582 Dorothy V.M. Bishop et al. Figure 3 Individual waveforms at electrode C4 to syllable stimuli in the time window 100 to 228 ms, for typically developing and SLI subgroups, subdivided according to ICC for speech at C4. SLI-E is expressive SLI; SLI-R is receptive SLI. The grey line shows the normative grand mean. Table 2 Characteristics of SLI sample, subdivided according to whether ICC to syllables at C4 is above 0 (average) or below this level (low) Average ICC N = 28 Low ICC N = 14 Variable N epochs tones N epochs syllables Age (months) Nonverbal IQ Auditory discrimination errors Frequency Duration Speech % male % non-right handed % receptive SLI Mean 753.0 759.3 95.2 101.1 10.7 17.8 9.7 % 21.4 22.2 39.3 SD 116.3 124.4 13.2 10.7 8.6 9.3 12.0 Mean 781.5 772.4 100.4 99.8 6.2 15.4 6.3 % 21.4 35.7 71.4 SD 86.4 83.7 13.3 11.4 6.1 9.2 6.0 t −1.18 .71 −.35 −.81 1.65 .38 .93 χ2 0 .86 3.86 d.f.* 40 34 40 40 36 40 32 1 1 p .243 .480 .726 .423 .108 .707 .360 p 1.00 .36 .05 * Some missing data for auditory discrimination tasks: Frequency: N1 = 25, N2 = 13; Duration: N1 = 25, N2 = 11; Speech: N1 = 22, N2 = 12. Topographic maps To gain further insight into the nature of the group differences, 2-D topographic maps were created for the grand average ERP to syllables over an interval encom- passing P1-N2 (48 to 328 ms) for typically developing and SLI children, subdivided according to ICC for syllables at electrode C4. These maps must be treated © 2007 The Authors. Journal compilation © 2007 Blackwell Publishing Ltd. cautiously, as distributions of activation can look very different depending on the amplitude and time ranges that are used, and amplitudes at surface electrodes are not a good indication of the source of underlying activity. Furthermore, the grand average may not be representative of individuals that contribute to it. Never- theless, they may elucidate topographic alterations in activation that are hard to see from the raw waveforms.