Categorical perception was investigated in a series of experiments on the perception of melodic musical intervals (sequential frequency ratios). When procedures equivalent to those typically used in speech-perception experiments were employed, i.e., determination of identification and discrimination functions for stimuli separated by equal physical increments), musical intervals were perceived categorically by trained musicians. When a variable-step-size (adaptive) discrimination procedure was used, evidence of categorical perception (in the form of smaller interval-width DL's for ratios at identification category boundaries than for ratios within categories), although present initially, largely disappeared after subjects had reached asymptotic performance. However, equal-step-size discrimination functions obtained after observers had reached asymptotic performance in the adaptive paradigm were not substantially different from those initially obtained. The results of other experiments imply that this dependence of categorical perception on procedure may be related to differences in stimulus uncertainty between the procedures. An experiment on the perception of melodic intervals by musically untrained observers showed no evidence for the existence of "natural" categories for musical intervals.

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The subjective relation “octave of” is demonstrated to be a valid interval for determining scales of musical pitch for pure tones presented successively. Octave judgments of trained musicians have a standard deviation averaging about 0.6 percent. Inter-observer variability is 2 to 5 times as great, increasing with frequency. Judgments vary significantly from day to day, but as the direction of shift at different frequencies for a single observer is random, the shifts cannot be attributed to changes in the octave criterion. Instead, this variability, and also (1) differences between right- and left-ear judgments of a given observer, (2) the change in difference between subjective and physical octaves as a function of frequency, and (3) the high inter-observer variability, all confirm the basic instability of pitch-frequency relations implied by the facts of binaural diplacusis. Individual and group scales of musical pitch are deduced. In these scales, the average rate of change of musical pitch with respect to frequency level is less than unity by a small but significant amount. Although this discrepancy not explained, tests show that it is not an obvious artifact of method. Simultaneous presentation raises variability, but affects means only slightly. The relation betwen the peculiarities of individual scales and binaural diplacusis discussed.

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