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Investigation of envelope rate and audio-frequency specific adaptation in the frequency following response (FFR)
Authors:
GOCKEL, H. E., Krugliak, A., Plack, C. J., & CaARLYON, R. P
Reference:
International Journal of Audiology, 53(9), 677-678.
Year of publication:
2014
CBU number:
7859
Abstract:
Using an adaptation paradigm, this study investigated whether the scalp-recorded frequency following response (FFR) in humans would show evidence (i) for neurons tuned to modulation rate (Experiments I+II), as has been previously shown in the inferior colliculus of the macaque using fMRI (Baumann et al, 2011), and (ii) for neurons tuned to audio-frequency (Experiment III), for which there is abundant evidence. A “vertical” montage (+ Fz, - C7, ground = mid-forehead) was used, for which the FFR is assumed to reflect phase-locked neural activity from generators in the rostral brainstem. All experiments compared the effect of different 200-ms adaptors on the FFR for a given 100-ms target, using ten subjects.
In experiments I and II, stimuli were harmonic tone complexes filtered between 3.9 and 5.4 kHz with alternating phase harmonics, and the 75-dB SPL target had an envelope rate of 213 Hz. In experiment I, the 75-dB SPL adaptor had an envelope rate of either 90, 213, or 504 Hz. Experiment II repeated two conditions of experiment I (adaptor rates of 213 and 504 Hz), but with the adaptor level increased to 80 dB SPL. The results showed significant adaptation effects in the spectral magnitude of the 213-Hz envelope-related component of the FFR. However, the identical-rate adaptor did not consistently produce more adaptation of the FFR to the target than the different-rate adaptors. Hence, the results showed no evidence for modulation-rate specific adaptation, i.e. adaptation of different neural populations that are tuned to different modulation rates, in the strength of the FFR.
Experiment III used unmodulated sinusoids. The target and adaptor frequencies were each either 213 Hz (80-dB SPL presentation level) or 504 Hz (75-dB SPL presentation level). The identical-frequency adaptor did not consistently produce more adaptation of the FFR to the target than the different-frequency adaptor. Instead, the high-frequency adaptor was generally more effective than the low-frequency adaptor. The latter finding is consistent with Dau’s (2003) suggestion that the FFR mainly stems from the activity of neurons on the upper skirt of the excitation pattern and not from neurons tuned to frequencies around the signal frequency.