Invited reviewClinical neurophysiology of language: The MEG approach
Introduction
In the clinical context, the question of language representation in the human brain is largely focused on pre-surgical mapping, in the form of noninvasive lateralization of language function and characterization of linguistic processes that are represented in the immediate vicinity of an area to be resected. Another topic of clinical interest is tracking of neural effects associated with rehabilitation after acquired language disorders, or with specific training programmes in the case of developmental disorders. All these applications critically depend on, or should depend on, information derived from basic research of the organization of language function in the human brain.
The successive and largely overlapping stages in language processing, from sensory analysis (visual, auditory, and tactile) to linguistic assessment, memory search, and motor function, can only be satisfactorily characterized and understood using combined spatial and temporal information. Whole-head magnetoencephalography (MEG) lends itself as an obvious tool in this endeavour as it allows fast tracking of brain activations at millisecond time resolution and reasonable spatial accuracy.
This paper discusses the neural organization of language function as it appears in MEG recordings. We will first outline neural processes of speech perception, reading, and speech production in healthy subjects and then proceed to consider how this information may be used in the clinical domain.
Section snippets
Sequence of activation
Speech perception is thought to proceed as follows: speech signals enter the ear as sound waves from which the brain extracts speech sounds and speech sound sequences which further activate the meaning of the word (Hickok and Poeppel, 2004).
At the level of the brain, activity is detected in the superior temporal cortex bilaterally (Fig. 1). All sounds evoke a prominent activation at about 100 ms, referred to as the N100 response, or N100m in MEG (Hari, 1990). When listening to natural speech,
Sequence of activation
It is usually assumed that when we see a familiar word basic visual features must be processed first before the analysis can proceed to the content, apparently first at the level of single letters and then as a whole word which further activates the word’s meaning and its sound form. According to the influential dual-route model (Coltheart et al., 1993) unfamiliar words or nonwords cannot be handled by this lexical route but, instead, we process them letter-by-letter, converting each grapheme
Speech production
Research into the neural basis of language production is complicated by the strong artefact signals that are generated by mouth and tongue movement and mask the cortical activity. Fortunately, those disturbing field patterns can often be removed from the MEG data (Salmelin et al., 1994, Salmelin et al., 2000b). Normally, there is considerable intertrial variability in timing from the trigger stimulus to actual speech production, and also from onset of mouth movement to actual speech onset
Language-dominant hemisphere
Based on the above, early stages of speech perception and analysis of written language would seem to show the clearest left-hemisphere lateralization in healthy right-handed individuals. Lateralization of the N100m response to speech vs. non-speech sounds indeed shows promise for a clinical paradigm, as it is fast and easy to perform and simple to analyze. Probably the simplest approach was proposed by Gootjes and colleagues (Gootjes et al., 1999) who presented healthy right-handed subjects
Language disorders
Published MEG research into the neural basis of developmental and acquired language disorders mostly falls in the realm of basic neuroscience rather than in the clinical domain. While there is genuine interest in understanding the neural underpinnings of these disorders, as a prelude to possible neuroscience-driven interventions, these deficits also provide essential information about brain areas and time windows that may be particularly relevant to successful language perception and
Conclusion
A reasonable amount of information has begun to be available about the cortical dynamics of basic processes of speech perception, reading, and speech production to support clinical MEG studies of language function. For estimation of the language-dominant hemisphere multiple experimental designs and analysis approaches have been proposed which will need to be further tested in practice. Ideally, the test should be fast and easy for both the subject and the experimenter, and extraction of the
Acknowledgements
Financial interests: This work was supported by the Centre of Excellence Programmes 2000–2005 and 2006–2011 of the Academy of Finland, the Sigrid Juselius Foundation, and the James S. McDonnell Foundation 21st Century Research Award.
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