| The auditory sense serves as an alerting system for visual attention and general awareness. We turn to look for the source of a new sound. Far less often do we listen for sound from something new that enters our visual field; listening for thunder following a lightning flash, or sounds from a supersonic jet streaking across the sky are examples of visual anticipation of sound onset, but they are clearly less frequent than instinctive orienting to auditory stimuli. The auditory system may be central to general awareness. Deaf people are not lacking in general awareness, but they depend upon alternatives to sound signals that most people rely on instinctively. Not hearing the honk of a horn makes activities like driving a car or riding a bicycle more hazardous for those with loss of auditory acuity. Deafness is a disabling condition, whether present from early childhood or part of the process of aging. Deafness in childhood requires special education for learning language. Deafness in old age often leads to withdrawal from social interactions, and inability to follow a conversation becomes an obstacle to full awareness of one's environment [1]. Those who maintain that deafness should not be considered a disability may be right, but need to go beyond euphemistic assertions to be fully convincing. Deafness is certainly not a sign of mental disability. Deafness is usually associated with impairment of structures of the ear or the cochlear nucleus. Loss of signal processing integrity higher up in the auditory pathway may interfere more with alerting functions and environmental awareness. Dysfunction in higher centers of the auditory pathway will also impact non- auditory structures to and from which they send and receive neural information. Deafness that comes with aging may involve higher auditory nuclei more than those close to the ear [1, 2]. The ability to focus on one of many sound sources, such as conversation, in a noisy environment becomes more difficult with advancing age. Some degree of auditory acuity begins to decline even in childhood; the ability to learn a foreign language by ear and without accent, the way young children do, is not easy past the first decade of life. Complex features of sound that makeup conversational streams of speech are more likely analyzed at higher levels of the auditory system. High metabolic activity in the inferior colliculi and other structures in the auditory pathway may lead to degrees of early decline beginning in late childhood. The auditory impairments of the young child with autism may possibly be of the same degree as those of an adolescent in a first year French class. Recognizing syllable and word boundaries of the teacher's spoken French is already more difficult for a 14-year-old student than it is for a preschool child learning French as a second language in a French speaking community. I would further raise the possibility that auditory impairments of the child with severe autism may be of the same degree as those of old age, when speech sounds can no longer easily be distinguished from background noise, and noise of any kind such as loud music can be distressing and provoke an angry reaction. At least twelve reports have been published (through December 2005) of adults who became deaf or lost understanding of spoken language following injury to the inferior colliculi [3-10]. Injury in all but one of these cases was detected through use of magnetic resonance imaging (MRI). A psychological cause for loss of hearing or responsiveness to speech had been thought responsible in several of these cases until the lesion in the inferior colliculi was found on MRI scans. Impaired subcortical processing of acoustic signals clearly can lead to problems that can be even more serious than deficits in loudness perception. |
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