Summary A forward masking technique was used to measure cochlear gain reduction which might be consistent with the medial olivocochlear reflex (MOCR). A 4-kHz signal was set at 20 dB SL, and an on-frequency forward masker adjusted to just mask the signal. Adding a pink noise precursor before the signal and masker increased the level of the masker needed to mask the signal, in contrast to what would be expected from theories such as additivity of masking. The magnitude and pattern of this increase was similar to the increase in signal threshold seen with an off-frequency masker following a precursor.

Auditory two-tone suppression is a nearly instantaneous reduction in the response of the basilar membrane to a tone or noise when a second tone or noise is presented simultaneously. Previous behavioural studies provide conflicting evidence on whether suppression changes with increasing age, and aging effects may depend on whether a suppressor above (high-side) or below (low-side) the signal frequency is used. Most previous studies have measured suppression using stimuli long enough to elicit the medial olivocochlear reflex (MOCR), a soundelicited reflex that reduces cochlear amplification or gain. It has a "sluggish" onset of approximately 25 ms. There is physiological evidence that suppression may be reduced or altered by elicitation of the MOCR. In the present study, suppression was measured behaviourally in younger adults and older adults using a forward-masking paradigm with 20-ms and 70-ms maskers and suppressors. In experiment 1, gain was estimated by comparing on-frequency (2 kHz) and off-frequency (1.2 kHz) masker thresholds for a short, fixed-level 2-kHz signal. In experiment 2, the fixedlevel signal was preceded by an off-frequency suppressor (1.2 or 2.4 kHz) presented simultaneously with the on-frequency masker. A suppressor level was chosen that did not produce any forward masking of the signal. Suppression was measured as the difference in on-frequency masker threshold with and without the suppressor present. The effects of age on gain and suppression estimates will be discussed.

Two-tone suppression reduces gain in the cochlea nearly instantaneously in a frequency-dependent manner. In speech perception, suppression may enhance spectral contrasts between regions of higher and lower energy. A few previous studies have investigated the effects of aging on suppression and correlations with speech perception in noise, but none have looked at the adaptability of suppression with preceding stimulation. In the present study, estimates of two-tone suppression and consonant perception were measured in younger and older adults. Suppression was measured with short tonal stimuli with and without preceding stimulation. Consonant vowel (CV) stimuli consisted of combinations of the consonants /b, g, d/ and the vowels /ɑ, i, u/ spoken by three male speakers. Broadband noise was filtered to match the long-term average spectrum of the speech stimuli used. CV onset was either 0 or 70 ms after onset of the noise. Participants were asked to identify the consonant by choosing /b/, /g/ or /d/. Percentage correct was calculated for CVs in quiet and in + 5, 0, −5, and −10 dB signal-to-noise ratio conditions. Suppression and adaptability were found to correlate with several aspects of speech perception in noise. Age-related results and implications will be discussed. [Research supported by NIH(NIDCD)F31 DC014395.]

Despite clinically normal audiometric thresholds, some older adults may experience difficulty in tasks such as understanding speech in a noisy environment. One potential reason may be reduced cochlear nonlinearity. A sensitive measure of cochlear nonlinearity is two-tone suppression, which is a reduction in the auditory system's response to one tone in the presence of a second tone. Previous research has been mixed on whether suppression decreases with age in humans. Studies of efferent cochlear gain reduction also suggest that stimulus duration should be considered in measuring suppression. In the present study, suppression was first measured psychoacoustically using stimuli that were too short to result in gain reduction. The potential effect of efferent cochlear gain reduction was then measured by using longer stimuli and presenting tonal or noise precursors before the shorter stimuli. Younger adults (ages 19-22 yr) and older adults (ages 57þ yr) with clinically normal hearing were tested. Suppression estimates decreased with longer stimuli or preceding sound which included the signal frequency, but did not decrease with preceding sound at the suppressor frequency. On average, the older group had lower suppression than the younger group, but this difference was not statistically significant. V

Psychophysical studies have shown that the ability to detect a signal in a masker may be improved by presenting a preceding sound (precursor) identical to the masker, an effect called overshoot. When the masker has no spectral notch around the signal frequency, overshoot has been predicted using a physiologically-realistic auditory nerve (AN) model in which gain is reduced by the precursor. This may simulate the medial olivocochlear reflex (MOCR). When there is a spectral notch in the masker at the signal frequency, the resulting improvement in signal detection is sometimes called enhancement. It has been suggested that enhancement may be due to adaptation of suppression, which could be related to gain reduction in the cochlea, or to adaptation of inhibition, which could occur more centrally. Physiological studies of the AN have shown either no adaptation of suppression [Palmer et al., J. Acoust. Soc. Am. 97, 1786-1799] or a reduction in suppression when the MOCR was elicited [Kawase et al., J. Neurophys. 70, 2533-2549]. The purpose of the present experiment is to use the AN model to investigate the relationship between gain reduction and suppression with stimuli used in previous studies of enhancement. [Research supported by NIH(NIDCD) R01 DC008327]