Frequency selectivity: reliability of electrocochleographic measures with iso-intensity masking

Frequency specificity of the human auditory brainstem and middle latency responses to brief tones. II. Derived response analyses

This study investigated the frequency specificity of the auditory brainstem (ABR) and middle latency (MLR) responses to 500- and 2000-Hz brief tones using narrow-band derived response analyses of the responses recorded in high-pass masking noise [Oates and Stapells, J. Acoust. Soc. Am. 102, 3597-3608 (1997)]. Stimuli were linear- and exact-Blackman-gated tones presented at 80 dB ppe SPI. Cochlear contributions to ABR wave V-V' and MLR wave Na-Pa were assessed by response amplitude profiles as a function of derived band center frequency. The largest amplitudes of waves V and Na-Pa occurred in the 500- and 707-Hz derived bands in response to the exact-Blackman- and linear-gated 500-Hz tones. The peak in the response amplitude profiles for wave V to both 2000-Hz stimuli was seen in the 2000-Hz derived band. For wave Na-Pa, the maxima in the amplitude profiles occurred in the 2000- and 1410-Hz derived bands for the exact-Blackman- and linear-gated tones. Smaller cochlear contributions to the ABR/MLR were also present at 0.5-1 octave above and below the nominal stimulus frequencies. The ABR/MLR to 500- and 2000-Hz 80 dB ppe SPL tones thus shows good frequency specificity, with no significant differences in the frequency specificity of: (1) ABR versus MLR; (2) these evoked potentials to 500-versus 2000-Hz tones; and (3) responses to exact-Blackman- versus linear-gated tones.

Pure-tone masking profiles for human auditory brainstem and middle latency responses

Several studies have compared the frequency selectivity of waves I and V of the auditory brainstem response (ABR) in humans, however little is known about the frequency selectivity of the middle latency response (MLR). Simultaneous recordings of ABRs and MLRs to 60 dB peSPL 2000-Hz probe tones were obtained in the presence of 0.5, 1.0, 1.41, 2.0, 2.83 and 4.0 kHz maskers presented at 40, 60, and 80 dB SPL. ABR/MLR iso-intensity masking profiles showing the percentage of the unmasked amplitudes as a function of frequency were constructed for ABR peak V-Vn and MLR peaks Na-Pa and Nb-Pb at each masker intensity. No significant differences were found between the frequency selectivity of the ABR and MLR, and the effects of masking on the amplitudes of these responses were similar. These results are consistent with the suggestion that frequency tuning is similar up to the level of the primary auditory cortex.

Auditory frequency selectivity and tinnitus

In the majority of cases tinnitus has a high pitch and can be matched rather precisely to an objective high frequency sound. However, it is well known that tinnitus can be masked by different tones without any frequency selectivity. We wondered whether objective tones matched to tinnitus would show a more significant deterioration of frequency selectivity than could be expected from any associated hearing loss. Psychoacoustic tuning curves were obtained using simultaneous pure-tone masking and revealed a broadening of frequency selectivity. In cases of comparable bilateral hearing losses with unilateral tinnitus, the broadening associated with the tinnitus was particularly conspicuous. This broadening strongly suggests that tinnitus originates in the cochlea, although some involvement of central pathways may also occur. Tinnitus-induced broadening of frequency selectivity as a possible sign of cochlear origin deserves further consideration in future studies.