The effect of noise bandwidth on the contralateral suppression of transient evoked otoacoustic emissions

Abstract

The acoustic reflex system (AR) and the influence of the olivocochlear bundle (OCB) affect the ear's response to sound. Both systems are controlled by efferent neurons that originate in the superior olivary complex (SOC). The AR system affects middle ear function through its action on the stapedius muscle, and the OCB affects cochlear function through its action on outer hair cells (OHC). The AR is sensitive to the loudness of a stimulus; that is to say, changes in sound pressure level (SPL) and bandwidth of a stimulus affect the threshold and magnitude of stapedius muscle contraction. Investigators have shown that the magnitude of the OCB effect is influenced by changes in SPL. Transient evoked otoacoustic emissions (TEOAEs) were collected from 10 women in quiet and in the presence of three different contralateral noise bands centered at 2000 Hz. The NB (100 Hz bandwidth) and WB (2200 Hz bandwidth) noises were at 60 dB SPL. The SPL of the EQ (100 Hz bandwidth) noise was adjusted such that it was equal in loudness to the WB noise. The results of the present study indicate that an increase in loudness as associated by an increase in noise bandwidth affects the contralateral suppression of TEOAEs. Only the WB noise was associated with a reduction of TEOAE amplitudes. It is believed that this effect results because the WB noise has greater effective energy representation across frequency on the basilar membrane. Although the spectrum level of the WB noise is lower than that of the NB noise at the transducer, it receives more gain from the action of the cochlear amplifier than the NB noise and thus has greater energy representation on the cochlear partition. While the effective energy level on the basilar membrane may have been essentially the same for the WB and EQ noises as a result of the action of the cochlear amplifier, the WB noise was the more effective suppressor because its energy was summed across multiple critical bands. The energy in the EQ noise was confined to a single critical band.