Rhythm appears to be a fundamental capacity of humans. Rhythm plays a role in the prenatal environment and the early socialization of infants (Bertoncini, et al., 1995; Fassbender, 1996; Hargreaves, 1986; Papoušek, 1996). It has been implicated in the coordination of motor activity and locomotion (Iverson & Thelen, 1999). Rhythmic processing is a late deteriorating function in neurodegenerative diseases, such as Alzheimer’s (Beatty, et al., 1999). Rhythm appears to be a basic element in the construction of more complex human behaviors and interactions, such as music and language (Iverson & Thelen, 1999; Patel, et al., 1998), and has been implicated in aspects of memory (Brower, 1993; Payne & Holzman, 1986; Patel, et al., 1998).
A greater understanding of rhythm processing will therefore benefit from joint explorations across these domains of human behavior, in particular in music and language because of their universal presence across cultures and throughout the lifespan. Both music and speech share the same acoustic medium. Both are processed by the same perceptual apparatus. I find it reasonable to assume that the cognitive heuristics used for making sense of music and speech are at least similar, because we lack sufficient evidence to suggest that humans have evolved two entirely different mental modules for music and for language. To the contrary, there is great evidence to suggest that the distinction between music and speech is only achieved at higher levels of processing (Patel, et al., 1998).
There are many aspects of temporal processing that are relevant for this examination, and which necessarily impact an understanding of the subject. Unfortunately, well-formed and agreed upon definitions are in short supply. Paul Fraisse (1982), for instance, has written: “The task of those who study rhythm is a difficult one, because a precise, generally accepted definition of rhythm does not exist.” (149) What’s more, the definitions that occasionally arise lack consistency in what they describe. In an attempt to clarify and tease apart the various aspects of temporal organization, I provide my own definitions of certain aspects, which I trust are no less nor more arbitrary than most. I make no attempt however to be exhaustive in these definitions, in part because there appear to be many equally valid ways to divide up the temporal domain. I merely seek a first approximation of terms to address those aspects which will most facilitate questions dealing jointly with music and speech.
It should be noted that these definitions are meant to account for a variety of naturally occurring phenomena in both speech and music. On the one hand this differs from most existing approaches that deal mainly with one or the other of these behaviors. The present attempt is meant to accommodate rhythmic events without preferencing the specific function (i.e. musical or linguistic) to which these events are put. An implicit motivation for this tack is the acknowledgement that, as Patel and colleagues put it (Patel, et al., 1998): “It is obvious that prosodic and musical processing share resources at certain neural levels: there are not separate cochlea for language and music. On the other hand, it is equally obvious that at some point melodic and rhythmic patterns and linguistic prosody follow separate processing pathways.” (136)
Any attempt to address the human capacity for organizing sound through time must confront the issue that the cognitive mechanism engaged in such organizing procedures is not normally presented with unambiguous stimuli, cleanly divided into musical and linguistic domains. The task of making sense of sound may, at least at an initial phase, be domain general in nature. In other words, the point at which sound stimuli are categorized in the mind as musical or linguistic is not yet known. Therefore, the assumption that rhythmic comprehension for either domain can be fruitfully studied independently is not warranted by the evidence. Further, it is only in examining realistic rather than idealized data that the subtlety required for this distinction between music and language processing will be properly achieved.
First, let me address some issues regarding that aspect of time most common in the literature. That is the question of periodicity, whether actual or perceptual. Some have argued that rhythm and meter arise out of the natural alternations of biorhythms (the beating of the heart, the pattern of breathing) or the ecological nature of interactions between an organism and the environment (the cycles of walking, or any repeated behavior—such as sweeping or hammering). While clearly there is an intuitive appeal to this supposition, the fact that these patterns exist and are perceived and produced by humans in a variety of contexts does not however account for the great flexibility with which humans are capable of recognizing and repeating far more complicated patterns through time. Otherwise, we would have great difficulty in explaining the uniqueness of human musical capacities. Surely other creatures exhibit such patterns in their circulatory and pulmonary behaviors, as well as in their motor activities (any locomotion for instance). A beaver is not musical or rhythmically adept merely because of cyclic repetitions in the gnawing of a log, even if that beaver is well equipped with the cognitive capacity to recognize the sounds of another beaver gnawing away downriver. It is not the mere capacity to produce and recognize temporal patterns, but the cognitive flexibility to willfully reproduce, manipulate, and compare these patterns that characterizes human rhythmic processing.
It is noted that periodic or oscillatory movement is fundamental in humans nearly from birth, and that these periodicities are a principal feature in the human organization of time (cf. Epstein, 1995; Fraisse, 1982). However, such oscillatory movements (rocking, kicking, sucking etc.), correlated with underlying endogenous biorhythms, have been shown not only to be prominent but transient features in the developmental trajectory of human infants toward voluntary control of movements (Iverson & Thelen, 1999; Pouthas, 1996), but also, as noted above, a feature of the locomotive, respiratory and other behaviors of many nonhuman animals (Iverson & Thelen, 1999; see also Port, Cummins & Gasser, 1996). What’s more, attempts to find true periodicity in speech have failed (Port, Cummins & Gasser, 1996; Cruttenden, 1997); Indeed, it has been found that even highly skilled musicians are extremely bad at reproducing and judging precise durations and ratios (Brower, 1993; Sternberg & Knoll, 1994). Even Epstein (1995), who relies so heavily on periodicity in his theories, is forced to acknowledge the discrepancy between actual and perceived periodicities: “If meter feels periodic, that is, it is periodic by standards of musical perception and judgment.” (28)
What Epstein fails to realize however is that evidence of periodicities in the body does not explain the feeling of periodicity that arises when none exists in the stimuli. If the biorhythms are as precise as Epstein’s citations suggest, and if perceptions of regularity, in music for instance, attach to widely varying stimuli and production, clearly it could not be a synchronization between the actual periodicities of the body and the lack of periodicities in the world that gives rise to our perception! The logical conclusion is that any perception of regularity in music and speech, at least at the conscious level, unlikely arises from periodic biorhythms. While clearly the timing of our biological processes do in some way constrain our cognitive processes, the interaction between the physiological and psychological domains in this regard is not well enough understood to draw strong analogies in terms of rhythmic processing. Because, if these analogies held, we would be dealing with perceived periodicities that corresponded to actual periodicities (not merely those that feel periodic as Epstein suggests); likewise the judgments and performance of even the least musical among us could be expected to be precise, which is clearly not the case. Epstein’s appeal for flexibility notwithstanding, it is difficult to conceive of a synchronization between events that do not share the same quality of regularity or irregularity.
It seems therefore that the principal matter to be explained, is not the human capacity for precise temporal calculations, but rather our likely unique capacity to recognize similarity despite surface irregularities, to extract the intended temporal relations from the noisy and imprecise surface of musical and linguistic stimuli. Merlin Donald (1991) has noted: “Rhythm is a uniquely human attribute; no other creature spontaneously tracks and imitates rhythms in the way humans do, without training.” (186) He has argued that rhythmic processing in this way is part of a central faculty of mimesis. I would argue further that our capacity to imitate, to create and reproduce events through time, is most remarkable because of our ability to recognize widely divergent events as similar. Rather than an ability for precision, therefore, human intellect (as reflected in musical and linguistic behaviors among others) is perhaps characterized more by fuzziness, a capacity to bring surface details in and out of focus, as we attend to varying aspects and degrees of similarity.
It is in light of this premise, and with an eye toward future research, that I turn to the task of defining some of the aspects of temporal organization. I recognize principally four interrelated aspects of time:
Beat refers to the perception of a regular pulsation through time. The beat is the regularity we perceive at the most salient level of repetition, the basic unit of measure for the flow of sound through time. At the level of beats, the flow of time is binary—there are beats and the duration between the beats. All beats are of equal value. It should be specified that there is no necessity for beats to be in actuality precisely periodic. Rather there is strong evidence to suggest the opposite. Most current theories that purport to describe linguistic rhythm deal more properly with the existence of beats, and to a lesser extent meter, but rarely if ever deal with the concept of rhythm presented below.
Tempo indicates the speed at which the beats (or prominences) are perceived to recur. As long as the tempo does not vary greatly over a short span of time, the perception of beats should not be disturbed, despite changes in the degree of their actual regularity.
Meter is how we organize beats, and how we create regularity in the grouping of these beats. At the level of meter, beats gain unequal values, as they are divided into repeated patterns of weak and strong alternations. The meter may determine various groupings of beats, for instance: strong-weak, weak-strong, strong-weak-weak, etc. It is most common to think of strength in two levels, though it is possible to recognize higher levels of distinction (i.e. weak-medium-strong, etc.).
In linguistic terms, the marked (i.e. strong) beats are normally described by accent or prominence. The matter is complicated in that prominence, in speech as well as in music, can arise by stress (i.e. increase of amplitude), lengthening of a given duration, or lengthening of the time between event (i.e. change in tempo). While it is certainly not necessary for the meter to remain unchanged (nor even for beats to remain constant throughout a given span of time) a semblance of regularity in beats and their arrangement must be evident for the perception of meter to arise, even momentarily. Necessarily there is inherent flexibility in the perception of regularity.
For perceptual purposes, many similar durations are considered the same duration. That is, many durations that can be distinguished in isolation (psychophysical thresholds) may not be distinguished in context. The mechanism by which such judgements of similar and same are made has not yet been thoroughly studied. While some aspects of meter may occur in natural spoken language (as opposed to recited poetry for instance), it is likely of a far more transient nature than is possible in music.
Rhythm refers to the relationships which obtain between time events (notes, syllables) of differing and similar durations1. Rhythm corresponds to the surface level of production, the perceived sounding durations at a given time and how they relate to one another in terms of ratios (i.e. 1:2, 1:3, 1:4, 2:3, etc.) or in relative terms (longer-shorter, short-medium-long, etc.).
This differs from beats and meter in part because pulsations and their groupings can be perceived despite their absence on the sound surface. For instance, a syncopation may occur wherein the rhythm provides sounds that do not correspond to the location of the perceived beats. Likewise, the meter might deem a strong beat to occur where no sound exists at a given moment in time. That is, the percepts of beat and meter once established can often persist despite countering evidence in the sound source. Further, the durations associated with beats (properly the intervals between beats) are conceived as always similar, and as abstractions, corresponding more to tendencies and expectancies than to actual events.
However, rhythm by definition always corresponds to the perception of sound events, not to their perceived underlying organization. Rhythm does not therefore presuppose the existence of meter, nor certainly does it preclude its presence. Rhythm is likewise not dependent upon the perception of beats, though such a perception may facilitate the conceiving of ratios between durations, by providing a basic unit of measure that is external to the actual durations (i.e. the duration between beats). Relative relationships do not appear to be as reliant upon such an external measure. It remains to be thoroughly studied just how these two approaches to durational relationships (relative vs. specific) interact in naturalistic processing of sound through time.
It will be noted that this definition of rhythm does not address the establishment of expectancies in predicting future events through time. Such predictions may arise from past experience with rhythms (as well as beats and meter), but sequences of events can be meaningfully parsed in terms of the relations between their durations without necessitating longer-term expectancies, and thus without requiring repetitions. While this definition does not require consecutive repetitions, implicit is the fact that rhythms thus conceived are repeatable. If the relations between durations can be described, they can likewise be repeated and recognized in these repetitions. Expectancies of this sort appear to be a separate though related feature of temporal processing.
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