Hearing

Teaching material for 2yr Medics and Physiology Module PL2207

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. AEP

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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