ELECTROPHYSIOLOGIC TECHNIQUES IN AUDIOLOGY AND OTOLOGY Human Auditory Steady State Responses: Effects of Intensity and Frequency*

Human auditory steady state responses were recorded in 41 normal subjects and 22 patients with hearing loss. The effect of intensity on the responses at different tonal frequencies was assessed using the sweep technique. The amplitude of the responses increases and the phase delay decreases with increasing intensity. Both the amplitude and the phase delay are smaller for high frequency tone bursts. Notched noise decreases the amplitude of the response by a factor of two but does not affect the phase of the responses. Thresholds were estimated in waking subjects using two techniques: intensity sweeps analyzed by linear regressions, and fixed intensities analyzed by Hotelling’s T2 test. Frequency-specific thresholds obtained with notched noise were less variable and more accurate with the fixed intensity technique. In patients with sensorineural hearing loss the amplitude increased more with increasing intensity above threshold than in patients with conductive hearing loss. The auditory evoked potentials occurring in the approximate latency range of 10 to 50 msec are called “middle latency responses” (MLR) (1). The components of the MLR decrease in amplitude and increase in latency with decreasing intensity (2, 3). In a waking subject the mean threshold for recognizing the MLR is 18.5 dB nHL (range 10 to 40 dB) for 1000 Hz tones (4), and between 10 and 20 dB nHL for 500 Hz tones (5,6). Some investigators (3, 7, 8) have therefore proposed using the MLR to evaluate hearing thresholds. In 198 1, Galambos et a1 (9) reported that the amplitude of the MLR was larger when stimuli were presented at 40/ sec than at lO/sec. The amplitude of a 40 Hz MLR to 500 Hz tones decreases linearly from about 1.6 pV at 90 dB nHL to about 0.3 p V at 20 dB nHL (10). These increased amplitudes could make the response more easily detectable at intensities near subjective thresholds. Table 1 shows the thresholds reported in the literature ‘The research was supported by the Medical Research Council of Canada. Dr. Rosendo Rodriguez was supported by Estado Mayor Presidencial and Servicio Midico-Quirurgico, Mexico City, Mexico. for the human 40 Hz MLR. Kileny and Shea (1 1) evaluated the MLR to 500 Hz tones at stimulus-rates of 9.1 and 40/sec. At 40/sec the thresholds varied from -5 to 9 dB nHL whereas at 9.l/sec the thresholds varied from -3.5 to 11.5 dB nHL. Szyfter et a1 (12) reported that the 40 Hz MLR thresholds in normal subjects were on average 15 dB (range 2.5 to 37.5) higher than those obtained with behavioral audiometry at 250, 500, and 1000 Hz. Sammeth and Barry (13) found that 40 Hz MLR thresholds in a group of normal-hearing subjects were on average 9 dB (range 0 to 30) higher than audiometric thresholds for frequencies between 500 and 2000 Hz and 16 dB at 4000 Hz. In 30 hearing-impaired patients, Dauman et a1 (14) found that 40 Hz MLR thresholds in response to tones of 250,500, and 1000 Hz were on average 12 dB (range 0 to 30) higher than audiometric thresholds. Lynn et a1 (15) compared 40 Hz MLR thresholds and behavioral audiometric thresholds at 500 and 1000 Hz in 40 hearingimpaired ears. They found that the thresholds for 40 Hz MLR were on average 3 dB higher than the audiometric thresholds, with a range from -20 to +35 dB. Sturzebecher et a1 ( 16) found that the 40 Hz response to a 2000 Hz tone was always detectable at 10 dB SL in 56 normal subjects and 10 subjects with a sensorineural hearing loss. When stimuli are presented at high rates, the response to one stimulus may overlap the response to preceding Table 1. Thresholds for the 40 Hz response”