Electrophysiological recording procedure ERPs were recorded as participants were passively pre- sented with either tones or speech syllables played in four blocks each of 333 stimuli with constant stimulus- onset asynchrony of 1 second. An oddball paradigm was used in which a standard stimulus was presented on 70% of trials and one of two deviants on the remaining trials; however, the analyses reported here are concerned solely with responses to standard stimuli. The standard tone stimulus was a 1000 Hz sinusoid (rise and fall time 10 ms) and the speech stimulus was a digitized syllable /da/ spoken by a female German speaker. Both were of 175 ms duration. Stimuli were presented to the right ear via earphones at 86 dB SPL. During the recording, chil- dren watched a silent videotape and were instructed to ignore the tones. Data were acquired using a Neuroscan system with sampling rate of 256 Hz, with recordings from silver silver-chloride electrodes positioned according to the International 10/20 system at 22 sites: FP1, FP2, F3, F4, F7, F8, FZ, FT9, FT10, T3, T4, T5, T6, C3, C4, CZ, P3, P4, PZ, O1, O2, OZ. (Note that in the more contem- porary 10/10 system, T3, T4, T5 and T6 are referred to as T7, T8, P7 and P8, respectively.) Electrodes were referenced to the right mastoid, and horizontal (HEOG) and vertical (VEOG) eye movements were recorded. An online bandpass ﬁlter was set with limits at 0.1 and 30 Hz. Automatic artefact rejection was applied ofﬂine to exclude all epochs with voltage exceeding ±80 µ v. Ofﬂine the recordings were re-referenced to the average of all electrodes (except for HEOG and VEOG) and a spline ﬁt was used to convert to a sampling rate of 250 Hz. Computation of the intraclass correlation (ICC) Computation of the ICC is described in detail by Bishop et al. (2007). In brief, the ICC is similar to the more familiar Pearson correlation co-efﬁcient, but is sensitive to amplitude as well as shape. The one-way random effects ICC (Shrout & Fleiss, 1979) was computed between datapoints between two waveforms: a normative wave- form, which was the grand average for the control group, and a comparison waveform, which is the waveform of an individual child. To ensure total independence of normative and comparison waveforms, the comparison waveform was never included in the average used for the normative waveform: i.e. when computing the ICC for a typically developing child, the normative grand average was based on all the other control children. Note, how- ever, that normative waveforms formed by dropping one subject from a sample of 47 were virtually identical. © 2007 The Authors. Journal compilation © 2007 Blackwell Publishing Ltd. Atypical auditory ERPs in SLI 579 Results Comparison of ICCs for SLI and typically developing children Children aged from 5 to 10 years were treated as a single group because there was no evidence of any develop- mental change in this age range (Bishop et al., 2007). Figure 1 shows the grand average auditory ERP for SLI and control groups at nine electrodes: F3, FZ, F4, C3, CZ, C4, PZ, T3 and T4, for both tone and speech stimuli. The overall waveform shape and peak latencies appear similar for the two groups, with only minor dif- ferences evident in amplitude at some electrodes. ICCs with control grand means were computed for all indi- viduals across the time scale 100 to 228 ms (33 data points) at these electrodes. This interval was selected so that we could directly compare results with those of Bishop and McArthur (2005), who selected this interval because it encompassed N1 and P2 for those individuals who had these peaks. ICCs were transformed to Fisher z to normalize the data, and then compared in a mixed ANOVA with two repeated measures (electrode: F3, Fz, F4, C3, Cz, C4, Pz, T3 and T4, and material: tone, speech) and one between-subjects variable, group (SLI vs. control). Mean Fisher-transformed ICCs at each electrode are shown in Figure 2. Mauchly’s test indicated that the assumption of sphericity was violated for the main effects of electrode (χ2 = 288.7, d.f. = 35, p < .001). Degrees of freedom were therefore corrected using Greenhouse-Geisser estimates of sphericity (ε = .59). The main effect of group was non signiﬁcant; F(1, 87) = 2.61, p = .109. There were signiﬁcant effects of stimu- lus, indicating that ICCs were generally higher for tone than for syllable stimuli: F(1, 87) = 67.3, p < .001. There was a signiﬁcant effect of electrode: F(4.71, 409.6) = 53.4, p < .001. In interpreting these main effects, note that the dependent variable is a measure of agreement between an individual’s waveforms and those of the grand average control group, rather than absolute ampli- tude of the ERP. A high ICC indicates greater uniform- ity in the waveforms of the sample, whereas a lower mean ICC indicates that the auditory ERP is more variable from child to child. The interaction between stimulus and group was not signiﬁcant (F ratio < 1). However, there was a signiﬁcant interaction between electrode and group: F(4.7, 409.6) = 2.93, p = .015. This was further inves- tigated using post-hoc one-tailed t-tests (87 d.f.) to compare the mean ICC (average of both conditions) of groups for each electrode, after ﬁrst establishing