(2005) argued for a maturational lag account on the basis that the auditory ERPs of adolescents with SLI were more similar to those of younger typically develop- ing children than to controls of their own age. We could not replicate this ﬁnding here, because all the children with SLI in the current study were aged between 5 and 10 years, an interval over which the ICC did not reveal any changes with age in the auditory ERP (see Bishop et al., 2007). Additional studies comparing children with SLI with younger typically developing children in the same paradigm would be necessary to establish whether the pattern seen in our low-ICC subgroups is immature, but this seems unlikely. In a study that was similar to ours in terms of stimuli (150 ms duration 1000 Hz tones) and presentation rate (SOA of 900 ms), Morr, Shafer, Kreuzer and Kurtzberg (2002) examined auditory ERPs in children aged from 2 to 47 months. In all groups, the dominant observation around 100 ms was a positivity which increased in magnitude over the observed age range. Thus the fronto-central negativity around 100 ms in children with receptive SLI appears to be abnormal rather than developmentally delayed. It is possible that N100 in these children is unmasked because other com- ponents are missing (cf. Ponton et al., 2000). There would be considerable interest in repeating our study using longer or shorter SOAs, to see whether group differences emerge more strikingly at slow presentation rates when N1 can be reliably elicited. Additional studies with children with SLI aged 12 years and over would also be of interest, as we know that the auditory ERP changes radically around adolescence (see Bishop et al., 2007). A maturational account suggests that children with SLI might show a shift to a more mature pattern of ERP at a later age than usual. Implications for ERP studies of developmental disorders It is noteworthy that the differences uncovered here would not have been detected had we relied on measures of peak amplitude and latency because so many children did not have the N1 and P2 peaks. As we have shown, mean amplitude measures also failed to detect the abnormalities found here, because they are insensitive to waveform shape. It is possible to get information on waveform shape without identifying peaks by dividing an interval (such as the 100–228 ms one used here) into a number of consecutive windows and computing mean amplitude in each of these. However, this method has several disadvantages: the size of window is arbitrary, yet can have a dramatic effect on results, and the high interdependence of consecutive windows creates prob- lems for statistical analysis. Perhaps most importantly, a © 2007 The Authors. Journal compilation © 2007 Blackwell Publishing Ltd. Atypical auditory ERPs in SLI 585 multiple-window approach does not provide a ready way of identifying those children who have atypical wave- forms, and will only reveal signiﬁcant group differences if atypical waveforms are all atypical in the same way. The ICC is especially useful when there is heterogeneity in the ERPs of individuals, as it can identify abnormal- ity, even when the speciﬁc type of abnormality differs from case to case. ICC is a global measure that does not specify the nature of differences between waveforms; however, we suggest that this study proves its usefulness in highlighting cases of atypical auditory ERPs, which may then be explored by more detailed consideration of waveform shape and topography. Our analysis highlights a drawback of grand averages which is that they obscure individual differences. Using the ICC we found subtle differences in the late auditory ERP in just a subset of cases. When individual wave- forms vary in the timing of peaks and troughs and in their amplitudes, group differences will be masked when a grand average is computed. The ICC provides a quantitative estimate of the abnormality of a waveform that has promise as a means of analysing individual differences. Acknowledgements We thank Christiana Leonard for helpful comments on an earlier draft. The ﬁrst two authors are supported by the Wellcome Trust. References Albrecht, R., von Suchodoletz, W., & Uwer, R. (2000). The development of auditory evoked dipole source activity from childhood to adulthood. Clinical Neurophysiology, 111, 2268–2276. American Psychiatric Association. (1994). Diagnostic and statistical manual of mental disorders (4th edn.). Washington, DC: APA. Benasich, A.A., Choudhury, N., Friedman, J.T., Realpe- Bonilla, T., Chojnowska, C., & Gou, Z. (2006). The infant as a prelinguistic model for language learning impair- ments: predicting from event-related potentials to behavior. Neuropsychologia, 44, 396– 411. Bishop, D.V.M. (2002). The role of genes in the etiology of speciﬁc language impairment. Journal of Communication Disorders, 35, 311–328. Bishop, D.V.M. (2004). Speciﬁc language impairment: diag- nostic dilemmas. In L. Verhoeven & H. Van Balkom (Eds.), Classiﬁcation of developmental language disorders (pp. 309– 326). Mahwah, NJ: Erlbaum. Bishop, D.V.M., Hardiman, M., Uwer, R., & von Suchodoletz, W. (2007). Maturation of the long-latency auditory ERP: step