Cortical deafness cannot account for the inability of Japanese macaques to discriminate species-specific vocalizations.

Bilateral ablation of the superior temporal gyrus in Japanese macaques results in a significant hearing loss (cortical deafness) as well as in an inability to discriminate between two types of their "coo" vocalizations. A two-part investigation was conducted to determine whether the hearing loss may itself affect the ability to discriminate vocalizations. First, four normal Japanese macaques were tested for their ability to discriminate coos which were filtered to simulate the effect of a cortical hearing loss. Second, four Japanese macaques with bilateral superior temporal gyrus lesions were tested for their ability to discriminate coos which were amplified and equalized to compensate for each animal's hearing loss. All four normal macaques were able to discriminate the filtered coos easily whereas compensating for the operated monkeys' hearing losses did not improve their performances. It appears that the inability of monkeys with bilateral superior temporal gyrus lesions to discriminate conspecific vocalizations is not simply due to the accompanying hearing loss, but is a separate auditory disorder.