Atypical auditory ERPs in SLI 581 Figure 2 Mean Fisher-transformed ICC values for SLI (white bars) and control children (grey bars) at nine electrode sites for tone and syllable stimuli. stimulus, and electrode with mean amplitude as depend- ent variable. The main effect of group and all inter- actions with group fell well short of signiﬁcance (F < 1). It is clear that the group differences observed with the ICC do not simply reﬂect group differences in mean amplitude in the N1 interval. Characteristics of children with atypical ERPs Although there were signiﬁcant differences in ICC values, there was nevertheless substantial overlap between groups, suggesting that only a subset of children with SLI have atypical ERPs. To explore whether this subgroup had speciﬁc characteristics, children were subdivided accord- ing to their ICC values at electrode C4 with speech stimuli, this being the electrode and condition that gave the largest group difference. Eight of 47 (17%) control children obtained an ICC of zero or less; this value was taken therefore as deﬁning the lower bound of normal limits (approx. 1 SD below the mean). Fourteen of 42 (33%) children with SLI had an ICC this low, and nine of these scored below the lowest ICC of any control (less than −0.3). This ‘low-ICC’ subgroup was compared with the remainder of the SLI group (‘average-ICC’ subgroup) and found not to differ signiﬁcantly on age, gender, handedness or nonverbal IQ, or on number of epochs included in the ERP averages (see Table 2). Uwer et al. (2002) had found higher error rates in children with SLI vs. controls in tests of discriminating tone frequency, tone duration, and syllable identity, but there was no © 2007 The Authors. Journal compilation © 2007 Blackwell Publishing Ltd. hint of higher error rates in the ‘low ICC’ subgroup on these behavioural measures. However, the subgroups did differ in their SLI classiﬁcation, with the excess of low-ICC cases being conﬁned to those with receptive SLI. Ten of 21 (48%) children with receptive SLI had a low ICC, com- pared with four of 21 (19%) of the expressive SLI children. The signiﬁcant group differences in ICC appear at ﬁrst sight to be at odds with the impression from the grand average waveforms shown in Figure 1, which suggest that any differences between SLI and control children are slight. However, further investigation indicated that the grand averages are misleading, because they mask quite substantial variability within the group. This was explored further by subdividing children according to their SLI classiﬁcation and their ICC for syllable stimuli at electrode C4. Figure 3 shows indi- vidual waveforms plotted against the normative grand mean for speech stimuli in each subgroup. On visual inspection, it can be seen that for most of those with an average ICC there is a downward slope in the waveform between 100 and 228 ms. This also appears to be the case for control children with low ICCs, where the low ICC seems to reﬂect large differences in amplitude from the normative grand mean, rather than differences in shape. However, for SLI children with low ICCs, indi- vidual waveforms over this time range do not show such a clear negative slope. Whereas virtually all children with average ICCs have positive amplitudes at the start of the interval, most of the SLI cases with low ICCs have negative amplitudes at this time point.