Onset overmasking of a brief amplitude increment in a pure tone and sensorineural hearing impairment

Hypothesis: The goal of this investigation was to determine, in patients wi th sensorineural hearing loss who may show an audiologic alteration in onse t overmasking, whether different pathologic conditions differ in this respe ct, and whether patients with a vestibular neurotomy damaging the cochlear efferents will be affected. Background: Auditory detection of brief signals, when presented at the begi nning of a simultaneous long masking sound,, may require a higher acoustic level than when presented after several hundred milliseconds. The proposed explanation, in terms of auditory nerve fibers adaptation has been based on the observation of a proportionally smaller increase of firing in response to an amplitude increment at the onset of a stimulus. However, this may no t explain all the data, and other underlying processes are certainly involv ed. The degree or type of sensorineural pathologic condition may be a contr ibuting factor. In addition, the cochlear efferent system, which exhibits a time course acid a high-frequency predominance compatible with that of ons et overmasking. could be involved. Methods: Onset overmasking of a brief amplitude increment in one pure tone was examined in 6 normal subjects, 12 patients who had undergone vestibular neurotomy, 8 subjects with Meniere's-like symptoms. 5 subjects with presby acusis, and 3 patients with a small neuroma. Both ears of all subjects were tested. Detection thresholds, amount of onset overmasking, and differences between the two ears were examined. Results: All results from subjects with presbyacusis and neuroma were withi n the range observed in the group of normal subjects. In the group of eight Meniere's syndrome patients, four had results outside the normal range, th ree had deteriorated detection, and one had better detection. Among the 12 subjects who had undergone neurotomy, 2 had better detection in the unopera ted ear. Conclusions: The results from Meniere's patients indicate that, in addition to the previously reported improved detection threshold for short onset de lay, a deterioration of detection thresholds may occur in some subjects. Ov erall, the results from neurotomized ears do not provide evidence for an in volvement of cochlear efferents in this tested psychoacoustic task.