Conductive hearing loss produces a reversible binaural hearing impairment

Conductive hearing loss, produced by otitis media with effusion, is widespr ead in young children. However, little is known about its short- or long-te rm effects on hearing or the brain. To study the consequences of a conducti ve loss for the perception and processing of sounds, we plugged the left ea r canal of ferrets for 7-15 months during either infancy or adulthood. Befo re or during plugging, the ferrets were trained to perform a binaural task requiring the detection of a 500 Hz tone, positioned 90 degrees to the righ t, that was masked by two sources of broad-band noise. In one condition ("c ontrol"), both noise sources were 90 degrees right and, in the second condi tion ("bilateral"), one noise source was moved to 90 degrees left. Normal f errets showed binaural unmasking: tone detection thresholds were lower (mea n 10.1 dB) for the bilateral condition than for the control condition. Both groups of ear-plugged ferrets had reduced unmasking; the mean residual unm asking was 2.3 dB for the infant and 0.7 dB for the adult ear-plugged anima ls. After unplugging, unmasking increased in both groups (infant, 7.1 dB; a dult, 6.9 dB) but not to normal levels. Repeated testing during the 22 mont hs after unplugging revealed a gradual return to normal levels of unmasking . These results show that a unilateral conductive hearing loss, in either i nfancy or adulthood, impairs binaural hearing both during and after the hea ring loss. They show scant evidence for adaptation to the plug and demonstr ate a recovery from the impairment that occurs over a period of several mon ths after restoration of normal peripheral function.