Binaural processing after corrected congenital unilateral conductive hearing loss.

Binaural processing was measured in a series of tests in patients before and after surgery to correct congenital unilateral conductive hearing losses. Data are presented from 19 patients between the ages of 6 and 33 years that had an abnormal external and/or middle ear on one side but normal hearing in the other ear. Surgery improved thresholds an average of 36 dB HL (from 56 to 20 dB HL). Patients were tested pre- and postoperatively for interaural temporal difference limens, alternate and simultaneous loudness balances, sound localization, binaural detection thresholds, and speech perception in noise. There was statistically significant improvement after surgery in all tests, and the amount of improvement varied along a continuum that appears to be related to the simplicity of the task. For example, most postoperative patients had normal or near-normal performance in a test of interaural temporal difference limens, while almost all had difficulty localizing sounds. Neither binaural performance (before or after surgery) nor the improvement in performance was correlated with age, pure-tone thresholds, or asymmetry. Limited available data show no significant changes in performance from four weeks to over 24 weeks after surgery. In conclusion, binaural ability following corrective surgery exists in varying degrees in these tasks, suggesting different effects of abnormal early experience on different aspects of binaural hearing.