Abnormal speaking-induced suppression in Alzheimer’s disease
We first analyzed group differences in peak amplitude for the speaking and listening conditions (Fig. 2C − F). While no overall main effect of GROUP was observed in the peak amplitude, significant interactions with GROUP were observed with peak component (GROUP x PEAK, F(4,212.8) = 16.999, P < 0.001), hemisphere (GROUP x HEMI, F(2,295.8) = 7.137, P < 0.001), and speaking/listening condition (GROUP x COND, F(2,295.8) = 4.247, P = 0.015), as well as a 3-way interaction with condition and peak component (GROUP x PEAK x COND, F(4,212.8) = 3.610, P = 0.007).
Further analyses within each of the three component peaks revealed that amplitude was, across condition, not statistically different between the two participant groups (GROUP; for M50, F(1,29.6) = 0.437, P = 0.514; for M100, F(1,33.4) = 0.662, P = 0.422; for M200, F(1,31.0) = 0.288, P = 0.595). However, when separating speaking (GROUP; for M50, F(1,32.8) = 0.739, P = 0.396; for M100, F(1,19.1) = 6.617, P = 0.019; for M200, F(1,29.0) = 0.100, P = 0.754) and listening (GROUP; for M50, F(1,28.0) = 0.030, P = 0.864; for M100, F(1,27.5) = 0.263, P = 0.612; for M200, F(1,28.8) = 0.378, P = 0.544) conditions, we observed significant group effects for M100 amplitude during speaking for the patient group as compared to healthy participants. No substantial M100 amplitude reduction during speaking is apparent in AD patients (Fig. 1A and B; Fig. 2C and D). We directly compare this effect at the M100 peak component using the SIS measure below.
To determine whether SIS amplitudes in AD differ from SIS amplitudes in healthy controls, the amplitude difference in auditory cortical responses was analyzed using SIS. Consistent with the findings above, a group difference in SIS was found across the three peaks of M50, M100, and M200 (GROUP x PEAK, F(4,111.8) = 4.245, P = 0.003), again with no hemispheric difference between AD patients and healthy controls (GROUP x HEMI, F(2,139.3) = 1.189, P = 0.308). Post hoc tests confirmed that reduced SIS for AD patients relative to healthy age-matched participants occurred specifically at the M100 (GROUP; for M50, F(1,30.0) = 0.326, P = 0.573; for M100, F(1,57.5) = 6.849, P = 0.011; for M200, F(1,30.0) = 0.259, P = 0.615). In healthy controls, SIS was extant at M100 in both hemispheres (Fig. 2A and B; Supplementary Table 1), confirming previous findings that SIS originated primarily from M100 responses [16, 34, 36, 37]. In the left hemisphere, SIS values at M100 were − 0.06 ± 0.10 (mean ± standard error (SE)) for AD patients and 0.41 ± 0.13 (mean ± SE) for healthy controls (Fig. 2A; Supplementary Table 1). Likewise, in the right hemisphere, healthy controls had a substantially higher SIS than AD patients (AD patients, − 0.04 ± 0.12 (mean ± SE); healthy controls, 0.16 ± 0.16 (mean ± SE); Fig. 2B; Supplementary Table 1). A diminished SIS in AD patients appears to be due to the substantial contribution of unsuppressed peak amplitudes at M100 during speaking, rather than the impact of decreased peak amplitudes at M100 during listening (see above).
Peak latency of auditory cortical activity
Group differences in peak latency between AD patients and healthy controls (Fig. 3C − F) varied across the three peaks (GROUP x PEAK, F(4,180.6) = 320.142, P < 0.001). However, this effect was largely due to differences in the within-group latency pattern across peak rather than a between-group difference at particular peaks (GROUP, M50, F(1,106.5) = 1.095, P = 0.298; M100, F(1,30.1) = 1.474, P = 0.234; M200, F(1,38.2) = 0.023, P = 0.879).
To examine the temporal relationship between the placement of peaks during speaking and listening conditions, we subtracted peak latency of the speaking condition from that of the listening condition for each peak component (Supplementary Table 1; Fig. 3A and B). The negative latency differences — occuring across all groups, peak components, and hemispheres — support an overall delay in peak activity during speaking relative to listening condition. Group differences in this temporal delay occurred across peaks (GROUP x PEAK, F(4,96.6) = 6.149, P < 0.001) and, as with un-subtracted latencies, reflected a different within-group pattern across peaks rather than differences between groups at the individual M50, M100, or M200 components.