further experiments ruled out the possibility that the MMN enhancement to words is due to bi- or trigram frequencies of the phonemes making up the words (PulvermŸller, Kujala et al., 2001). We call this enhancement of the MMN in word context relative to pseudo-word context the lexical enhancement of the MMN (see the figure below). These results show that word-related brain responses can be elicited when subjects do not focus their attention on the language input. The activation of the memory traces for words does not appear to require that subjects engage in a linguistic task.
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Figure: Words elicited larger magnetic MMNs than pseudowords (left side). This lexical enhancement effect was present at ~150 ms after the stimulus information necessary for word recognition. The locus of the main cortical source, as revealed by calculating the Equivalent Current Dipole, was found in the left superior temporal cortex. Source locus did not distinguish between words and pseudowords. From PulvermŸller, Kujala et al., 2001). |
The lexical enhancement effect was originally obtained with Finnish stimuli, and we have since replicated this with English stimuli (Shtyrov & PulvermŸller, 2002b). In a study using monosyllabic English words, deviant and standard CVC stimuli were distinguished by their final phoneme. Word-word and word-pseudoword pairs were included, as in pairs like type-tight and pipe-pite. An item ending in a [t] was always used as the frequent standard stimulus, whereas an item terminating with a [p] was the infrequent deviant stimulus that elicits the MMN. Physical differences between stimuli were minimized by cross-splicing word-initial CV syllables and word-final phoneme sounds so that all standard stimuli shared their final sound and the same was true for the deviants. The unexpected word-final phoneme elicited a larger MMN than the same phonetic signal terminating a CVC pseudoword, thereby replicating a lexical enhancement effect for the English words. An additional result was that the enhancement effect was dependent on the lexical status of the deviant stimulus but not of the standard stimulus (Shtyrov & PulvermŸller, 2002b).
In more recent research (PulvermŸller, Shtyrov, Ilmoniemi, & Marslen-Wilson, submitted) we have even more direct evidence for the sensitivity of the MMN to linguistically and cognitively relevant processing events. The concept of "word-recognition point" is well established in the Cohort Model of Marslen-Wilson, and claims that spoken words can be identified by the listener as soon as the speech input diverges from other possible words in the language. Thus, for example, the fragment "crocodå..." might be sufficient to identify the word crocodile, and the recognition point would be set in the region of the /d/. In experiments run in Finnish, and again using the MEG laboratory in Helsinki, we were able to measure the magnetic Mismatch Negativity (MMN) elicited by spoken words, for pairs of words like tuon and tuot. These are forms of the verb tuo 'bring', with either the inflectional ending [-n], meaning "I bring" or with the ending [-t], meaning "you bring". The participants were independently tested to determine their individual recognition points for these pairs, with recognition point being consistently slower for tuot. In the MMN experiment itself, the two words alternated as standard and deviant across conditions.
The results were striking, with the latency and detailed timing of the MMN responses primarily determined by word recognition points (see the figure below). Both ECD (Equivalent Current Dipole) and MCE (Minimum-Norm Current Estimate) calculations of the spatio-temporal properties of the MMN responses showed (a) that these peaks occurred about 100-150 ms after the information in the acoustic input was sufficient for word recognition, and (b) that the peak was significantly delayed for the word (tuot) with a later recognition point. The neural generators for these responses were located in the left superior temporal cortex, which is known to play a role in mapping sound onto lexical meaning. Even more telling was the further result that the recognition-points computed in the gating task for the individual participants correlated significantly (r = 0.66) with the latency of these activity peaks in superior temporal cortex. We conclude that the latency of the magnetic MMN elicited by spoken words reflects an early brain process that underlies the recognition of individual lexical items in individual subjects.
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Figure: Minimum Current Estimates of cortical sources activated during the presentation of the spoken Finnish words "tuon" and "tuot". Gating experiments revealed average word recognition points for "tuon" at 350 ms and for tuot at 420 ms after word onset. Starting at ~100 ms after the respective recognition points, cortical activation was observed in the left superior temporal lobe, correlated in time with individual subject's recognition points. |
This is not only an important result in itself, but also it validates the use of the MMN to probe the higher-level structure of language processing mechanisms in the brain. In future research, we plan to exploit this vigorously, using the MMN to examine more generally the spatio-temporal patterns of cortical activation invoked by linguistic material