Auditory frequency discrimination in the squirrel monkey

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Abstract
Four squirrel monkeys (Saimiri sciureus) were tested for their frequency discrimination capacity using an eyeblink classical conditioning procedure, with air puff against the eye as unconditioned stimulus and 600-ms pure tones 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 higher 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 increased 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 squirrel monkey's frequency discrimination function thus shows a severe deviation from Weber's law. The frequency difference thresholds are comparable to human'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.

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