Two-tone suppression in the basilar membrane of the cochlea: mechanical basis of auditory-nerve rate suppression

Abstract

1. The vibratory response to two-tone stimuli was measured in the basilar membrane of the chinchilla cochlea by means of the Mössbauer technique or laser velocimetry. Measurements were made at sites with characteristic frequency (CF, the frequency at which an auditory structure is most sensitive) of 7-10 kHz, located approximately 3.5 mm from the oval window. 2. Two-tone suppression (reduction in the response to one tone due to the presence of another) was demonstrated for CF probe tones and suppressor tones with frequencies both higher and lower than CF, at moderately low stimulus levels, including probe-suppressor combinations for which responses to the suppressor were lower than responses to the probe tone alone. 3. For a fixed suppressor tone, suppression magnitude decreased as a function of increasing probe intensity. 4. The magnitude of suppression increased monotonically with suppressor intensity. 5. The rate of growth of suppression magnitude with suppressor intensity was higher for suppressors in the region below CF than for those in the region above CF. 6. For low-frequency suppressor tones, suppression magnitude varied periodically, attaining one or two maxima within each period of the suppressor tone. 7. Suppression was frequency tuned: for either above-CF or below-CF suppressor tones, suppression magnitude reached a maximum for probe frequencies near CF. 8. Cochlear damage or death diminished or abolished suppression. There was a clear positive correlation between magnitude of suppression and basilar-membrane sensitivity for responses to CF tones. 9. Suppression tended to be accompanied by small phase lags in responses to CF probe tones. 10. Because all of the features of two-tone suppression at the basilar membrane match qualitatively (and, generally, also quantitatively) the features of two-tone rate suppression in auditory-nerve fibers, it is concluded that neural two-tone rate suppression originates in mechanical phenomena at the basilar membrane. 11. Because the lability of mechanical suppression parallels the loss of sensitivity and frequency tuning due to outer hair cell dysfunction, the present findings suggest that mechanical two-tone suppression arises from an interaction between the outer hair cells and the basilar membrane.

The Role of Suppression in the Upward Spread of Masking

“…An estimate of 0.17 is comparable to the compression exponent of 0.16 derived from GOM functions with high-pass noise (Oxenham and Plack 1997;Nelson et al 2001). These higher compression estimates with notched or high-pass noise are consistent with the higher compression estimates from animal data (Sellick et al 1982;Yates et al 1990;Ruggero et al 1992). The average compression exponent estimate of 0.26 is comparable to the compression exponent of 0.3 from BM I/O functions derived from forward-masking thresholds with notched noise (Yasin and Plack 2003).…”

“…The results show great intersubject variation in the estimated suppression, as has previously been noted for both human (e.g., Oxenham and Plack 1998) and animal (e.g., Ruggero et al 1992) data. The fact that this variability is also found in the physiology suggests that it may be a consequence of differences in the cochlear response itself, rather than a consequence of measurement variability.…”

“…Some (Ruggero and Rich 1991;Ruggero et al 1992;Oxenham and Plack 1997;Gregan et al 1998;Bacon et al 1999;Nelson et al 2001;Yasin and Plack 2003), although not all (Bacon and Viemeister 1985;Murugasu and Russell 1995;Ruggero et al 1997) physiological and psychophysical studies show that the slope of the I/O function or GOM function returns to a value of 1 dB/dB for high-level signals. The implication being that for high-level signals the BM response to both the signal and masker may grow linearly at the signal place, as is the case for low-level signals (Delgutte 1990a;Yates et al 1990).…”

“…The characteristics of cochlear nonlinearity can be measured in the response of the BM (Rhode 1971(Rhode , 1977Sellick et al 1982;Robles et al 1986;Ruggero et al 1992;Rhode and Recio 2001). The active mechanism is responsible for the compressive growth of the BM response for a mid-level signal with a frequency close to the characteristic frequency (CF) of the BM place of recording (Ruggero et al 1997).…”

“…Although nonlinear growth of suppression may be the major cause of USM for low-level signals (Delgutte 1990a), physiological (Robles et al 1987;Ruggero et al 1992) and psychophysical (Yasin and Plack 2003) studies suggest that for mid-level signals suppression may reduce the USM by increasing the slope of the BM I/O function and hence decreasing the compression in the signal response. Because suppression requires simultaneous presentation of the suppressor and suppressee (Arthur et al 1971;Houtgast 1972Houtgast , 1974Fastl and Bechly 1983), simultaneous-masking experiments may underestimate the amount of compression that occurs in the absence of a suppressor (Stelmachowicz et al 1987).…”