Topography of auditory evoked cortical potentials in children with severe language impairment

Topography of auditory evoked cortical potentials in children with severelanguage impairmentIngerTonnquist-UhlenDepartment of Audiology, Karolinska Hospital, Stockholm, SwedenDevelopment of normal speech and language functions is closely related to normal hearing.However, most children with delayed or disturbed speech development show normal tone thresh-olds and here the investigation has to include higher levels in the auditory system. There isevidence for a connection between language impairment and a central auditory processingdisorder, but the underlying mechanisms are not well understood. There is also a need forobjective diagnostic methods of central auditory function. In the present study, a computerised method of mapping the scalp topography of long-latency (cortical) auditory evoked potentials (LAEP) was used for assessment of the functionin central auditory pathways . Topographic mapping of the LAEP component N I, in adultsand normal children, showed reproducible and valid results. In adults, a focal negativity,focus of NI (FNI), with a frontocentral position and contralateral to the stimulated earwas observed. The N I maps in normal children showed a pattern similar to that in adults,but with some age-related changes. The Nl latencies declined significantly with age innormal children and reached adult values at the ages of 14-16 years. The topography of the LAEP components N I, P2, N2 and the T complex was investi-gated in 20 children with severe language impairment (LI). The study also included audi-tory brainstem responses (ABR), electroencephalography (EEG), quantitative EEG (qEEG)and magnetic resonance imaging (MRI). Twenty normal children served as controls (C). A similar topographic pattern was found in the LI and the C children, but with a higherproportion of deviating and non-focal maps in the LI group. The latencies of all componentswere significantly longer in the LI than in the C children. The diagnostic value of LAEP topog-raphy, latency and amplitude was estimated with a scoring system, whereby significantlyhigher scores were found in the LI group than in the C group. With all three parameterstogether the sensitivity was 65%, with a specificity of 90%. There was a high degree of pathological EEGs in the LI group. ABR abnormalities were seenin some LI subjects. MRI was normal in all but two LI children. There was no significantcorrelation between the results of EEG,ABR, MRI and the total score of LAEP, but someLI children showed a wide pathological pattern. In 17 of the 20 LI subjects a pathologi-cal result was obtained in one or more of the investigations. In conclusion, our results may indicate that language impairment has a dualpathophysiology, a specific auditory disorder (LAEP, ABR) and a non-specific generalcerebral disturbance (EEG, MRI). The highly varying results among the present LI chil-dren, with specific and/or non-specific deviations, may be due to heterogeneity of thegroup with different aetiologies of their language impairment, or to a general develop-mental disturbance with a varying distribution and penetrance. The scoring systern ofLAEP proved to be the most sensitive method in separating the LI children from thecontrols. This may be a promising model for individual diagnostic criteria and for clas-sification of language impairment.Keywords: Auditory evoked potentials - Brain map - Topography - Child - Languageimpairment - Auditory perceptual disordersISBN 91-628-2037-