[Top]
II. THE AUDITORY SYSTEM

12 - Metabolic Rank Order
Myers (1972) described involvement of a "monotonous rank order" of brainstem nuclei in the pattern of damage caused by asphyxia at birth. This rank order is comparable to the metabolic rank order of brainstem nuclei revealed by the autoradiographic techniques for measuring cerebral blood flow and metabolism.

The autoradiogram picture in Figure 1 (above) is part of data gathered in experiments done nearly half a century ago (Landau et al. 1955) to investigate cerebral circulation [3]. A radioactive tracer was injected into a laboratory animal (cats were used in this initial investigation). Distribution of the tracer was measured one minute later.

Measurements of the aerobic enzymes alpha-ketoglutarate and cytochrome oxidase further confirm that high blood flow supports aerobic metabolism, which is highest in the same hierarchy of brain areas [12-14].

The auditory system is susceptible to injury because its components have greater metabolic needs than most other areas of the brain. On the other hand this most active system is clearly often spared. But after a few minutes of sudden total circulatory arrest, and if resuscitation is possible, the inferior colliculus incurs severe damage. This has been found true in adult human cases as well as children [111-117].
[Top]

13 - The Auditory System
Figure 11 is a diagram of the auditory system that shows the location of the darkly labeled structures in figure 1.

Roth and Barlow (1961) employed an autoradiographic technique modeled after that used for measuring blood flow to investigate distribution of drugs in the brain [122]. They found that the fast-acting anesthetic thiopental was quickly distributed to the inferior colliculus. Thiopental is used for rapid induction of general anesthesia (loss of consciousness). That thiopental goes directly to the inferior colliculi suggests that high metabolic rate in this pair of auditory nuclei may be important for maintaining the conscious state.

Deafness is not a handicap of consciousness or general awareness. But deafness is the result of impairment at the level of the cochlear nucleus, or mechanical components of the ears. Autism is a handicap of general awareness, or of multiple attention deficits at least. Lack of social awareness and diminished capacity for "shared attention" are manifestations of environmental obliviousness in children with autism. That these characteristics may stem from impairment of midbrain auditory alerting functions is worth exploring.
[Top]

14 - Auditory Dysfunction
That autistic children are hypersensitive, hyper-reactive, and confused by some sounds is common knowledge (though largely based on anecdotal accounts). Normal children carry on conversations in school cafeterias and gymnasiums as easily as anywhere else, even with music blaring from the loudspeakers. An autistic child in such a noisy setting may exhibit extreme distress. My son Conrad would cover his ears and often refuse to go into a room where he saw a telephone – another anecdotal account. But anecdotes should be collected and taken more seriously as sources of useful data.

A parent on an internet email exchange (autism@list.feat.org, 3 Oct 2002) asked for advice on what to do about classroom aides who were controlling his 13-year-old son's behavior by holding him down in front of a vacuum cleaner. An outpouring of sympathy, outrage, similar stories, and ways to help overcome the child's fear followed. Kanner (1943) described two of the eleven children in his original report as being afraid of the vacuum cleaner, one so much that she would not go near the closet where it was kept [123]. Analysis of sound patterns emitted by vacuum cleaners, ability to recognize words and other sounds with component sounds as background noise, and research on auditory evoked potentials to signals of interest presented in noisy surroundings might yield useful data.

Research to date on auditory evoked potentials suggests that acoustic signals from ear to temporal lobes may be slowed or distorted in some children with autism [124-134]. These investigations have been controversial but they do provide indication of auditory dysfunction. Further, monkeys asphyxiated at birth were not deaf, but measurement of auditory evoked potentials revealed a delay in auditory signal transmission similar to that found in some children with autism [135].

Confusion in noisy environments points to problems processing multiple incoming sounds, and suggests that alternatives to simple click and tone stimuli should be used in testing for disorders of hearing. For example, tests of word recognition in quiet (WRIQ) and word recognition in noise (WRIN) described by Church et al. (1997) could be used to assess verbal children and even adapted for use with low functioning children with autism [136].

Researchers at the molecular level have found that inhibitory as well as excitatory neurotransmitters work together to modulate responses of neurons that detect sound onset; ongoing signals of the same frequency and intensity are detected but not transmitted further [137, 138]. The hypersensitivity to sounds displayed by some autistic children may represent loss of inhibitory function – why the sound of a vacuum cleaner might be distressing beyond the imagination of most of us. Inability to distinguish sound onset then relegate it to background awareness could also be part of the difficulty in recognizing boundaries between words and syllables in spoken language.

Caspary et al (1995) provided data showing decline with advancing age of neurotransmitter function in the inferior colliculus that may lead to loss of the capacity to detect and extract meaningful signals from background noise [139]. They pointed out that this leads to difficulty following a conversation in a noisy environment and may be the reason some elderly people withdraw from participation in society. The same or similar disability may lead children with autism to avoid social contact.
[Top]