Abstract
It is generally held that the right and left middle ears of mammals are acoustically isolated from each other, such that mammals must rely on neural computation to derive sound localisation cues. There are, however, some unusual species in which the middle ear cavities intercommunicate, in which case each ear might be able to act as a pressure-difference receiver. This could improve sound localisation at lower frequencies. The platypus Ornithorhynchus is apparently unique among mammals in that its tympanic cavities are widely open to the pharynx, a morphology resembling that of some non-mammalian tetrapods. The right and left middle ear cavities of certain talpid and golden moles are connected through air passages within the basicranium; one experimental study on Talpa has shown that the middle ears are indeed acoustically coupled by these means. Having a basisphenoid component to the middle ear cavity walls could be an important prerequisite for the development of this form of interaural communication. Little is known about the hearing abilities of platypus, talpid and golden moles, but their audition may well be limited to relatively low frequencies. If so, these mammals could, in principle, benefit from the sound localisation cues available to them through internally coupled ears. Whether or not they actually do remains to be established experimentally.
Highlights
If sound can reach a tympanic membrane both directly from the external environment and from the contralateral ear, via an internal acoustic pathway, the membrane will respond to the instantaneous difference in pressures applied to its external and internal surfaces
Mammals are generally regarded as having acoustically isolated middle ears, which means that in order to localise sound effectively they require, and have, excellent high-frequency hearing compared with nonmammals of similar body size. Is this true of all mammals though? The present paper considers the question of whether pressure-difference sound localisation could be possible in any mammalian species
Maximum sensitivity was at around 3 kHz. These results suggest that the hearing of these moles is restricted to relatively low frequencies, largely within the sonic range
Summary
If sound can reach a tympanic membrane both directly from the external environment and from the contralateral ear, via an internal acoustic pathway, the membrane will respond to the instantaneous difference in pressures applied to its external and internal surfaces. This results in a directional tympanic membrane response which can increase both interaural time and amplitude differences over a certain frequency band, improving the sound localisation abilities of the organism in question (Christensen-Dalsgaard 2011; Köppl 2009). Sound localisation which makes use of pressure-difference receivers is referred to as pressure-difference sound localisation
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