Evolution of Sound Localization in Land Vertebrates
稿件作者：唐业忠
通讯作者：
刊物名称：Washington,November,2008,2008 J.B.Johnston Club Meeting Abstracts
发表年份：2008
卷：
期：
页码：83-83
影响因子：
文章摘要：
Current theories suggest that there are major differences between directional hearing in birds and mammals .Information from other tetrapods therefore becomes increasingly necessary if we are to understand the evolution of sound localization circuits.A reevaluation of auditory evolution is also needed because tympanic hearing emerged not once,but independently in at least five major tetrapod groups:amphibians,lizards and snakes,turtles,archosaurs and mammals［Clack,1997,Brain Behav Evol 50:198-212］.It can therefore no longer be assumed that their central auditory nuclei are homologous;rather,each of the groups must be regarded as an independent‘experiment in hearing’,with the central processing highly influenced by the peripheral processing.
Tympanic ears emerged in the Triassic, with earlier ears probably resembling those of fish ancestors: able to respond to soundinduced vibrations of the skull and substrate.On land, the impedance mismatch between the air and tissue causes most sound energy to be reflected, and physical constraints on detection and increased importance of detecting air-borne sound might have led to the emergence of the eardrum and middle ear. The evolution of the early eardrums would have increased both the frequency range and hearing sensitivity.Additionally, the ears must have been acoustically coupled through the mouth cavity and therefore directional, acting as pressure difference receivers. A similar arrangement is found in modern lizards, where acoustical coupling can generate a 50-fold directional difference, usually at relatively high frequencies(2-4 kHz).Because a part of the binaural processing already is taking place at the eardrum, a pressure gradient ear reduces the amount of neural computation needed to determine sound location, although modern lizards appear to have generally similar neural circuits to those in birds and mammals.
The closure of the middle ear cavity in mammals and some birds is a derived condition, and would have profoundly changed the operation of the ear by decoupling the eardrums, improving their low-frequency responses, and leading to a requirement for additional neural computation of directionality in the central nervous system. Once the pressure gradient mechanisms were lost or diminished, there would also have been selection for cues that would improve localization, such as increased high-frequency sensitivity(for example by incorporating two jaw bones in the mammalian three-ossicle middle ear), and the addition of structure such as external ears that could increase interaural cues. Additional neural processing could also compensate for the loss of directional information from the eardrum, leading to specialization for central processing of sound source localization information.
Many of the features of avain and mammalian central auditory systems might reflect modifications of existing structures in already functioning tetrapod systems, such as those in geckos. The circuits mediating sound localization in archosaurs and mammals appear as examples of parallel solution to similar problems.
Tympanic ears emerged in the Triassic, with earlier ears probably resembling those of fish ancestors: able to respond to soundinduced vibrations of the skull and substrate.On land, the impedance mismatch between the air and tissue causes most sound energy to be reflected, and physical constraints on detection and increased importance of detecting air-borne sound might have led to the emergence of the eardrum and middle ear. The evolution of the early eardrums would have increased both the frequency range and hearing sensitivity.Additionally, the ears must have been acoustically coupled through the mouth cavity and therefore directional, acting as pressure difference receivers. A similar arrangement is found in modern lizards, where acoustical coupling can generate a 50-fold directional difference, usually at relatively high frequencies(2-4 kHz).Because a part of the binaural processing already is taking place at the eardrum, a pressure gradient ear reduces the amount of neural computation needed to determine sound location, although modern lizards appear to have generally similar neural circuits to those in birds and mammals.
The closure of the middle ear cavity in mammals and some birds is a derived condition, and would have profoundly changed the operation of the ear by decoupling the eardrums, improving their low-frequency responses, and leading to a requirement for additional neural computation of directionality in the central nervous system. Once the pressure gradient mechanisms were lost or diminished, there would also have been selection for cues that would improve localization, such as increased high-frequency sensitivity(for example by incorporating two jaw bones in the mammalian three-ossicle middle ear), and the addition of structure such as external ears that could increase interaural cues. Additional neural processing could also compensate for the loss of directional information from the eardrum, leading to specialization for central processing of sound source localization information.
Many of the features of avain and mammalian central auditory systems might reflect modifications of existing structures in already functioning tetrapod systems, such as those in geckos. The circuits mediating sound localization in archosaurs and mammals appear as examples of parallel solution to similar problems.
