Teaching material for 2yr Medics and Physiology Module PL2207
Auditory evoked potentials
Sound-evoked potentials can be obtained between an electrode simply placed
on the scalp over the vertex of the head and one above the mastoid (with an
earth on the chin or forehead). At least 15 waves have been identified in
response to click stimuli, and are conventionally labelled as in the diagram.
Waves l to Vl are the so-called brain-stem evoked potentials, and some of them
have been tentatively identified as indicating activity in a specific location,
e.g. l: cochlear nerve; ll: cochlear nucleus; lll: superior olive; lV/V:
inferior colliculus. The sites of origin of the so-called middle latency waves
(No-Nb) and the long latency potentials (P1-N2) are very uncertain. The medial
geniculate nucleus and the auditory cortex have been implicated in the
generation of the waves No and Na respectively. However, with the exception of
the cochlear nerve, it is unlikely that the various waves can represent
components specific to the nuclei. It is possible that the waves represent the
synchronous activity of these neurones with the shortest latency in the nerve
fibre tracts connecting the nuclei.
The recording of these responses, particularly the early waves l to V, is
proving a useful clinical tool. First, it provides information of value for
localising lesions within the central auditory pathways. Second, it allows
objectives measurements of hearing threshold to be made without invasive
procedures in non-communicating children and adults. The long latency waves are
markedly affected by the state of arousal or sedation, and are therefore less
useful clinically. However, the longest latency waves (e.g. N2 and later) are
systematically affected by cognitive processes and may turn out to be of value
in the investigation of language disorders.
Taken from: The Senses, by Barlow, H.B. and Mollon, J.D. Cambridge
University Press, 1982.
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