Differences between psychoacoustic and frequency following response measures of distortion tone level and masking

Abstract

The scalp-recorded frequency following response (FFR) in humans was measured for a 244-Hz pure tone at a range of input levels and for complex tones containing harmonics 2-4 of a 300-Hz fundamental, but shifted by ±56 Hz. The effective magnitude of the cubic difference tone (CDT) and the quadratic difference tone (QDT, at F(2)-F(1)) in the FFR for the complex was estimated by comparing the magnitude spectrum of the FFR at the distortion product (DP) frequency with that for the pure tone. The effective DP levels in the FFR were higher than those commonly estimated in psychophysical experiments, indicating contributions to the DP in the FFR in addition to the audible propagated component. A low-frequency narrowband noise masker reduced the magnitude of FFR responses to the CDT but also to primary components over a wide range of frequencies. The results indicate that audible DPs may contribute very little to the DPs observed in the FFR and that using a narrowband noise for the purpose of masking audible DPs can have undesired effects on the FFR over a wide frequency range. The results are consistent with the notion that broadly tuned mechanisms central to the auditory nerve strongly influence the FFR.

Fig. 1

(Color online) Magnitude spectra of FFRs averaged across six subjects with FFRs for the two polarities added (dashed line) or subtracted (solid line). Panels (a) and (b) show the FFR spectra for a frequency-shifted complex tone, presented at 75 dB SPL, for which harmonics 2+3+4 of a 300-Hz F0 were shifted down by 56 Hz in the absence and presence of a narrowband noise (140–440 Hz, presented at 85 dB SPL), respectively. Panels (c) and (d) are as (a) and (b), but with the harmonics shifted up by 56 Hz and a noise in the range of 160–460 Hz. Panels (e) and (f) show the FFR spectra for a 244-Hz 75-dB SPL pure tone in the absence and presence, respectively, of a narrowband noise (84–404 Hz) presented at 90 dB SPL.

Fig. 2

Top (a): Peak height (in dB) at the QDT frequency in the averaged spectra of the FFR waveforms for the two polarities minus the peak height at the QDT frequency in the spectrum of the addition waveform. Bottom (b): Peak height (in dB) in the FFR at the CDT frequency in the averaged spectra of the FFR waveforms for the two polarities minus the peak height at CDT in the spectrum of the subtraction waveform. The frequency-shifted complex was presented at 75 dB SPL. The 300-Hz wide noise had an rms level of 85 dB SPL, and was centered on 290 and 310 Hz for the downward- and upward-shifted complex tones, respectively.

Fig. 3

Mean peak magnitude at 244 Hz (and corresponding standard error) in the spectrum of the subtraction waveform of the FFR for the 75-dB-SPL pure tone condition in the absence and in the presence of a narrowband noise (84–404 Hz) presented at levels of 86 and 90 dB SPL.

Fig. 4

Mean reduction of peak magnitudes in the FFR, due to the addition of a narrowband noise, at the QDT frequency (spectra of addition waveforms), the frequencies of the CDT, and the primary components (spectra of subtraction waveforms), averaged across conditions with the downward- and the upward-shifted complex. Complex tones were presented at 75 dB SPL. The 300-Hz wide noise had an rms level of 85 dB SPL and was centered on 290 and 310 Hz for the downward- and upward-shifted complex tones, respectively.

Fig. 5

Peak magnitudes at 244 Hz (spectra of subtraction FFRs) for individual subjects for the 244-Hz pure tone presented without a masker, as a function of level. Downward-pointing arrows indicate cases in which the peak value for the time interval when the tone was presented did not exceed the baseline. In these cases, the corresponding (empty) symbol gives the baseline value.

Fig. 6

Excitation patterns for the stimuli in the complex tone conditions calculated using the model of Moore et al. (1997). It was assumed that the sound delivery system had a flat response at the eardrum. Excitation patterns are shown for the complex tone alone (short-dashed line), the narrowbandnoise masker alone (long-dashed line), and the two together (solid line). Excitation patterns for the downward- and upward-shifted complexes are shown at the top (a) and bottom (b), respectively.