The conventional view in auditory science is that whatever the role of the otolith organs in lower vertebrates, in mammals hearing is mediated entirely by the cochlea. Evidence accumulated over the last few decades, though, supports the case that the sacculus has conserved an acoustic sensitivity throughout vertebrate phylogeny. The question remains, however, whether saccular acoustic sensitivity in humans has any perceptual or behavioural significance. It is quite possible that acoustic sensitivity of the sacculus does not have a role or function in higher vertebrates, but is simply an epiphenomenon of evolution, a consequence of there being no selection pressure for it to disappear. An alternative hypothesis proposed by Todd et al. (2000) is that saccular acoustic sensitivity contributes to the affective quality of loud sounds. Circumstantial evidence for this in humans comes from the coincidence of saccular acoustic sensitivity with the distribution of frequencies and intensities of natural acoustic signals (Todd and Cody 2000; Todd et al. 2000). In particular given that humans seek out pleasurable sensations of self-motion by normal inertial stimulation, such as at 'fun parks', it is possible that human compulsion to exposure to loud, low-frequency sounds is a kind of acoustic equivalent of vestibular self-stimulation.
In this paper I discuss an experiment which indicates that there is a significant change in the quality of sensation above the saccular acoustic threshold (Todd 2001). In order to account for this I outline a new theory of the evolutionary biology of hearing. The evolutionary significance of saccular acoustic sensitivity in higher vertebrates, according this theory, is that a primitive central mechanism has also been conserved, through which vocally mediated sexual selection has continued to operate by providing a direct pathway to reward centres in the brain (which is not provided by cochlear pathways). The existence of loud vocalisations in primates is almost a universal. Such loud synchronised vocalisations in primates are considered to be the precursor of loud music in human culture. Whilst amplified music is clearly a modern invention, both human vocalisations and percussive instruments are of sufficient acoustic power to activate the sacculus (Todd et al. 2000). Thus saccular acoustic sensitivity may still play an important role in perceptual and behavioural responses to loud music.
Todd, N.P.McAngus and Cody, F. (2000) Vestibular responses to loud dance music: A physiological basis for the "rock and roll threshold"? Journal of the Acoustical Society of America. 107(1), 496-500.
Todd, N.P.McAngus, Cody, F. and Banks, J. (2000) A saccular origin of frequency tuning in myogenic vestibular evoked potentials?: Implications for human responses to loud sounds. Hearing Research. 141, 180-188
Todd, N.P.McAngus (2001) Evidence for a behavioural significance of saccular acoustic sensitivity in humans. Journal of the Acoustical Society of America. 110(1), 380-480.
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