Hearing in the ferret (Mustela putorius): effects of primary auditory cortical lesions on thresholds for pure tone detection.

1. Pure tone thresholds were determined for five adult male ferrets before and after bilateral ablation of primary auditory cortex. Complete audiograms ranging from 0.016 to 48 kHz were obtained for two animals. The remaining three animals were tested at five frequencies selected to assess hearing throughout the audible range (0.125, 0.5, 2.0, 8.0, and 32.0 kHz). 2. Shortly after surgery one animal had elevated thresholds across the entire frequency range with the most pronounced hearing loss above 12.0 kHz. Four other animals had no elevation of thresholds at low and midrange frequencies but suffered a hearing loss at very high frequencies (32 kHz). 3. Repeated testing over a period of several months revealed substantial recovery of sensitivity. There was complete recovery of sensitivity in the low- and middle-frequency range of the audiogram. Some hearing loss persisted at the extreme upper end of the audiogram (32 kHz), but in two cases there was evidence of recovery at this frequency as well. 4. Following determination of absolute thresholds all animals were assessed for their ability to localize sound in space. Minimum audible angles were obtained on midline as well as within both left and right hemifields, i.e., around 0, -60 and +60 degrees azimuth. All animals had severe and persistent deficits in their ability to localize brief sounds within the lateral fields, but were still capable of midline localization.