Cognitive and Academic Testing ()
Although in the normal range, both full scale and verbal mental ability scores from the WASI were significantly lower in the children with ASD (t(37) = 2.42, p = .021, d = .82 and (t(37) = 3.11, p = .004, d = 1.01, respectively), whereas performance mental ability did not differ between groups (t(37) = 1.09, p = .282, d = .35). On the CELF test, children with ASD scored significantly lower on indices of core and receptive language ability (t(37) = 2.77, p = .01, d = .87 and t(37) = 3.58, p = .001, d = 1.12, respectively), but not expressive language ability (t(37) = 1.80, p = .08, d = .59).
Click-evoked brainstem responses
Wave V latency ranged from 5.15-5.90 ms (M
= 5.56 ms, SD
= .178), consistent with the previously reported normal range (Gorga et al., 1985
; Hood, 1998
; Jacobson, 1985
). Wave V latency did not differ between groups (t
(37) = 1.46, p
= .149, d
Speech-evoked response fidelity
The brainstem response to /da/ () consists of 7 transient response peaks (V-O). In the quiet condition, waves were identifiable 100% of the time in the TD group; however 6 children with ASD were missing a wave, specifically in the FFR region, although not always the same wave. Because onset response components are known to become abolished or diminished by background noise even in normal subjects (Russo et al., 2004
), these measures were omitted from analyses. Analyses of responses in background noise were restricted to those waves (F and O) which were reliably present in background noise (~90% typical responses). Nevertheless, all sustained response measures (RMS, FFT, and correlations) were evaluated. Despite normal click-evoked responses, the ASD group showed pervasive deficits transcribing speech in quiet and background noise, as was evident in both the onset and FFR portions of the response. Significant group differences are described below. Due to the large number of dependent variables, only the means and standard deviations of measures that were significantly different between groups are reported in .
Significant speech-evoked auditory brainstem response measures
Transcription of phonetic (filter) aspects of speech
Wave V and A latencies were significantly delayed in children with ASD (wave V: F(1,36) = 4.45, p = .042, d = .87; wave A: t(37) = 3.45, p = .001, d = 1.18) (). Onset response duration was also significantly prolonged in the ASD group (F(1,36) =4.67, p = .037, d = .75).
Comparison of grand average onset responses to /da/ in quiet in TD children (n=18; black line) and children with ASD (n=21; gray line)
Transcription of fundamental frequency (source) aspects of speech
Quiet response wave D and F latencies were significantly delayed in children with ASD (wave D: t(37) = 2.47, p = .018, d = .81; wave F: t(37) = 2.62, p = .013, d = .87), and wave F amplitude showed a trend toward being reduced (t(37) = 1.91, p = .064, d = .61). Wave F amplitude in background noise was smaller in children with ASD (t(37) = 2.14, p = .039, d = .70). These differences are shown in . In the frequency domain, there were no between group differences in F0 magnitude.
Comparison of grand average frequency-following responses to /da/ in quiet in TD children (black line) and children with ASD (gray line)
Neural synchrony in background noise: the sustained response
Stimulus-to-response-in-background-noise correlations over the FFR (13-34 ms) were lower (t(37) = 2.41, p = .021, d = .78) and the maximum correlation occurred at a shorter lag (t(37) = 2.03, p = .050, d = .66) in the ASD group. Quiet-to-noise inter-response correlations over the entire response (0-49 ms) and restricted to the FFR range (11-40 ms) were also significantly lower in the ASD group (p < .018, d > .80, both ranges) indicating poorer response fidelity in the ASD group. These findings are consistent with excessive response degradation by background noise in the ASD group relative to the TD controls.
Overall, subcortical transcription of sound was pervasively disrupted in ASD children as was evidenced by both phonetic/filter (delayed onset) and F0/source deficits (delayed FFR waves D and F and smaller amplitude of wave F).
Relationship between neurophysiology and behavior
Individual measures Quiet
Performance mental ability was related to wave C amplitude (r(37) = .38, p = .018) while all scores of mental ability (performance, verbal and full scale) were also related to offset wave O amplitude (performance and verbal: r(37) = .35, p = .028; full scale: r(37) = .38, p = .018), such that better scores on tests of mental ability were indicative of larger amplitudes of these transient responses.
Higher performance and full scale mental ability scores were associated with earlier wave F latency in background noise (r(37) = .35, p = .04 and r(37) = .38, p = .22, respectively). Higher core and receptive language indices were associated with greater quiet-to-noise inter-response correlations (11-40 ms range; r(37) = .36, p = .025 and r(37) = .35, p = .027, respectively).
Based on the variables that differed significantly between groups, we computed four composite scores: onset synchrony in quiet (waves V and A latencies and VA duration), transient responses in quiet (waves V, A, D and F latencies, and VA duration), phase locking in quiet (waves D and F latencies), and neural synchrony in noise (wave F amplitude, stimulus-to-response-in-background-noise correlations and lag, and quiet-to-noise inter-response correlations). Composite scores were significantly worse in children with ASD compared to TD children on all measures (onset synchrony in quiet: t(37) = 3.59, p = .001, d = 1.13; transient responses in quiet: t(37) = 3.92, p < 0.001, d =1.23; phase locking in quiet: t(37) = 3.26, p = .003, d = 1.03; neural synchrony in noise: t(37) = 3.21, p = .003, d = 1.04).
Of the composite scores, neural synchrony in noise was the only variable that correlated significantly with behavior. Higher core and receptive language indices were indicative of better resilience in background noise (i.e., greater neural synchrony score) (r(37) = .53, p < .001 and r(37) = .36, p = .02, respectively).