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middle ear must have been a safety engineer.” By this, he meant that the flexibility of the two joints between the three ossicles affords some degree of protection to the cochlea. This has indeed been shown to be the case. In everyday life, our middle ears cushion the effects of very large and sudden pressure changes, such as when we cough or sneeze. Also, when there is a large difference in static pressure between the middle ear cavity and the environment, the flexible ossicular chain and eardrum are able to expand outward or squeeze together until pres- sure equalization can occur, thus reducing the amount of stress exerted on the cochlea, which might otherwise receive damage to its delicate hair cells (Brownell, 2017).
Impulsive Sounds
It is known that sudden impulsive sounds tend to cause far more damage to the cochlear hair cells than sounds with the same energy spread out over time. It was recently shown that flexibility of the malleus–incus joint might provide a protective means of spreading out the energy of impulsive sounds over time as they travel toward the cochlea (Gottlieb et al., 2018). Experimental measure- ments with impulsive stimuli showed that the shape of the impulse reaching the cochlea becomes higher in amplitude and more sharply focused in time after the malleus–incus joint is fused. This suggests that the normal flexible joint disperses the energy over time, thus broadening the impulse and lowering its peak amplitude.
Two Muscles, Two Joints
Another potential advantage of this design could be the need for independent activity by the two middle ear mus- cles. These are the tensor tympani muscle, which attaches to the malleus handle and pulls it inward, and the stape- dius muscle, which attaches to the head of the stapes and pulls it to the side (Figure 1). Recent evidence for the evolutionary coadaptation of the two muscles and joints comes from the observation that rodents like the guinea pig and chinchilla that have a fused malleus–incus joint also have a reduced or complete lack of stapedius muscle function (Mason, 2013). The fused malleus–incus joint and relatively large middle ear cavities of these animals may be important for improving low-frequency hearing.
The stapedius muscle has been studied extensively, and it is generally agreed that it forms part of an acoustic reflex arc that operates with a latency of about 20 ms, after which point it is able to attenuate sounds below a few kilohertz.
This reflex arc is thought to help protect the cochlea against loud, long-duration sounds. However, its onset is too slow to protect against sudden impulsive sounds, such as a gun- shot, that can be very damaging to the cochlea.
The function of the tensor tympani muscle is not well- established. Contractions of the tensor tympani muscle are elicited by tactile stimulation of facial areas, a puff of air against the eyes, and during speaking. Many subjects can voluntarily contract their tensor tympani muscles and perhaps also their stapedius muscles.
A Possible Connection Between Hearing
and Seeing
Recently, an intriguing new idea has been posited for the role of the tensor tympani muscles. It was shown that when people move their eyes to one side, pressure waves of opposite polarities can be measured in the two ear canals, with the polarities reversing when the eyes move to the other side (Gruters et al., 2018). One interpreta- tion of this is that the pressure waves are produced by the left eardrum being pulled inward by the tensor tympani muscle and the right eardrum relaxing outward as the eyes move to the right and vice versa as the eyes move to the left. Because a tense eardrum is thought to reduce transmission time through the middle ear, it is possible that linking eardrum tension to eye movement could allow the interaural time difference cues used for sound localization (Heffner and Heffner, 2016) to be adjusted as the eyes move, such that the brain perceives a shift in the location of the sound in accord with the new visual field, even though the ears themselves have not moved. One can speculate that this might be one of the reasons for the existence of substantial delay in the middle ear to begin with. Many of the middle ear biomechanics concepts we have encountered above have found their way into sev- eral application areas that are now discussed.
The Breadth and Depth of Middle
Ear Biomechanics
The middle ear apparatus comes into play in several areas spanning scientific, surgical, and technological fields. Many aspects of middle ear research have not been cov- ered here, but review articles can be found elsewhere (e.g, Puria et al., 2013), including ongoing work on develop- mental biology (Anthwal et al., 2013), evolutionary biology (Manley, 2010), and comparative anatomy (Rosowski, 2013). Much of the present treatment relates to hearing
Fall 2020 • Acoustics Today 33