Anatomical and Physiological Origins of Auditory Brain Stem Responses (ABR)
The development of far field recording techniques to measure the activity of the auditory pathway in its course from the cochlea to the cortex has had important clinical applications. In man there are up to seven vertex positive waves that occur in the first ten msec after a click signal (Fig. 1). The largest of these components, designated variously as the IV–V complex, 4a and 4b, or N4 and N5, is usually 0.5 µV in amplitude and occurs at a latency of 5.6–6.0 msec for a 65 dB (H.L. re normal) click. Since the components of the auditory brain stem responses (ABR) change in latency in an orderly manner with signal intensity (Fig. 1) the measure can provide objective definition of hearing threshold in difficult-to-test subjects such as newborn infants or mentally impaired patients (Davis and Kirsch, 1977; Hecox and Galambos, 1974; Mokotoff et al., 1977; Shulman-Galambos and Galambos, 1975; Sohmer and Feinmesser, 1973; Starr et al., 1977; Yamada et al., 1975). The ABR evoked by clicks primarily reflects high frequency hearing capacities since it depends on the activity of the basilar or high frequency end of the cochlea. However, there are several methods under evaluation that will enable the ABR to serve as a reliable measure of hearing threshold across a wide range of signal frequencies; these include the use of filtered clicks (Davis, 1976) and narrow band masking noise (Don and Eggermont, 1978).
KeywordsBrain Stem Inferior Colliculus Cochlear Nucleus Auditory Pathway Electrolytic Lesion
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