Rapid measurement of auditory filter shape in mice using the auditory brainstem response and notched noise

Abstract

The notched noise method is an effective procedure for measuring frequency resolution and auditory filter shapes in both human and animal models of hearing. Briefly, auditory filter shape and bandwidth estimates are derived from masked thresholds for tones presented in noise containing widening spectral notches. As the spectral notch widens, increasingly less of the noise falls within the auditory filter and the tone becomes more detectible until the notch width exceeds the filter bandwidth. Behavioral procedures have been used for the derivation of notched noise auditory filter shapes in mice; however, the time and effort needed to train and test animals on these tasks renders a constraint on the widespread application of this testing method. As an alternative procedure, we combined relatively non-invasive auditory brainstem response (ABR) measurements and the notched noise method to estimate auditory filters in normal-hearing mice at center frequencies of 8, 11.2, and 16 kHz. A complete set of simultaneous masked thresholds for a particular tone frequency were obtained in about an hour. ABR-derived filter bandwidths broadened with increasing frequency, consistent with previous studies. The ABR notched noise procedure provides a fast alternative to estimating frequency selectivity in mice that is well-suited to high through-put or time-sensitive screening.

Fig. 1

Schematic of the notched noise method of estimating auditory filter bandwidth. Thresholds are measured for a probe tone presented in the presence of broadband noise containing a spectral notch centered on the probe tone frequency. The shaded portions of the figure represent the noise energy falling inside the filter that masks the probe tone frequency. As the notch is widened, less and less energy falls within the auditory filter rendering the tone more detectible.

Fig. 2

(A) Representative ABR waveform in response to an 80 dB click stimulus. The ABR peaks occur approximately 2–10 ms after stimulus onset. The magnitude of the ABR was determined by calculating the maximum peak-to-peak amplitude. Baseline noise levels are sampled during the end of the recording epoch when no tone stimulus is being presented. (B) ABR threshold calculation method. Closed circles represent the ABR magnitude at each stimulus level. The dashed line represents two standard deviations (SD) above the average baseline noise level. Open circles represent the average baseline noise level for each averaged response. Threshold is defined as the point at which the ABR magnitude exceeds the baseline noise magnitude by 2 SD. (C) Example ABR waveforms in response to a click presented at descending levels.

Fig. 3

(A) Averaged ABR masked threshold data comparing CBA/CaJ mice at 8 kHz, (B) 11.2 kHz, and 16 kHz probe frequencies. Notch widths of 0, 0.125, 0.25, 0.5, 1 and 2 octaves correspond to normalized deviations of 0, 0.04, 0.083, 0.159, 0.293 and 0.5, respectively. (B) Estimated roex filter functions plotted against the interpolated averaged masked ABR threshold functions at 8, 11.2 and 16 kHz center frequencies.

Fig. 4

Equivalent rectangular bandwidths (ERB) for both ABR-derived and behaviorally-derived filters (May et al., 2006) compared with human ERB measures as described by Moore and Glasberg (1983) and Glasberg and Moore (1990).