Cochlear Place Specificity of the Auditory Brainstem Response to Narrowband Chirp Versus 2-1-2 Stimuli: High-Pass Noise/Derived Responses

Objective In recent years, many researchers have recommended using narrowband chirp (NBchirp) stimuli for Auditory Brainstem Response (ABR) audiometry instead of more-standard 2-1-2 cycles linear-gated tones, primarily because NBchirps often result in larger ABR wave V amplitudes. However, the acoustic frequency spectra of currently recommended NBchirps are wider than those for 2-1-2 tones, and it is currently not known whether ABRs to these NBchirps have similar (or poorer) cochlear place specificity compared to 2-1-2 tones. The current study used the high-pass noise/derived response technique to assess the cochlear regions contributing to ABRs evoked by NBchirp versus 2-1-2 stimuli. Design A total of 24 adults with normal hearing participated (N=12 for each stimulus frequency). Stimuli were 60-dB peSPL 500- and 2000-Hz NBchirps and 2-1-2 tones mixed with high-pass (HP) filtered masking noise. The level of broadband (pink) noise required to mask the ABR was determined individually, then the broadband noise at this level was HP filtered at ½-octave intervals. Three ABR replications were obtained for each condition, with recordings stopped when the residual noise level of each replication was reduced to 40 nanovolts. Derived responses (DRs) representing 1-octave-wide or ½-octave-wide cochlear regions were calculated by subtracting ABRs recorded in HP noise. Results Non-masked ABR amplitudes in response to NBchirps were significantly larger than those to 2-1-2 stimuli, averaging 55% larger for 500 Hz and 81% larger for 2000 Hz. For both 500- and 2000-Hz stimuli, HP noise masking produced significant amplitude decreases, occurring 1 to ½ octave higher for ABRs to NBchirps versus 2-1-2 tones. One-octave-wide and ½-octave-wide DR amplitude profiles for the ABRs to 2-1-2 tones showed good cochlear place specificity, as described in previous studies. DR results for the NBchirps were similar but showed important differences. The profiles for the 2000-Hz NBchirps showed significantly larger amplitudes in the 4- and 1-kHz DRs compared to the 2-1-2 stimuli. Many more responses were seen 1-octave away for the 2000-Hz NBchirp compared to 2-1-2 tone. DR results for 500-Hz tones showed similar patterns but differences did not quite reach statistical significance, except amplitudes to NBchirps were larger at DR354, DR500 and DR707. A measure of the width of the 1-octave-wide and ½-octave-wide DR amplitude profiles (BW 0.075 , in Hz) showed the 500- and 2000-Hz NBchirp profiles were significantly wider (32% to 77%) compared to those for 2-1-2 stimuli. As the cochlear area able to respond decreased, wave V amplitudes to NBchirp stimuli decreased more than those for 2-1-2 stimuli, with no difference between stimuli for ½-octave-wide responses. Conclusion ABRs to narrowband chirps reflect wider cochlear contributions than those to 2-1-2 tones. Responses to NBchirps arise from cochlear regions as far as one octave away from the stimulus frequency. In contrast, responses to 2-1-2 tones arise from cochlear regions primarily within approximately ±0.5 octaves of the stimulus frequency. Further research in individuals with hearing loss is required to determine whether the wider bandwidths for NBchirps result in threshold mis-estimations, and whether NBchirp amplitude advantages over more-standard stimuli remain with hearing loss.