|MadSci Network: Zoology|
Three anatomical areas are likely responsible for the differences between human and canine hearing. In order of importance, they are the external ear, middle ear, cochlea and auditory central nervous system. External Ear High frequency sounds are much more directional than those at low frequency. This is why many speaker systems in the past 10 years have gone to using a single subwoofer for low frequencies. It is also why animals with mobile external ears, including many dogs, have better sound location abilities than humans. By changing the shape and direction of their ears they are able to better transmit high frequency sounds to the ear drum. The length of the ear canal may also have some effect on its resonant frequency with shorter canals being better for high frequencies. Middle Ear The area of the skull between the ear drum (tympanic membrane technically) and the inner ear (the cochlea is the hearing portion) is called the middle ear. This contains the bones of hearing. It also contains an air filled portion of the skull. The size of this air filled space (tympanum and mastoid cavities) and the stiffness of the bones of hearing, have an effect on the sounds that are transmitted through the middle ear. Animals such as the kangaroo rat and chinchilla have a middle ear with similar volumes to those found in humans. Their range of hearing is very similar to humans. This makes these animals a favorite of hearing researchers. Inner ear The canine cochlea has 3 1/4 turns compared to the human cochlea's 2 1/2 turns. The additional length may provide some additional space for sensory cells. Auditory nervous system The nerves directly attached to the cochlea are similar in dogs and humans. In the brain stem and thalamus, most of the same nuclei or processing centers are present in humans and dogs, but they may vary in size. There are also connections with motor nuclei controlling ear movement, not present in humans. The biggest difference is in the auditory cortex. Humans have much more cortical tissue in general. Recent functional imaging studies using PET and fMRI imaging have let us see some of areas activated when performing some functions, but little beyond the primary auditory cortex is likely frequency specific. Refrences http://www.barkbytes.com/medical/med0057.htm http://www.lsu.edu/deafness/HearingRange.html
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