Abstract
BackgroundLanguage comprehension requires decoding of complex, rapidly changing speech streams. Detecting changes of frequency modulation (FM) within speech is hypothesized as essential for accurate phoneme detection, and thus, for spoken word comprehension. Despite past demonstration of FM auditory evoked response (FMAER) utility in language disorder investigations, it is seldom utilized clinically. This report's purpose is to facilitate clinical use by explaining analytic pitfalls, demonstrating sites of cortical origin, and illustrating potential utility.ResultsFMAERs collected from children with language disorders, including Developmental Dysphasia, Landau-Kleffner syndrome (LKS), and autism spectrum disorder (ASD) and also normal controls - utilizing multi-channel reference-free recordings assisted by discrete source analysis - provided demonstratrions of cortical origin and examples of clinical utility. Recordings from inpatient epileptics with indwelling cortical electrodes provided direct assessment of FMAER origin. The FMAER is shown to normally arise from bilateral posterior superior temporal gyri and immediate temporal lobe surround. Childhood language disorders associated with prominent receptive deficits demonstrate absent left or bilateral FMAER temporal lobe responses. When receptive language is spared, the FMAER may remain present bilaterally. Analyses based upon mastoid or ear reference electrodes are shown to result in erroneous conclusions. Serial FMAER studies may dynamically track status of underlying language processing in LKS. FMAERs in ASD with language impairment may be normal or abnormal. Cortical FMAERs can locate language cortex when conventional cortical stimulation does not.ConclusionThe FMAER measures the processing by the superior temporal gyri and adjacent cortex of rapid frequency modulation within an auditory stream. Clinical disorders associated with receptive deficits are shown to demonstrate absent left or bilateral responses. Serial FMAERs may be useful for tracking language change in LKS. Cortical FMAERs may augment invasive cortical language testing in epilepsy surgical patients. The FMAER may be normal in ASD and other language disorders when pathology spares the superior temporal gyrus and surround but presumably involves other brain regions. Ear/mastoid reference electrodes should be avoided and multichannel, reference free recordings utilized. Source analysis may assist in better understanding of complex FMAER findings.
Highlights
Language comprehension requires decoding of complex, rapidly changing speech streams
Principal Components Analysis (PCA) [24,30] on the entire electrode set showed one primary, 4 Hz sine wave dominant component which by source analysis decomposed into two major responses with origins in left and right posterior superior temporal gyri and with source orientation directed toward the central vertex region
This report demonstrates the potential utility of the FM auditory evoked response (FMAER) for study of children with language disorders
Summary
Language comprehension requires decoding of complex, rapidly changing speech streams. The technique failed to achieve the wide use and clinical acceptance that it may deserve To some degree this may reflect: (1) lack of clinicians’ intuitive understanding of the FM process - most familiar to electrical and acoustic engineers and research audiologists; (2) lack of a general appreciation of the relevance of FM to language processing; (3) lack of simple, readily available equipment to perform such testing; (4) frequent observation that the maximal response amplitude to FM evoked responses is seen over the frontal-central regions, rather than over language associated temporal regions [1,2]; (5) mixed results in the literature with some authors reporting negative [5] and others positive [6,7] findings with FM stimulation in childhood language disorders, and (6) lack of data driven demonstration of FM’s utility in various language disorders
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