Fig. 26.1
Mean FFR spectral magnitude at 504 Hz and the corresponding standard errors across ten subjects. Panel a shows the data for the no-gap condition. The three left-hand groups of two bars show the FFR strength during three time ranges of the stimulation with the adaptor: 12–62 ms after onset (A–start), 62–112 ms after onset (A–mid), and 150–200 ms after onset (A–end). The two right-hand groups of two bars show the FFR strength during two time ranges of the stimulation with the target: 12–62 ms after onset (T–start) and 50–100 ms after onset (T–end). The black and the white bars show results when the adaptor led in the left and right ears, respectively. Panel b is as (a), but for the with-gap condition. n = p > 0.05; * = p < 0.05; ** = p < 0.01; *** = p < 0.001
To assess whether there was ITD-specific adaptation, the FFR strength during period T-start was compared across the conditions where the adaptor had the same ITD as the target (LL) and where it did not (RL). A paired-sample t-test showed no significant difference between the FFR strength for these two conditions [second group of two bars from the right; t(9) = 1.11, p = 0.30]. Thus, although the FFR response to the target was reduced due to the preceding adaptor, there was no evidence that this adaptation depended on whether the ITD of the adaptor matched that of the target.
Consider next the with-gap conditions (panel (b), bottom). Again, adaptation over time was visible in the FFR. However, the short silent gap between the adaptor and the target largely abolished the reduction in FFR strength during period T-start. The results of the ANOVAs were as follows. First, the FFR strength was significantly smaller during period A-mid than during period A-start [F(1,9) = 28.67, p < 0.001]. Second, the FFR strength was significantly smaller during period A-end than during period A-mid [F(1,9) = 5.57, p < 0.05]. Third, the FFR strength was significantly larger during period T-start than during period A-end [F(1,9) = 7.47, p < 0.05]. The effect of time period indicates significant recovery from adaptation. Fourth, the FFR strength was not significantly larger during A-start than during T-start [F(1,9) = 2.13, p > 0.18]. In all of these ANOVAs, the effect of adaptor ITD and its interaction with time period were both nonsignificant. Thus, while there was significant adaptation over the course of the adaptor, the larger FFR at T-start than at A-end meant that a silent gap of only ten cycles duration was sufficient to allow significant recovery from adaptation. In addition, a paired-sample t-test comparing FFR strength during T-start across conditions LL and LR showed no significant difference [t(9) = −0.26, p = 0.80]. This was not entirely surprising, as the short silent gap before T-start allowed significant recovery from adaption.
4 Discussion and Conclusion
The FFR strength in response to the left-leading target was not smaller when it was preceded by the left-leading adaptor than when preceded by the right-leading adaptor. Thus, the observed adaptation effect of the adaptor on the response to the target could have been dominated by the adaptation of monaural components in the auditory pathway. Several (not mutually exclusive) factors may contribute to this finding.
1.
Not all neurons in the IC, and not all neurons that contribute to the FFR, are ITD sensitive. The responses of such neurons would dilute a differential adaptation effect of the two adaptors.
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