Asymmetrical frequency discrimination in primates

Abstract

Auditory frequency difference limens at 0.5, 1.0, 2.0, and 4.0 kHz were measured monaurally in humans and in four Old World monkey species: Japanese and rhesus macaques (Macaca fuscata, Macaca mulatta) and vervet and deBrazza monkeys (Cercopithecus aethiops, Cercopithecus neglectus). A go, no‐go, repetitive‐standard discrimination procedure was employed with positive reinforcement. At certain frequencies and sensation levels, humans and macaques exhibit superior discrimination for frequency increments, while under these same conditions the Cercopithecus species exhibit superior discrimination for frequency decrements. Possible coding mechanisms to account for these results will be discussed, including monitoring rate increases versus rate decreases in asymmetrically tuned auditory nerve fibers. In one species, the Japanese macaque, there is some evidence that superior discrimination of frequency increments may relate to the design features of the animal's vocal communication system. [Supported by NSF, Sloan Foundation, and Deafness Research Foundation.]