Authors:

GOCKEL, H.E., CARLYON, R.P., Mehta, A. & Plack, C.J.

Reference:

Year of publication:

2012

CBU number:

7389

Abstract:

The frequency following response (FFR), a scalp-recorded measure of phase-locked brainstem activity, is often assumed to reflect the pitch of sounds as perceived by humans. In two experiments we investigated the characteristics of the FFR evoked by complex tones in the absence of the fundamental frequency component. The FFR was measured in five subjects, using a “vertical” electrode montage (+=Fz, -=C7, ground=mid-forehead). FFR waveforms to alternating-polarity stimuli were averaged for each polarity and added, to enhance envelope, or subtracted, to enhance temporal fine structure information. In experiment 1, frequency-shifted complex tones that are derived from a harmonic complex tone by shifting all harmonics by the same amount in Hertz were presented diotically. In contrast to Greenberg et al (1987), no spectral peak at a frequency corresponding to a possible pitch match was observed. Only the autocorrelation functions (ACFs) of the subtraction-FFR waveforms showed a peak at a delay shifted in the direction of the expected pitch shifts. The expected pitch shifts were also present in the summary ACFs of the output of an auditory-nerve model. In experiment 2, the FFR was measured during the last 350 ms of a 450-ms 3-tone complex with a 244-Hz F0. All components were ramped on together in both ears; some were turned off gradually so that in the last 400 ms the harmonics presented were (i) 2+3+4 to the left ear (“mono”); (ii) 2+4 to the left and 3 to the right (“dichotic”); (iii) 2+4 to the left (“2+4”); (iv) 3 right (“3”). Stimuli “mono” and “dichotic” had the same pitch. Importantly, however, the subtraction and the addition waveforms showed: (i) FFR magnitude spectra for “dichotic” were similar to the sum of the spectra for the two monaural control conditions (“2+4” and “3”), and lacked peaks at the fundamental frequency and other distortion products visible for “mono”; (ii) ACFs for “dichotic” were similar to those for “2+4” and dissimilar to those for “mono”. Overall the results indicate that the neural responses reflected in the FFR preserve a lowpass-filtered version of monaural temporal information that may or may not be important for pitch. However, they provide no evidence for processing over and above that already present in the auditory periphery, and do not reflect the pitch of dichotic stimuli. Supported by the Wellcome Trust Grant 088263.

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