The use of high stimulus rate auditory brainstem responses in the estimation of hearing threshold.

This normative study investigates the efficiency of using the maximum length sequence (MLS) technique applied to auditory brainstem evoked response (ABR) testing to estimate hearing thresholds. Using a commercially available system, ABRs were recorded in sixteen subjects at two conventional rates--9.1 and 33.3 clicks/s--and six MLS rates between 88.8 and 1000 clicks/s. Each subject was tested at five stimulus levels from 60 down to 10 dBnHL. The wave JV amplitude input-output (I/O) functions, relative signal to noise ratio (SNR) and speed of test were calculated for all conditions. The JV amplitude and detectability decrease as the stimulus rate increases and level decreases. The latency of JV increases as the stimulus rate increases and the intensity decreases. While the slope of the amplitude I/O function was maximal at 200 clicks/s, at 300 clicks/s it was comparable with that obtained at conventional rates. At higher rates, the slope of the I/O function decreases. When compared with the conventional recording rate of 33.3 clicks/s there is a small improvement in SNR for MLS rates between 200 and 600 clicks/s at levels above 30 dBnHL. The calculated speed improvement at 300 clicks/s is a factor between 1.4 to 1.6 at a screening level of 30-40 dBnHL. It is felt therefore that there may be a small advantage to using MLS in screening and that the optimal rate for this lies at around 200 to 300 clicks/s. However even at these rates, as a consequence of the adaptation of the response with both rate and level, the improvement in SNR or speed of test would be modest when estimating threshold.