DIATONIC CATEGORIZATION IN THE PERCEPTION OF MELODIES

Previous research has shown that musicians skilled at interval identification categorize intervals according to their approximate size in semitones to perform interval discrimination tasks. The hypothesis that listeners have a similar strategy, diatonic categorization (a categorization scheme with seven rather than twelve categories within the octave), available to them for a range of other musical tasks neatly explains some phenomena of musical practice and some previous experimental results. Diatonic categorization would be a particularly effective strategy for music based on diatonic scales, and because it is a more efficient representation, it might be useful to listeners with less musical training and for musical tasks that are more complex than simple interval identification. I tested the specific hypothesis that musicians with moderate training would use diatonic categorization as a strategy in a short term memory task for transposed “melodies” (tone sequences) using two types of stimuli, one with diatonic stimuli in a familiar twelve-tone equal tuning (12-tET) and one with mock-diatonic stimuli in an unfamiliar thirteen-tone equal tuning (13-tET). The results show that subjects used diatonic categorization in the unfamiliar tuning condition as the only effective strategy to recall the interval content of melodies. In the familiar tuning, diatonic categorization appeared to be accompanied by other strategies, though more intersubject variability made these results less clear. 1. BACKGROUND AND AIMS Auditory categorical perception was first observed by Liberman et al. (1957) in the perception of speech consonants. Since then categorical perception has been observed in numerous aspects of speech perception and the concept has been greatly refined and modified. (Lindblöm 1996, Macmillan 1987, Rosen & Howell 1987) The basic elements of categorical perception research are identification and discrimination paradigms. Where there is categorical perception along some physical continuum, subjects (1) are able to use categories to identify where stimuli occur on the continuum, and (2) are able to discriminate stimuli that fall into different categories, but not ones that fall within the same category. Burns & Ward (1978) found an instance of categorical perception in a specifically musical context, the task of interval identification, and other studies have confirmed their basic findings (Siegel & Siegel 1977, Burns 1999). The categories in this case are the twelve semitonal interval sizes between unison and octave. In identification, subjects classify intervals presented in sinusoidal tones in familiar musical categories (“minor second,” “major third,” etc.). In discrimination, subjects hear pairs of intervals presented at different pitch levels and classify them as same or different. It is important in both cases that the intervals are transposable. Even though the discrimination task should be simpler from a signal theory perspective, musicians are better at the more familiar task of identification (Burns & Campbell 1994), suggesting strong categorical perception: i.e., discrimination can only be performed on the basis of identification. Yet the phenomenon is only observed for very skilled musicians who have experience in interval identification. The untrained subjects in Burns & Ward 1978 showed no evidence of categorical perception in addition to performing poorly on the task. Therefore, while twelve-tone categorization may be the only strategy available for the precise discrimination of isolated intervals, other strategies must exist for the discrimination of melodies, since presumably both musicians and non-musicians can perform such tasks to some extent. The hypothesis motivating the present study is that a different type of interval categorization strategy, using only seven interval categories per octave, helps listeners recall the relative pitch characteristics of melodies. I will call this strategy diatonic categorization. Because diatonic categorization reduces the number of categories, it is a more efficient representation for melodies that demonstrate a relatively high degree of conformity to diatonic scales (or other relatively even seven-note scales). Because of the reduced cognitive load of the diatonic representation, this type of categorization may be available to less trained listeners who cannot always accurately identify the semitone categories of intervals. There is a great deal of circumstantial evidence in support of the diatonic categorization hypothesis. Most obviously, it is intrinsic to Western interval nomenclature, where the generic name for each semitonal interval size includes its diatonic category (“second,” “third,” et c.—the exception being “tritone” which is the generic name for an interval that falls on a diatonic category boundary). Empirical research provides additional circumstantial evidence. For instance, in interval identification experiments subjects more often confuse diatonic-category equivalents than other intervals differing in width by a semitone. (Plomp, Wagenaar, & Mimpin, 1973; Killam, Lorton, & Schubert, 1975) In a series of experiments, Balzano (1977) found that subjects could identify intervals more quickly when using generic (e.g., “second”) rather than specific (e.g., “major second”) interval names. If they were identifying intervals in semitonal categories, than the use of generic names should have required an extra cognitive step and therefore should have taken longer. (See also Balzano, 1982a–b). Finally, numerous studies on transposed-melody discrimination demonstrate that melodies that differ only in mode are particularly difficult to discriminate, especially for musically untrained listeners. (Dowling and Bartlett 1981, Dewitt & Crowder 1986, Halpern 1984, Halpern, Bartlett, & Dowling 1998, and Leaver & Halpern 2003) Changes in mode preserve the diatonic categories of all intervals in a melody. The present study looks for evidence of diatonic categorization in a melody discrimination task. Melody discrimination should be more complex as a short-term memory task than interval identification but it also more closely resembles familiar musical tasks.