Extracting the Beat: An Experience-dependent Complex Integration of Multisensory Information Involving Multiple Levels of the Nervous System

In a series of studies we have shown that movement (or vestibular stimulation) that is synchronized to every second or every third beat of a metrically ambiguous rhythm pattern biases people to perceive the meter as a march or as a waltz, respectively. Riggle (this volume) claims that we postulate an "innate", "specialized brain unit" for beat perception that is "directly" influenced by vestibular input. In fact, to the contrary, we argue that experience likely plays a large role in the development of rhythmic auditory-movement interactions, and that rhythmic processing in the brain is widely distributed and includes subcortical and cortical areas involved in sound processing and movement. Further, we argue that vestibular and auditory information are integrated at various subcortical and cortical levels along with input from other sensory modalities, and it is not clear which levels are most important for rhythm processing or, indeed, what a "direct" influence of vestibular input would mean. Finally, we argue that vestibular input to sound location mechanisms may be involved, but likely cannot explain the influence of vestibular input on the perception of auditory rhythm. This remains an empirical question for future research

Extracting the beat: An experience-dependent complex integration of multisensory information involving multiple levels of the nervous system

ABSTRACT: In a series of studies we have shown that movement (or vestibular stimulation) that is synchronized to every second or every third beat of a metrically ambiguous rhythm pattern biases people to perceive the meter as a march or as a waltz, respectively. Riggle (this volume) claims that we postulate an "innate", "specialized brain unit" for beat perception that is "directly" influenced by vestibular input. In fact, to the contrary, we argue that experience likely plays a large role in the development of rhythmic auditory-movement interactions, and that rhythmic processing in the brain is widely distributed and includes subcortical and cortical areas involved in sound processing and movement. Further, we argue that vestibular and auditory information are integrated at various subcortical and cortical levels along with input from other sensory modalities, and it is not clear which levels are most important for rhythm processing or, indeed, what a "direct" influence of vestibular input would mean. Finally, we argue that vestibular input to sound location mechanisms may be involved, but likely cannot explain the influence of vestibular input on the perception of auditory rhythm. This remains an empirical question for future research. KEYWORDS: rhythm, meter, vestibular, experience-dependent, multisensory, sound location IN his paper "A simpler explanation for vestibular influence on beat perception: No specialized unit needed", Riggle (this volume) misinterprets our papers In the series of studies cited above, we have shown that when played a metrically ambiguous auditory rhythm (i.e., one with no physical accents), both adults and infants can be biased to perceive it as a march (with perceived accents every second beat) or as a waltz (with perceived accents every third beat) by engaging them in rhythmic movement synchronized to every second or every third beat of the ambiguous auditory rhythm, respectively. Furthermore, we have shown that stimulation of the vestibular nerve in such a way as to give rise to the sensation of head movement synchronized to every second or to every third beat of the metrically ambiguous rhythm (in the absence of actual movement) also biases metrical interpretation for a march or a waltz. We therefore conclude that rhythmic movement can influence auditory rhythm perception, and that the vestibular system appears to play a critical role in this interaction. Riggle's article is centered on the claim that we postulate a "specialized brain unit" for "beat perception" that is "directly" influenced by vestibular input. The first point to note is that our experiments do not simply concern "beat perception" but rather hierarchical metrical perception. And, 32 Empirical Musicology Review Vol. 4, No. 1, 2009 contrary to Riggle's claims, we do not discuss a "specialized brain unit" for "beat perception"; indeed, we state that the physiological locus of the interaction between auditory and movement rhythms remains unknown, and that auditory and vestibular information converges at various subcortical and cortical areas (e.g., see As for Riggle's statement that we claim a "direct" connection between vestibular and auditory input in rhythm perception, we are not even sure what a "direct" connection would mean for a complex task such as extracting the metrical structure of a rhythm pattern. Indeed, vestibular and auditory inputs interact at a number of levels, both subcortically (e.g., dorsal cochlear nucleus; Riggle also claims that we postulate an "innate direct mechanism". However, nowhere do we claim that the interaction between auditory and vestibular systems in processing musical rhythm is innate. In fact, we suggest that experience likely plays a large role in the development of the neural connections that support auditory-vestibular interactions, as infants experience concurrent movement and sound from the onset of hearing. For example, caregivers rock infants while they sing or talk to them, and infants are typically carried for substantial periods of time every day, during which they experience correlated auditory (e.g., sound of footsteps) and vestibular (e.g., movement of the walking) input. Furthermore, a recent study indicates that the degree of influence of movement on the interpretation of ambiguous auditory rhythms in infancy is affected by specific experiences in Kindermusik classes In sum, Riggle's statement that, if beat perception originates from body movement, "then [this] implies the existence of a specialized function in the brain that tracks musical beats and is directly influenced by vestibular system input rhythms" does not make sense. Interactions between the auditory and vestibular systems in metrical perception in no way imply the existences of an "innate" specialized "brain unit" that is "directly" influenced by vestibular input. Now to the interesting part of Riggle's paper. He discusses research by Graybiel Riggle suggests that the results of our 2009 paper Riggle suggests that when the vestibular nerve is stimulated in the absence of physical movement, as in our 2009 paper, it could give rise to a change in the perceived location of a sound. If so, subjects could perceive different beats as coming from slightly different spatial locations and this could bias which beats they hear as accented, and hence whether they perceive the ambiguous rhythm as a march or as a waltz. We think that this explanation is unlikely and, furthermore, even if true, it does not contradict our interpretation of our results. First, the location changes in question are not large, and spatial location is a rather weak cue for determining which beats are accented in music, the main auditory cues being duration, intensity and pitch (e.g., see In the experiment involving stimulation of the vestibular nerve Although sound location mechanisms might be an attractive basis for the interaction between vestibular and auditory rhythms, one further piece of evidence argues against it. Mammals in general are of course good at locating objects in space using vision and hearing in interaction with vestibular input to deal with effects of body movement on location cues. Although we know of no such studies, it would be very surprising if these animals did not show the location illusions that people show when tricked about their perceived self-movement, as integration of vestibular and auditory information is necessary for locating sounds in space. Yet very few species are capable of entraining rhythmic movement to an externally provided auditory beat. Indeed, it appears that this ability is only present in vocal learners such as cockatoos, seals and elephants In summary, we do not postulate a "specialized brain unit" for rhythm that is "directly" influenced by vestibular input. Rather, our data are consistent with the claim that movement in general, and vestibular stimulation in particular, affect the interpretation of metrically ambiguous auditory rhythms in humans. Whether or not auditory-vestibular mechanisms for sound location play a role in 34 Empirical Musicology Review Vol. 4, No. 1, 2009 this interaction is an empirical question for future research. ACKNOWLEDGMENT

The material politics of damming water: An introduction

First paragraph: Hydroelectric power is on the rise. Both developed and emerging economies establish hydroelectric dams in order to make use of natural water resources, contribute to electrification, and supply energy to national industries. For their advocates, dams are a silver bullet combining three pillars of sustainable development that are often perceived as being in mutual conflict, namely, economic growth, social welfare, and ecological sustainability. Dams are often in line with donors' funding priorities relating to low‐carbon energy production and therefore attract major investments by private companies. In short, dam building is seen as a pathway to a bright and promising future, a road to modernity, progress, and—to take the most prevalent prescription for a desirable future—sustainable development