Effects of varying signal and noise levels on pure-tone frequency discrimination in humans and monkeys.

This study examined the effects of presenting various signal and noise levels on pure-tone frequency difference limens (FDLs) in humans and monkey. Signal levels for two frequencies of 500 Hz and 4 kHz were varied both in quiet and in noise. For the lower frequency of 500 Hz, the two species exhibited qualitatively divergent FDLs as level increased in quiet: FDLs decreased at high levels for humans, but increased for monkeys. In noise, differences between human and monkey FDLs were less pronounced. At 500 Hz, monkeys exhibited slightly lower FDLs in noise than in quiet. At the higher frequency of 4 kHz, under all conditions, FDLs for both species were qualitatively similar. Results are discussed in relation to (1) different use of "temporal" coding mechanisms by humans and monkeys at 500 Hz, and (2) similar use of "rate-place" coding mechanisms at 4 kHz.