Understanding speech in noise after correction of congenital unilateral aural atresia: effects of age in the emergence of binaural squelch but not in use of head-shadow.
ABSTRACT Unilateral hearing loss causes difficulty hearing in noise (the "cocktail party effect") due to absence of redundancy, head-shadow, and binaural squelch. This study explores the emergence of the head-shadow and binaural squelch effects in children with unilateral congenital aural atresia undergoing surgery to correct their hearing deficit. Adding patients and data from a similar study previously published, we also evaluate covariates such as the age of the patient, surgical outcome, and complexity of the task that might predict the extent of binaural benefit--patients' ability to "use" their new ear--when understanding speech in noise.
Patients with unilateral congenital aural atresia were tested for their ability to understand speech in noise before and again 1 month after surgery to repair their atresia. In a sound-attenuating booth participants faced a speaker that produced speech signals with noise 90 degrees to the side of the normal (non-atretic) ear and again to the side of the atretic ear. The Hearing in Noise Test (HINT for adults or HINT-C for children) was used to estimate the patients' speech reception thresholds. The speech-in-noise test (SPIN) or the Pediatric Speech Intelligibility (PSI) Test was used in the previous study.
There was consistent improvement, averaging 5dB regardless of age, in the ability to take advantage of head-shadow in understanding speech with noise to the side of the non-atretic (normal) ear. There was, in contrast, a strong negative linear effect of age (r(2)=.78, selecting patients over 8 years) in the emergence of binaural squelch to understand speech with noise to the side of the atretic ear. In patients over 8 years, this trend replicated over different studies and different tests. Children less than 8 years, however, showed less improvement in the HINT-C than in the PSI after surgery with noise toward their atretic ear (effect size=3). No binaural result was correlated with degree of hearing improvement after surgery.
All patients are able to take advantage of a favorable signal-to-noise ratio in their newly opened ear; that is with noise toward the side of the normal ear (but this physical, bilateral, head-shadow effect need not involve true central binaural processing). With noise toward the atretic ear, the emergence of binaural squelch replicates between two studies for all but the youngest patients. Approximately 2dB of binaural gain is lost for each decade that surgery is delayed, and zero (or poorer) binaural benefit is predicted after 38 years of age. Older adults do more poorly, possibly secondary to their long period of auditory deprivation. At the youngest ages, however, binaural results are different in open- and closed-set speech tests; the more complex hearing tasks may involve a greater cognitive load. Other cognitive abilities (late evoked potentials, grey matter in auditory cortex, and multitasking) show similar effects of age, peaking at the same late-teen/young-adult period. Longer follow-up is likely critical for the understanding of these data. Getting a new ear may be--like multitasking--challenging for the youngest and oldest subjects.