Auditory frequency discrimination in the squirrel monkey

Four squirrel monkeys (Saimiri sciureus) were tested for their frequency di scrimination capacity using an eyeblink classical conditioning procedure, w ith air puff against the eye as unconditioned stimulus and 600-ms pure tone s as conditioned stimuli. Absolute frequency difference thresholds showed a minimum (20-41 Hz, mean 30 Hz) at 4,000-8,000 Hz and increased towards hig her as well as lower frequencies (70-90 Hz, mean 80 Hz at 300 Hz; 44-120 Hz , mean 82 Hz at 16,000 Hz). Relative frequency difference thresholds increa sed from higher to lower frequencies, with values as low as 0.3-0.8% (mean 0.5%) at 16,000 Hz and as large as 24-30% (mean 27%) at 300 Hz. The squirre l monkey's frequency discrimination function thus shows a severe deviation from Weber's law. The frequency difference thresholds are comparable to hum an's in the 4,000-8,000 Hz range, but are 65-80 times higher in the 500- to 300-Hz range. Individuals with high auditory thresholds do not necessarily also have high frequency difference thresholds.