| Auditory agnosia is the term used by Pan et al (2004) to describe the loss of speech understanding caused by a cancerous growth into the inferior colliculi of the patient described in their case report [1]. Auditory agnosia, impairment in sound recognition with seemingly adequate hearing, is according to Pan et al, usually associated with lesions in the auditory cortex; but they cite five earlier reports in which agnosia for language was also caused by traumatic or cancerous injury within the inferior colliculi [2-5]. Additional case reports have been published by Hoistad & Hain (2003), Kimiskidis et al (2004), and Musiek et al (2004) [6-8]. Some of these papers cited an earlier report by Howe and Miller (1975), who found damage in the inferior colliculi after the patient's death [9]. With MRI scanning, these small nuclei in the midbrain can be clearly seen, and more reports of functional loss following injury can be anticipated in the future. The inferior colliculi are more than simple way-stations in the auditory pathway. Auditory agnosia and loss of speech understanding following bilateral lesions of the inferior colliculi, in people who had previous full command of language, indicates the importance of these small nuclei. How much more serious the effect of damage in infancy must be, before the beginning of language development. The bilateral destruction of the inferior colliculi caused by asphyxia at birth should not be overlooked as a potential cause of developmental language disorder. In addition to awareness and attention, Fisch (1983) discussed the role of the auditory system as the evolutionary basis of language [11]. Language continues to be viewed by many as a higher cognitive function that (beyond the way a particular language is learned by a young child) has little to do with hearing. Fisch, an audiologist, on the other hand maintained that the raw material of language is sound and that it is practically impossible to separate language from its phonetic elements. Speaking and hearing are a major part of daily life, reading and writing much less so. According to Fisch (1983), in West European languages there are on average four phonetic elements per word and speech delivered at three words per second can still be understood. He pointed out that the auditory system does not act just as a cable but as a mechanism for transforming the signals to be used by language centers of the cerebral cortex. Fisch further noted that in the auditory system rapid extinction of signals increases the rate at which sequential sounds can quickly be replaced, new following old. As described by Caspary et al. (1995), rapid extinction of sounds is accomplished by inhibitory neurotransmitters [11]. Speech sounds last about 40 milliseconds with rapid transitions from vowels to plosives, with the sensation created by any one signal not persisting long enough to interfere with recognition of the next. Phylogenetically the auditory system is a late development [12]. Fisch suggested that the auditory system is therefore more complex, with greater metabolic activity, and is thus also more vulnerable to unfavorable metabolic influences. The auditory system has clearly evolved beyond being a simple alerting mechanism for visual attention. The capacity for language has developed along with, and as a result of, an increasing capability to detect meaningful short-lived changes in the acoustic environment. Fisch noted that in birds, the auditory system ends in the torus semicircularis, the auditory center within the optic tectum. Birds use auditory communication, for which the system terminating in the tectum suffices nicely. Textbooks of comparative anatomy discuss how in mammalian species the torus semicircularis of lower vertebrates evolved into the paired inferior colliculi, posterior to the superior (optic) colliculi [12]. These are the corpora quadrigeminae of the midbrain tectum, with connections to later evolutionary structures of the thalamus and temporal and frontal lobes of the cerebral cortex. |
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