Frequency discrimination as a function of frequency, measured in several ways

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Abstract
Frequency discrimination was measured for a wide range of center frequencies (0.25–8 kHz) using three different tasks. In the first (difference limens for frequency, DLFs) subjects were required to indicate which of two successive tone pulses was higher in frequency. In the second (difference limens for change, DLCs), two successive pairs of tone pulses were presented; one pair had the same frequency and the other pair differed in frequency. Subjects were required to indicate which pair differed in frequency. In the third (frequency-modulation difference limens, FMDLs), subjects were required to indicate which of two successive tone pulses was frequency modulated. Modulation rates were 2, 5, or 10 Hz. For frequencies up to 2 kHz, DLFs and DLCs were small (less than 0.6% of the center frequency) and were similar to one another. For frequencies of 4 kHz and above, both DLFs and DLCs increased markedly, but the increase was greater for DLFs. Thus the worsening of performance at high frequencies is greater when subjects are required to indicate the direction of a frequency change than when they just have to detect any change. FMDLs, when expressed relative to the carrier frequency, varied much less with frequency than DLFs or DLCs. At 2 kHz and below, FMDLs were larger than DLFs or DLCs. Above 4 kHz, FMDLs were smaller than DLFs or DLCs. At 2 kHz and below, FMDLs usually worsened with increasing modulation frequency. Above 4 kHz, FMDLs improved with increasing modulation frequency. The pattern of results suggests that two mechanisms play a role in frequency discrimination, one based on changes in the excitation pattern (a ‘‘place’’ mechanism) and one based on phase locking in the auditory nerve (a ‘‘temporal’’ mechanism). The temporal mechanism only operates below about 4 kHz and, within this range, it determines DLFs and DLCs. The temporal mechanism is probably sluggish, and it affects FMDLs only for very low modulation rates. The place mechanism dominates for high frequencies, and for lower carrier frequencies when stimuli are frequency modulated at high rates.

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