Effect of unilateral and bilateral auditory cortex lesions on the discrimination of vocalizations by Japanese macaques.

Ten Japanese macaques were trained to discriminate between two types of Japanese macaque coo vocalizations before and after auditory cortex ablation. Five of the animals were tested following left unilateral ablation, whereas the other five were tested following right unilateral ablation. After postoperative testing, symmetrical lesions were made in the remaining hemisphere in two animals from each group and the effect of bilateral lesions was assessed. The animals were tested using a shock avoidance procedure. Unilateral ablation of left auditory cortex consistently resulted in an initial impairment in the ability to discriminate between the vocalizations with the animals regaining normal performance in 5-15 sessions. In contrast, right unilateral ablation had no detectable effect on the discrimination. Bilateral auditory cortex ablation rendered the animals permanently unable to discriminate between the coos. Although the monkeys could learn to discriminate the coos from noise and from 2- and 4-kHz tones, they had great difficulty in discriminating between the coos and tones in the same frequency range as the coos (i.e., 500 Hz and 1 kHz). The initial impairment following left unilateral lesions indicates that the ability to perceive species-specific vocalizations is lateralized to the left hemisphere. The observation that bilateral lesions abolish the discrimination indicates that the recovery in the left lesion cases was the result of the right hemisphere mediating the discrimination.