Effect of restricted cortical lesions on absolute thresholds and aphasia-like deficits in Japanese macaques.

The effect of small bilateral cortical lesions on pure-tone audiograms and on the ability to discriminate between two types of Japanese macaque coo vocalizations was determined in four Japanese macaques (Macaca fuscata). A lesion that included the middle portion of the superior temporal gyrus of both hemispheres, that is, the primary and secondary auditory areas, resulted in a partial hearing loss as well as an inability to discriminate the vocalizations. Lesions that included the ventral portions of the superior temporal gyrus of both hemispheres but spared auditory cortex on one side also resulted in a partial hearing loss but had either a small effect or no effect on the ability to discriminate the vocalizations. Bilateral ablation of the dorsal superior temporal gyrus and adjacent parietal and occipital areas did not appear to result in a hearing loss and had no effect on the ability to discriminate the vocalizations. These results suggest that a hearing loss may be produced by lesions that involve small portions of the ventral two-thirds of the superior temporal gyrus bilaterally although the resulting loss is not as great as that observed with larger lesions. However, the aphasia-like deficit appears to result from a lesion of primary and/or secondary auditory cortex.