Clinical evaluation of 40-Hz middle-latency responses in adults: frequency specific threshold estimation and...

40-Hz responses were evaluated for objective threshold estimation and frequency specificity at 500, 1,000, 2,000, and 4,000 Hz and compared to normal middle-latency response (MLR) recordings. Disturbing influences were examined. Stimulus-intensity-dependent amplitude characteristics were investigated. 30 adults with normal hearing and 45 with sensorineural hearing loss were included. On the whole, 40-Hz responses gave better threshold estimation than MLR as compared with pure-tone audiograms. In about 80%, deviation was less than 10 dB. For both the 40-Hz method and the MLR, myogenic potentials and changes in the level of vigilance are disturbing influences which cause amplitude variations. Amplitude-intensity functions can be described and their possible contribution to an objective identification of recruitment was examined. Additionally, an improvement of threshold estimation was achieved by narrow-band filtering of the response curve around 40 Hz.