Volume 66, Issue 1-2

A general account of rhythm in human behaviour is provided, according to which rhythm inheres in the affordance that a signal provides for the entrainment of movement on the part of a perceiver. This generic account is supported by an explication of the central concepts of affordance and entrainment. When viewed in this light, rhythm appears as the correct explanandum to account for coordinated behaviour in a wide variety of situations, including such core senses as dance and the production of music. Speech may appear to be only marginally rhythmical under such an account, but several experimental studies reveal that speech, too, has the potential to entrain movement.

Like any other aspect of spoken language, rhythm needs to be, and has been, studied from four different perspectives for a comprehensive and insightful account of its nature and functioning in speech communication: symbolic representation, production, perception, communicative function. The paper first gives an overview of the milestones in the analysis of rhythm under the headings of these four approaches over the past 70 years. This survey of the development of scientific ideas in rhythm research prepares the ground for the integration of the four strands in an interrelated framework of linguistic and speech signal analysis. On the basis of a definition of rhythm derived from the theoretical and methodological discussion, a new paradigm is outlined for future research, centred on the listener and on communicative function.

This article reviews the evidence for rhythmic categorization that has emerged on the basis of rhythm metrics, and argues that the metrics are unreliable predictors of rhythm which provide no more than a crude measure of timing. It is further argued that timing is distinct from rhythm and that equating them has led to circularity and a psychologically questionable conceptualization of rhythm in speech. It is thus proposed that research on rhythm be based on the same principles for all languages, something that does not apply to the widely accepted division of languages into stress- and syllable-timed. The hypothesis is advanced that these universal principles are grouping and prominence and evidence to support it is provided.

The Pairwise Variability Index (PVI) has been widely used as a metric for quan-tifying rhythm in languages, often with a view to placing them on a continuum between notional categories of stress-timing and syllable-timing. We review the history of and rationale for the PVI, and point out three potential anomalies in the way the PVI has been applied. Following up one of these we apply the PVI to the level of the foot, and argue that stress-timing and syllable-timing are not points at either end of a continuum but orthogonal dimensions, so that a language can be (for instance) both syllable-timed and stress-timed. Results from Estonian, English, Mexican Spanish, and Castilian Spanish are presented which give some support for this view.

In a production study, Bulgarian, English and German verses with regular poetic metrical metres of different types and elicited prose utterances with varied accentual patterns are produced in textual and iterative ( dada ) form and measured at syllable level according to the pairwise variability index (PVI) principle. Systematic differences in PVI values show that the measure is sensitive to metrical differences. But variations for utterances with the same metrical structure and comparable measures for accentually different utterances show the measure to be insensitive to the temporal distribution of accents. A perceptual experiment with Bulgarian, English and German subjects confirms the hypothesis that the perceived strength of rhythmicity in a line of verse is determined not only by its temporal structure, but also by other acoustic properties, most clearly by F₀ change within the metrical foot.

A perception experiment shows for German that different global, F₀-based speech rhythms in the context section of stimuli influence the local prominence position in the target section. This effect may be conceptualized as a perceptual adjustment of the syllables in the target section to the ones of the global rhythmic context with regard to both the prominence and the F₀ patterns. Two conclusions were drawn on this basis. First, listeners use speech rhythm to predict the perceptual properties of syllables, which is in line with the guide function that speech rhythm is assumed to have in German and other Western Germanic languages. Secondly, speech rhythm is a perceptual phenomenon, generated by a cyclic construction process that involves repetitive patterns in multiple dimensions. Thus, although speech rhythm is initiated by changes in acoustic parameters, it cannot be soaked up by acoustic measurements, especially, if these measurements refer to duration alone.

This study was motivated by the prospective role played by brain rhythms in speech perception. The intelligibility – in terms of word error rate – of natural-sounding, synthetically generated sentences was measured using a paradigm that alters speech-energy rhythm over a range of frequencies. The material com-prised 96 semantically unpredictable sentences, each approximately 2 s long (6–8 words per sentence), generated by a high-quality text-to-speech (TTS) synthesis engine. The TTS waveform was time-compressed by a factor of 3, creating a signal with a syllable rhythm three times faster than the original, and whose intel-ligibility is poor (<50% words correct). A waveform with an artificial rhythm was produced by automatically segmenting the time-compressed waveform into consecutive 40-ms fragments, each followed by a silent interval. The parameters varied were the length of the silent interval (0–160 ms) and whether the lengths of silence were equal (‘periodic’) or not (‘aperiodic’). The performance curve (word error rate as a function of mean duration of silence) was U-shaped. The lowest word error rate (i.e., highest intelligibility) occurred when the silence was 80 ms long and inserted periodically. This is also the condition for which word error rate increased when the silence was inserted aperiodically. These data are consistent with a model (TEMPO) in which low-frequency brain rhythms affect the ability to decode the speech signal. In TEMPO, optimum intelligibility is achieved when the syllable rhythm is within the range of the high theta-frequency brain rhythms (6–12 Hz), comparable to the rate at which segments and syllables are articulated in conversational speech.