Perspectives on Rhythm Processing In Motor Regions of the Brain

Abstract

The field of music therapy is expanding, both in terms of research into mechanisms underlying therapy and strategies to improve practical implementation. In recent decades, neuroscientific investigations into the relationship between music processing and other perceptual and cognitive processes have accelerated, yielding insights that may interest music therapists. Although pitch, melody and intensity are all elements of music that have been investigated, here we discuss the neuroimaging and neuropsychological investigations of rhythm, focusing first on conceptual issues and definitions, and then moving to studies that have often highlighted the importance of two subcortical brain structures: the basal ganglia and the cerebellum.One of rhythm's most fascinating features is the way it can spontaneously induce movement, even in very young infants (Hannon & Trainor, 2007; Hannon & Trehub, 2005; Soley & Hannon, 2010). Sensitivity to musical rhythm appears to be unique to humans; in particular, we are unusual in our ability to rapidly identify a central feature of rhythm called the 'beat.' The beat is the regular time interval that we may tap along to and against which other time intervals in the rhythm can be measured (Large & Palmer, 2002; London, 2004). The presence of beat structure in rhythm is important for perception and behavior, as it improves timing performance accuracy and the ability to discriminate changes in rhythmic sequences (Drake & Gerard, 1989; Grube & Griffiths, 2009; Hebert & Cuddy, 2002; Patel, Iversen, Chen, & Repp, 2005; Ross & Houtsma, 1994). Because of humans' special rhythmic abilities, a variety of researchers are investigating how the brain responds to rhythm, with particular interest in the role of activation of functional motor areas.As many of the words associated with rhythm and timing are used in different ways across different fields, we first clarify how they are used here. 'Rhythm' is defined as the pattern of time intervals in a stimulus sequence. A rhythmic pattern is generally indicated by the onset of a stimulus (a tone, click or other sound). Inter-onset-intervals (the time intervals between stimulus onsets) define the lengths of the time intervals in the sequence. The relative importance of onsets compared to offsets is illustrated by the fact that we can recognize the same rhythm whether played by plucking a string (which produces a brief sound) or bowing (which produces a more sustained sound). Listening to a rhythmic pattern often gives rise to a sense of 'pulse,' sometimes termed the 'beat.' The pulse or beat is a series of regularly recurring, equivalent psychological events that arise in response to a musical rhythm (Cooper & Meyer, 1960; Large, 2008). The beat is psychological because it is not defined as a stimulus property, even though it generally arises in response to a rhythm (Benjamin, 1984; Lerdahl & Jackendoff, 1983; London, 2004; Palmer & Krumhansl, 1990). The psychological internalizing of pulse is why we can sense a pulse even when music is rhythmically complex or has offbeats (many note onsets not occurring on the beat). Individual pulses or beats are frequently perceived to possess differing degrees of accent, or stress, which gives rise to meter, sometimes termed metrical structure. Meter refers to the temporal organization of beats, in which some beats are perceived as more salient than others, on multiple time scales (Epstein, 1995; Lerdahl & Jackendoff, 1983). For example, in a march rhythm, every other beat is accented (1 2 1 2), whereas for a waltz it is every third beat ( 1 23 1 2 3). These patterns therefore differ in their perceived meter: The 1s are strong beats, and the others are weak beats. Most beat-based rhythms are also metrical. In summary, rhythms can induce an internal pulse, and internal organization of these pulses can lead to the perception of a recurring pattern of relative pulse strengths called meter. …