Unilateral auditory cortex ablation in macaques results in a contralateral hearing loss.

1. The behavioral audiograms of four Japanese macaques (Macaca fuscata) were assessed before and after unilateral ablation of auditory cortex. The tones were presented via insertion earphones so that each ear could be tested separately. 2. Each animal had a hearing loss in the ear contralateral to the lesion, whereas the ipsilateral ear showed no change in sensitivity. The hearing loss initially appeared as a large shift in thresholds followed by rapid but incomplete recovery during the first 3-5 wk after surgery. The initial hearing loss ranged as high as 68 dB at some frequencies, although thresholds at other frequencies were occasionally unchanged. A threshold shift could be demonstrated with broadband noise as well as with tones. Although thresholds for some tones returned to normal within a few weeks, most were still elevated 16 wk after surgery when testing was discontinued. The largest long-term hearing losses occurred at frequencies from 4 to 25 kHz. 3. Analysis of the animals' psychophysical functions suggested that the hearing loss resembled a sensory deficit, as opposed to a nonsensory deficit in attention or vigilance. 4. Testing with binaural stimuli indicated that the hearing loss could best be described as a contralateral "ear" deficit, as opposed to a contralateral "auditory field" deficit. 5. It is suggested that a similar hearing loss occurs in humans after unilateral damage to auditory cortex.