ON THE ROLE OF THE OLIVOCOCHLEAR BUNDLE IN HEARING - A CASE-STUDY

A young patient with normal pure-tone thresholds in both ears underwen t a unilateral vestibular neurotomy in January 1992 to relieve severe vertigo ascribed to Meniere's disease. Evidence is provided that the w hole vestibular nerve including the olivocochlear bundle (OCB) was sec tioned. Just prior to the surgery, the patient was examined in several psychoacoustic tests involving mainly signal detection and selective attention. Over the next 20 months, he was reexamined in those same te sts. The patient's ability to detect expected tones in the quiet (incl uding audiograms) or in noise was the same as before the surgery. The one change was a marked improvement in the detection of unexpected sig nals in noise, which appears to reflect impaired selective attention. During those 20 months, new tests were also performed on discriminatio n, loudness, pitch, lateralization, and temporary threshold shift. On these tests, the only differences between the operated and unoperated ears concerned binaural diplacusis and loudness adaptation close to th reshold, but these differences may well have been present prior to the surgery. Except with respect to what is probably selective attention, we uncovered no other clear role for the OCB in hearing. This outcome agrees with limited measurements on other patients, with their subjec tive reports, and with a number of published neurophysiological observ ations.