Masculinization of the mammalian cochlea

Abstract

Otoacoustic emissions (OAEs) differ between the sexes in humans, rhesus and marmoset monkeys, and sheep. OAEs also are different in a number of special populations of humans. Those basic findings are reviewed and discussed in the context of possible prenatal androgen effects on the auditory system. A parsimonious explanation for several outcomes is that prenatal exposure to high levels of androgens can weaken the cochlear amplifiers and thereby weaken otoacoustic emissions (OAEs). Prenatal androgen exposure apparently also can alter auditory evoked potentials (AEPs). Some non-hormonal factors possibly capable of producing sex and group differences are discussed, and some speculations are offered about specific cochlear structures that might differ between the two sexes. KeywordsOtoacoustic emissions; Auditory evoked potentials; Sex differences; Prenatal development; Masculinization; Testosterone; Estradiol Address for Corresponding Author and manuscript correspondence: Dennis McFadden, Department of Psychology, Seay Building, University of Texas, 1 University Station A8000, Austin, TX 78712-0187, (512) 471-4324, (512) 471-5935 (fax), E-mail: mcfadden@psy.utexas.edu. 1 In science, the words sex and gender have distinctly separate meanings and should not be used as synonyms (e.g., Wizemann and Pardue, 2001, p. 176). Sex alludes to a person's body morphology, and gender alludes to a person's self-image as belonging to one (or neither) of the two sexes. Assessing gender requires substantially different procedures from assessing morphological sex. Many investigators who have used the term gender actually did not measure it. 2 Pairwise comparisons like sex differences are commonly characterized using effect size, a d′-like measure that expresses the difference between means in standard-deviation units. We calculate effect size as the difference between the means of the two groups of interest divided by the square root of the weighted mean of the two variances. By convention, effect sizes of 0.2, 0.5, and 0.8 are viewed as small, medium and large differences, respectively (Cohen, 1992). I have provided effect sizes in the figure captions. 3 One possible reason for this inconsistency in the sex differences for DPOAEs and CEOAEs is the effective level of the stimuli used to elicit the two forms of OAE. DPOAEs typically are measured with primary tones of 50 dB SPL or greater, whereas CEOAEs typically are measured with clicks of 60 -80 dB peSPL (peak-equivalent sound-pressure level). This means that the spectrum level of the clicks is quite weak at each of the successive locations along the length of the cochlea contributing to the reflection-based component of the CEOAE, while the energy in the primary tones used to evoke DPOAEs is relatively more concentrated. Accordingly, sex differences typically are measured using locally weak stimuli for CEOAEs and locally strong stimuli for DPOAEs, and this difference eventually may prove relevant. 4 Sometimes it is worthwhile to say the obvious. Having a masculinized coch...

Comparing behavioral and physiological measures of combination tones: Sex and race differences

“…Here, DPOAE strength was about 60 dB weaker than the primary tones [condition (2), Table III], which is about typical for humans (see McFadden et al, 2009); the DPOAEs of other mammalian species tend to be considerably stronger than those in humans. For CEOAEs, the sex differences seen in Table III were generally in accord with the literature (McFadden, 2009(McFadden, , 2011, but with a twist. As expected, the CEOAEs of females were stronger than those of males [conditions (3) and (4)], and this was true for both phases of the menstrual cycle.…”

“…(10) For CEOAEs, the White subjects exhibited substantial sex differences but the sex differences were essentially zero for the Non-White subjects. If replication confirms this outcome, it will raise interesting questions about the origins of sex differences in OAEs (see McFadden, 1998McFadden, , 2009McFadden, , 2011. It is likely that the past reports on this topic were based on largely White samples, but that is impossible to verify from the published details provided.…”

On Sex-Related Differences in Auditory and Visual Sensory Functioning

“…Converging evidence for this hypothesis comes from studies on organizational and activational effects of sex hormones on measures of auditory sensitivity, such as auditory brain-stem responses, otoacoustic emissions or pure tone thresholds (e.g., McFadden, 2002McFadden, , 2009. While organizational effects refer to permanent structural changes initiated primarily by prenatal hormone exposure, activational effects relate to the response elicited by the presence of a hormone after differentiation and organization (cf.…”

Sex Steroid-Dependent Modulation of Acoustic Communication Systems in Fishes

“…In A. burtoni, subordinate males have smaller testes with lower plasma levels of all sex-steroids compared to dominant males, raising the possibility that low androgens (or removal of an androgen-dependent inhibition in the auditory system) contribute to improved hearing at the high frequencies observed in the AEP experiments. In contrast to estrogens (Al-Mana et al 2008;Sisneros 2009a;Maney and Pinaud 2011;Caras 2013), however, relatively little is known about how androgens influence auditory processing and hearing thresholds in vertebrates (McFadden 2009;Forlano et al 2010;Snihur and Hampson 2012). As mentioned above, A. burtoni does express androgen receptors in the saccule of the inner ear, and in several auditory processing nuclei of the brain (Maruska and Fernald 2010c;Munchrath and Hofmann 2010;, which may serve as neural substrates for changes in androgen-related hearing ability associated with social status.…”