Clinical neurophysiology of language: The MEG approach

doi:10.1016/j.clinph.2006.07.316

Clinical Neurophysiology

Volume 118, Issue 2, February 2007, Pages 237-254

https://doi.org/10.1016/j.clinph.2006.07.316 Get rights and content

Abstract

Clinical evaluation of language function and basic neuroscience research into the neurophysiology of language are tied together. Whole-head MEG systems readily facilitate detailed spatiotemporal characterization of language processes. A fair amount of information is available about the cortical sequence of word perception and comprehension in the auditory and visual domain, which can be applied for clinical use. Language production remains, at present, somewhat less well charted. In clinical practice, the most obvious needs are noninvasive evaluation of the language-dominant hemisphere and mapping of areas involved in language performance to assist surgery. Multiple experimental designs and analysis approaches have been proposed for estimation of language lateralization. Some of them have been compared with the invasive Wada test and need to be tested further. Development of approaches for more comprehensive pre-surgical characterization of language cortex should build on basic neuroscience research, making use of parametric designs that allow functional mapping. Studies of the neural basis of developmental and acquired language disorders, such as dyslexia, stuttering, and aphasia can currently be regarded more as clinical or basic neuroscience research rather than as clinical routine. Such investigations may eventually provide tools for development of individually targeted training procedures and their objective evaluation.

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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Invited reviewClinical neurophysiology of language: The MEG approach

Abstract

Introduction

Section snippets

Sequence of activation

Sequence of activation

Speech production

Language-dominant hemisphere

Language disorders

Conclusion

Acknowledgements

Neuroimage

Epilepsy Behav

Neuroimage

Neurosci Lett

Neuron

Neuroimage

Brain Res

Trends Cogn Sci

Neuroimage

J Physiol (Paris)

Neuroimage

Trends Neurosci

Cognition

Neuroimage

Cognition

Neuroimage

Clin Neurophysiol

Brain Res Cogn Brain Res

Trends Cogn Sci

Neuroimage

Neuron

Neuroimage

Neuropsychologia

Brain Research

Neuroimage

Trends Cogn Sci

Brain Lang

Neuroscience

Neuroimage

A Handbook on Stuttering

Time course of top-down and bottom-up influences on syllable processing in the auditory cortex

Cereb Cortex

Categorizing sounds and learning to read – a causal connection

Nature

A typical language representation in patients with chronic seizure disorder and achievement deficits with magnetoencephalography

Epilepsia

Abnormal activation of temporoparietal language areas during phonetic analysis in children with dyslexia

Neuropsychology

Language dominance in children as determined by magnetic source imaging and the intracarotid amobarbital procedure: a comparison

J Child Neurol

Lateralization of cerebral activation in auditory verbal and non-verbal memory tasks using magnetoencephalography

Brain Topogr

Lateralization of activity associated with language function using magnetoencephalography: a reliability study

J Clin Neurophysiol

Language dominance determined by magnetic source imaging: a comparison with the Wada procedure

Neurology

Lexical organization of nouns and verbs in the brain

Nature

The visual word form area – spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients

Brain

Models of reading aloud: dual-route and parallel-distributed-processing approaches

Psychol Rev

Event-related potential components reflect phonological and semantic processing of the terminal word of spoken sentences

J Cogn Neurosci

Adult brain plasticity elicited by anomia treatment

J Cogn Neurosci

Cortical effects of shifting letter-position in letter-strings of varying length

J Cogn Neurosci

Renewal of the neurophysiology of language: functional neuroimaging

Physiol Rev

Specific and nonspecific effects of transcranial magnetic stimulation on picture-word verification

Eur J Neurosci

Magnetic and electric brain activity evoked by the processing of tone and vowel stimuli

J Neurosci

Systematic research in experimental phonetics: 1. A theory of the speech mechanism as a servosystem

J Speech Hear Disord

Neuroimaging studies of word reading

Proc Natl Acad Sci USA

Left-hemisphere dominance for processing of vowels: a whole-scalp neuromagnetic study

Invited review
Clinical neurophysiology of language: The MEG approach