The Ears Emit Sounds as Part of the Hearing Process

The cochlea is not only the first stage in the analysis of sounds; in most people it also produces sounds. If a brief sound—a click or a short burst of tone—is sent into the external ear canal, a few milliseconds later a similar sound comes back from the inner ear. This is not just an echo from the eardrum or middle ear, although such echoes exist. The cochlea produces this sound. The sounds that the cochlea produces in response to acoustic stimulation are called evoked otoacoustic emissions (EOAEs). They occur in all people who have normal hearing, even in infants, and are thought to reflect the action of the cochlear amplifier discussed earlier, selectively boosting the response to particular frequencies. Alone or in combination with other techniques, the measurement of EOAEs is useful for detecting hearing impairments in newborns (Watkin, 2001).

In addition to evoked otoacoustic emissions, the ears of many people produce continuous low-level sounds at one or more frequencies; these sounds are called spontaneous otoacoustic emissions (SOAEs) (D. T. Kemp, 1979; Zurek, 1981). These spontaneous emissions are usually less than 20 dB above threshold; they can be detected by sensitive microphones in quiet environments, but the people who produce these sounds do not perceive them.

About two-thirds of women and half of men below the age of 60 produce SOAEs (McFadden, 1993a). Because they are observed in infants as well as in adults, SOAEs don’t seem to require prior auditory experience. Usually people who produce SOAEs have more-sensitive hearing than people who don’t—a difference that is consistent with the idea that these emissions are part of the cochlear amplifier (McFadden, 1993b).

Surprisingly, women who have a twin brother produce significantly fewer SOAEs, and weaker EOAEs, than do singleton females or females who have a female twin (McFadden, 1993b). A possible explanation for the lower incidence of SOAEs in females with male twins is that, in the womb, the female is exposed to androgens secreted by the male—a prenatal masculinizing effect (see Chapter 12). Lesbians, on average, produce weaker EOAEs than heterosexual women (Loehlin and McFadden, 2003; McFadden and Pasanen, 1998); given the possible link between prenatal androgen exposure and SOAEs, perhaps there is also a link between prenatal androgen exposure and an increase in the probability of homosexuality in females.