The brain generates its own sentence melody: A Gestalt phenomenon in speech perception
Introduction
Auditory encoding, prosodic analysis, and linguistic evaluation constitute the brain processes required to understand spoken language. Prosodic cues, like sentence melody or intonation, are probably evaluated by subsequent processes of linguistic analysis, such as syntactic parsing and semantic integration. Both of these types of analyses result in individual event-related potentials (ERPs) in the human electroencephalogram (EEG): syntactic violations lead to the so-called early left anterior negativity (ELAN) (Friederici, 1997). However, the early syntax-related anterior negativity, although being lateralized to the left in a number of studies (Friederici, Mecklinger, & Hahne, 1996; Friederici, Pfeifer, & Hahne, 1993; Hahne & Friederici, 1999), not always demonstrates a clear left maximum but sometimes shows a bi-lateral distribution (Friederici, von Cramon, & Kotz, 1999; Knösche, Maess, & Friederici, 1999). An early anterior negativity with a right hemisphere dominance (ERAN) was found for violations of syntax-like disharmonic patterns in music (Kölsch, Gunter, & Friederici, 2000). Musical features such as frequency, rhythm, and intonation also appear in natural speech and are called the prosodic cues of spoken language which have been shown to be important factors of speech comprehension (Kimberly, Lindfield, Wingfield, & Goodglass, 1999) at the segmental and suprasegmental level. Pitch (F0 frequency), especially, influences the correct classification of spoken words in languages such as German and English (Pell & Baum, 1997). These processes of prosodic analysis are believed to be mediated by right-hemispheric mechanisms (Pell, 1999). The right prefrontal cortex was shown to support pitch discrimination in speech syllables (Zatorre, Evans, Meyer, & Gjedde, 1992). At the suprasegmental level pitch modulations appear to affect brain activation in the right more than in the left hemisphere (Lattner, Maess, Wang, Friederici, & Alter, 2001). Thus, it is conceivable that the degree to which the early syntax-related anterior negativity is lateralized depends upon the parsing system: if the latter mainly relies upon syntactic processes it would be lateralized to the left hemisphere, if, however, it additionally considers prosodic information it would be lateralized to the right hemisphere.
The interdependence of these two types of information was demonstrated in a recent electrophysiological study indicating that prosodic information is used to guide early syntactic structure building (Steinhauer, Alter, & Friederici, 1999).
In previous MEG experiments, we found that syntactic violations in spoken language lead to an early anterior syntax component, distributed bi-laterally in some experiments (Herrmann, Oertel, Wang, Maess, & Friederici, 2000; Knösche et al., 1999) but lateralized to the left hemisphere in others (Friederici, Wang, Maess, Herrmann, & Oertel, 2000). A recent dipole analysis of the magnetic data for the early syntax-related component (ELAN) revealed a fronto-lateral and a temporal dipole in each hemisphere with a tendency to larger amplitudes within the left hemisphere (Friederici et al., 2000). In that study, dipoles were constrained by anatomical locations obtained in an earlier fMRI experiment (Meyer, Friederici, & von Cramon, 2000). From these studies we can conclude that both the left and right hemisphere are involved in auditory sentence processing. This conclusion is supported by recent fMRI studies investigating spoken sentence comprehension (Friederici, Meyer, & von Cramon, 2000; Meyer et al., 2000; Müller et al., 1997). The specific contribution of the right hemisphere, however, is still to be determined.
The present experiment was designed to test the hypothesis that the degree of lateralization of the early syntax-related negativity depends on the reliance on prosodic cues available in the auditory sentence input.
For the experiment reported here, we flattened the pitch of the sentences used in a previous experiment (Herrmann et al., 2000). With this procedure, optimal prosodic cues such as the normally present global pitch contour, i.e., the typical rising and falling F0 pattern over the whole sentence, were eliminated resulting in monotonously sounding sentence material. Subjects had to listen to correct and syntactically incorrect sentences and judge their grammatical correctness.
According to the assumption that the processing of prosodic cues is carried out by the right hemisphere (Pell, 1999), we expected our pitch-flattening to decrease the right-hemispheric component of the magnetic syntax-related component.
Section snippets
Methods
Eleven student subjects (4 female), aged 19–29 (mean age 23.2), were investigated. All subjects were right-handed (laterality index 100). All subjects gave written informed consent and showed no signs of neurological, psychiatric, or hearing disorders. Two subjects of an initial set of 13 subjects had to be excluded from analysis due to artifacts. Three types of experimental sentences were presented. Correct sentences comprised a noun phrase, an auxiliary and a past participle (e.g., ‘Der Fisch
Results
Fig. 1 shows one of the sentences in the time domain (top) and its F0 frequency over time before (middle) and after (bottom) the pitch had been flattened.
An ANOVA of the surface derivative of the event-related magnetic fields in the time interval 120–200 ms after the critical word yielded a significant main effect of condition (F(1,10)=10.98,p<.01), indicating larger amplitudes for syntactic violations than for correct sentences (cf. Fig. 2 top). In addition, the interaction condition ×
Discussion
In line with our hypothesis, pitch flattening affected the laterality of the magnetic early syntax-related component, however, in the opposite direction. In the present study we found a condition effect only over the right hemisphere, while condition effects were found in both hemispheres for sentences with normal prosody (Herrmann et al., 2000).
Following the hypothesis that the right hemisphere supports prosodic processes, the direction of the change in laterality seems to be counterintuitive.
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