The researchers played recorded coos, grunts, and other vocalizations made by macaques, but also other animals and natural sounds like thunder and running water. A small area of the macaques' temporal lobes turned on only in response to macaque voices, being insensitive to other sounds.

The nucleus could differentiate the voices of individual macaques: its activity decreased when the researchers played several times a monkey's voice, but it was boosted by a new played voice.