Electrophysiological signatures: magnetoencephalographic studies of the neural correlates of language impairment in autism spectrum disorders

Abstract

While magnetoencephalography (MEG) is of increasing utility in the assessment of pediatric patients with seizure disorders, this reflects only a part of the clinical potential of the technology. Beyond epilepsy, a broad range of developmental psychiatric disorders require the spatial and temporal resolution of brain activity offered by MEG. This article reviews the application of MEG in the study of auditory processing as an aspect of language impairment in children. Specifically, the potential application of MEG is elaborated in autism spectrum disorders (ASD), a devastating disorder with prevalence of 1 in 150. Results demonstrate the sensitivity of MEG for detection of abnormalities of auditory processing in ASD ('electrophysiological signatures') and their clinical correlates. These findings offer promise for the comprehensive assessment of developmental neuropsychiatric disorders.

Figures

Figure 1

Latency of neuromagnetic evoked response, M100, as a function of tone frequency in a typical child with autism. Both left and right hemisphere responses show a characteristic latency inverse dependence on tone frequency, but note that the dynamic range (latency difference between M100 responses to low vs high frequency tones) is low compared with literature values in adults and typically developing children.

Figure 2

Age dependence of M100 latency. Left hemispheric M100 latency responses to 1kHz sinusoidal tones in (a) typical development, and (b) children with an ASD, both tend to show maturational shortening with increasing age, but note that (c) overall latencies (both left and right hemispheres) are prolonged for children with an ASD compared to age-matched controls.

Figure 3

In a rapid temporal processing paradigm, it is noted that left hemispheric responses to closely spaced tones predict clinical language function assessment. Such correlation was absent in right hemispheric responses.

Figure 4

MEG and EEG responses to a 200Hz sinusoidal tone presented to the right ear of a healthy adult volunteer. 275-channel MEG and simultaneous 64-channel EEG was used. Responses are shown for right MEG temporal sensors (top), left MEG temporal sensors (middle), and electrode Cz (bottom). The peak of the left hemisphere 100 ms contralateral response is observed at 116 ms (first solid vertical line). As expected, the right hemisphere ipsilateral response is delayed, occurring ~20 ms later (second solid vertical line). Maximal 100 ms activity at Cz is observed at 119 ms. Although the latency of Cz response is similar to the latency of the MEG left hemisphere response, Cz does not provide a clean measure of the left hemisphere activity, as left and right hemisphere activity linearly sum at the EEG midline sites.

Figure 5

The surface Laplacian emphasizes superficial, localized sources. The surface Laplacian for the 100 ms EEG auditory response shows clear source and sink peaks over each hemisphere. As such, in this subject, separate scoring of left and right STG activity is possible.

Figure 6

A strategy for use of MEG in studying language impairment in developmental disorders. Use of paradigms spanning sound perception, processing and linguistic computation directs focus to progressively later latencies in evoked response data.

Figure 7

In subjects with verbal communication difficulties, a visual aid may help ensure their head is placed correctly in the helmet.