Question
At what level does a unilateral hearing loss begin to noticeably interfere with hearing in noise and localizing sound?
Answer
We have long known that, in general, the greater the degree of unilateral hearing loss, the more difficulty an individual will have with sound localization and speech perception abilities. For example, years ago Bess and colleagues demonstrated that children with severe-to-profound unilateral hearing loss had significantly poorer speech recognition in noise scores than children with mild-to-moderate degrees of unilateral hearing loss (Bess, Tharpe, & Gibler, 1986). Furthermore, they found that the greater the degree of unilateral hearing loss, the poorer the localization ability (Humes, Allen, & Bess, 1980). In fact, the one subject with unilateral hearing loss who performed essentially normal on the localization task had the mildest degree of hearing loss of the group. These results were consistent with those of earlier researchers who also observed that as the degree of unilateral hearing loss increased, the size and number of localization errors increased. This effect was noted in both quiet and noise conditions (e.g., Viehweg & Campbell, 1960).
However, it should be noted that even though, on average, the degree of unilateral hearing loss is positively correlated to difficulties with speech recognition and localization tasks, much individual variability remains. Despite continued attempts at illuminating the specific factors contributing to poor localization ability in patients with unilateral hearing loss (e.g., degree, etiology, type, and length of hearing loss, or age of individual), the results remain puzzling (Newton, 1983; Wilmington, Gray, & Jahrsdoerfer, 1994). That is, one cannot easily predict the localization or speech recognition ability of a given individual on the basis of an audiogram.
References:
Bess FH, Tharpe AM, & Gibler AM (1986). "Auditory Performance of Children with Unilateral Sensorineural Hearing Loss". In Bess FH et al., Children with Unilateral Hearing loss. Ear and Hearing Monograph, Jan Feb.
Humes LE, Allen SK, & Bess FH (1980). Horizontal sound localization skills of unilaterally hearing -impaired children, Audiology, 19: 508-518.
Newton VE (1983). Sound localization in children with a severe unilateral hearing loss. Audiology, 22: 189-198.
Viehweg R, & Campbell RA (1960). Localization difficulty in monaurally impaired listeners. Annals of Otology, 69: 622-634.
Wilmington D, Gray L, & Jahrsdoerfer R (1994). Binaural processing after corrected congenital unilateral conductive hearing loss. Hearing Research, 74: 99-114.
Anne Marie Tharpe, Ph.D., has been an audiologist for over 20 years. She is on the faculty in the Department of Hearing and Speech Sciences at the Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences in Nashville Tennessee. She can be contacted at anne.m.tharpe@vanderbilt.edu.
