shows the mean and range of accepted deviant and standard trials for each group. In general, ASD subjects had slightly noisier data, resulting in fewer accepted epochs in the ASD than TD group for both standard and deviant stimuli. Although the TD group had more accepted trials than the ASD group, examination of shows that the difference between groups was small and, as such, it is unlikely that the MMF findings are due to group differences in trial numbers (i.e., the number of accepted trials between groups is not sufficient to result in significant between-group signal-to-noise ratios).
Means and ranges of accepted trial data for each stimulus.
Of the accepted trials the MMF was computed as the difference between each stimulus presented as a standard and as a deviant. In the tone condition, a viable MMF was scored 44% and 61% of the time in the ASD/-LI and ASD/+LI groups, and 65% of the time in the TD group. For vowels, the MMF was accepted 61% and 75% of the time in ASD/-LI and ASD/+LI groups, and 71% of the time in the TD group.
The age-covaried Group X Hemisphere X Frequency X Stimulus linear mixed model showed a significant main effect of Group on MMF latency, F(2,301), p<0.001. Simple-effects analyses revealed MMF latency differences between all groups: TD versus ASD/-LI (p<0.001), TD versus ASD/+LI (p<0.001), and ASD/-LI versus ASD/+LI (p<0.01). Examining effect sizes, Cohen's d for each pairwise comparison were: ASD/-LI vs TD=1.89; ASD/+LI vs TD=3.11; ASD/+LI vs ASD/-LI=1.37. shows the grand average of tone MMF waveforms for the three groups (collapsing across hemisphere and frequency).
Figure 1 Grand-averaged source difference waveforms for 300 and 700 Hz tone stimuli collapsed across hemispheres and frequencies. Mismatch activity was present in both hemispheres 170 to 300 ms following stimulus onset and clearly shows prolongation in ASD subgroups. (more ...)
shows MMF latencies for each group, collapsing across Hemisphere, Stimulus, and Frequency (the group age-corrected means were: ASD/+LI 228.73±5.82ms; ASD/-LI: 208.68±3.26ms; TD: 177.27±2.72ms). No other MMF latency main effects or interactions were observed. Re-running linear mixed model analyses with perceptual reasoning index (PRI) as a covariate (reflecting non-verbal IQ), the simple effects group findings remained unchanged with significant (p<0.001) differences between all pairwise TD, ASD/-LI and ASD/+LI contrasts.
Figure 2 Age-corrected mean MMF latencies (with standard error bars) for each group. Latency is prolonged in ASD/-LI and in ASD/+LI compared with typically developing peers (TD: 177.27±2.72ms; ASD/-LI: 208.68±3.26ms; ASD/+LI: 228.73±5.82ms, (more ...)
Given the unequal ratio of males to females between groups, additional linear mixed model analysis of MMF latency was conducted for the TD group. In particular, to assess an effect of gender on MMF latency in the TD group, a Type III fixed-effects model was fitted to the data for within-subject factors GENDER, HEMISPHERE, and FREQUENCY, with AGE as a covariate. GENDER did not account for changes in latency measurements, F(1,111) = 1.29, p =.26. In addition, GENDER did not interact with other factors to modulate mismatch latency, p's >.39.
Although indicated by the lack of any Group interaction terms, lower-level analyses confirmed that the main effect of Group on MMF latency was observed when examining both the high (700Hz and /a/) and the low (300Hz and /u/) frequency stimuli. In addition, the main effect of Group on MMF latency was also observed when examining both tone (300Hz, 700Hz) and vowel stimuli (/u/, /a/). Comparing tone and vowel stimuli, there was a slight (<10ms) MMF latency prolongation for vowel as compared to tone stimuli; this small latency difference is likely due to vowel stimuli onset characteristics (vowel onsets ramped 10ms slower than pure tones).
No main effects or interactions were observed for MMF amplitude. Across Hemisphere, Frequency, and Stimulus, the mean MMF amplitudes in each group were: TD =16.99±1.02nAm; ASD/-LI =16.36±1.05nAm; ASD/+LI = 18.44±2.14nAm.
Finally, as shown in , receiver operator characteristic (ROC) analysis of the mean MMF latency as a predictor of LI found significant area under the curve, AUC = 0.86, p<0.001, with a sensitivity of 82.4% and a specificity of 71.2%.
Receiver operator characteristic (ROC) analysis of the mean MMF latency as a predictor of LI found significant area under the curve, AUC = 0.86, p<0.001, with a sensitivity of 82.4% and a specificity of 71.2%.