Effects of varying signal duration on pure-tone frequency discrimination in humans and monkeys

This study examined the effects of varying signal duration and pulse repetition rate on pure tone frequency difference limens (FDLs) at 500 Hz and 4 kHz in humans and monkeys. Two experiments examined FDLs as a function of tone duration from 12-400 ms, using stimuli presented either with a constant pulse rate, or with a constant interstimulus interval. In both cases, at 500 Hz, human FDLs increased more than monkey FDLs as duration decreased from 400 to 12 ms. This effect resulted in monkey FDLs that were about 14 times larger than human FDLs at 400 ms, but only about 4 times larger at 12 ms. At 4 kHz, human and monkey FDLs showed more similar rates of increase as duration decreased. A third experiment examined FDLs at 500 Hz for a 100-ms tone as a function of pulse rate (1 per 250-2000 ms). Here, FDLs for humans and monkeys varied in a similar fashion, indicating that both species' sensitivity was similarly affected by manipulating temporal variables relating to memory load, rather than sensory coding. These data provide evidence that, at 500 Hz, humans have a qualitatively different pure tone frequency analyzing mechanism from monkeys, possibly based on better access to phase-locking mechanisms.