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Rhythmic entrainment as a musical affect induction mechanism


One especially important feature of metrical music is that it contains periodicities that listeners’ bodily rhythms can adapt to. Recent psychological frameworks have introduced the notion of rhythmic entrainment, among other mechanisms, as an emotion induction principle. In this review paper, we discuss rhythmic entrainment as an affect induction mechanism by differentiating four levels of entrainment in humans—perceptual, autonomic physiological, motor, and social—all of which could contribute to a subjective feeling component. We review the theoretical and empirical literature on rhythmic entrainment to music that supports the existence of these different levels of entrainment by describing the phenomena and characterizing the associated underlying brain processes. The goal of this review is to present the theoretical implications and empirical findings about rhythmic entrainment as an important principle at the basis of affect induction via music, since it rests upon the temporal dimension of music, which is a specificity of music as an affective stimulus.



1. Introduction
1. はじめに

Music is a powerful form of art, because it is able to elicit strong affective experiences, which may not only result in a brief emotional episode or change of mood in the listener, but may also induce motoric behavioral changes in the form of rhythmic motion or even rhythmic dance movements in a social context. Because of these phenomena, music may even manifest in long-term therapeutic effects (de Dreu et al., 2012; Loewy et al., 2013; Thaut et al., 2015). In fact, this social aspect is believed to play a major role in music, by bringing people together to enjoy or celebrate important events in the community, for example. In this sense, music as a social activity promotes common actions between individuals and helps to communicate and even to synchronize actions in a group. Common phenomena include spontaneous synchronization of hand clapping or dancing with the rhythm of the music. Who has not experienced this urge to move with the music when their favorite song comes up? Dance, which is often based on a rhythmic activity in synchronization to the music, has a long tradition in many cultures and across the evolution of mankind (Clayton, Will & Sager, 2005). Moving to music in dance and music listening are thus popular behaviors that seem to be an integral part of our culture. One potential explanation for why such activities are so popular might be that music induces a pleasant state and helps to regulate moods and emotions. Indeed, according to Todd and Lee (2015), these behaviors are intrinsically rewarding and self-reinforcing partly thanks to their stimulation of the vestibular system and its connections with the limbic system. Taking these roles of music together, music can be understood as a social activity that fosters synchronized behavior and stimulates our reward system. This dual role of music raises the question of whether there is a close link between synchronization processes and the affective experiences that music induces. In this review, we suggest that such entrainment processes play an important role in music performance, as well as in music perception, and that these processes might contribute to the emotional power of music. Specifically, we present four different levels in the music listening context in which such entrainment processes can take place. We describe the subjacent brain mechanisms and discuss the links of these processes to the induction of affective experiences in musical contexts.
音楽は強力な芸術の一形態である。なぜなら、音楽は強い効果体験を引き出すことができ、リスナーの短い感情エピソードや気分の変化をもたらすだけでなく、社会的文脈におけるリズム運動やリズムダンスの動きという形で運動行動の変化を誘発する可能性もあるからである。こうした現象から、音楽は長期的な治療効果を現すことさえある(de Dreu et al., 2012; Loewy et al., 2013; Thaut et al., 2015)。実際、音楽には、人々が集まって地域社会の重要な出来事を楽しんだり、祝ったりするなど、この社会的側面が大きな役割を果たすと考えられている。その意味で、社会的活動としての音楽は、個人間の共通行動を促進し、コミュニケーションを図り、さらには集団内の行動を同期させるのに役立つ。よくある現象としては、音楽のリズムに合わせて手拍子をしたり、ダンスをしたりと、自発的な同期が見られる。好きな曲が出てくると、音楽に合わせて動きたくなる、そんな衝動を経験したことがない人はいないのではないだろうか。ダンスは、音楽と同期したリズミカルな活動を基本とすることが多く、多くの文化圏で、また人類の進化を越えて長い伝統を持っている(Clayton, Will & Sager, 2005)。このように、ダンスで音楽に合わせて動くことや、音楽を聴くことは、私たちの文化に欠かせない人気のある行動であると思われる。なぜこのような行動がポピュラーなのか、その理由の1つとして、音楽が心地よい状態を誘発し、気分や感情を調整するのに役立つということが考えられるかもしれない。実際、Todd and Lee(2015)によると、これらの行動は、前庭系への刺激と大脳辺縁系とのつながりのおかげで、部分的に内発的な報酬と自己強化があるそうです。音楽のこれらの役割を総合すると、音楽は、エントレインメント行動を促進し、報酬系を刺激する社会的活動として理解することができる。このような音楽の二重の役割は、音楽が引き起こすエントレインメントプロセスと効果的な経験の間に密接な関連性があるかという問題を提起する。本総説では、このようなエントレインメント過程が音楽知覚のみならず、音楽演奏においても重要な役割を果たしていること、また、これらの過程が音楽の感動的な力に寄与している可能性があることを示唆するものである。具体的には、音楽聴取の文脈の中で、このようなエントレインメント過程が起こりうる4つの階層を提示する。また、隣接する脳メカニズムを説明し、これらのプロセスと音楽文脈における効果的な経験の誘導との関連について議論する。

1.1. Defining entrainment
1.1. エントレインメントの定義

Entrainment is the process through which two physical or biological systems become synchronized by virtue of interacting with each other. However, the term entrainment describes not only the case of perfect period and phase synchronization between two oscillators, but also the tendency toward that state that can be observed between two or more systems capable of emitting periodic outputs. Therefore, perfect synchronization is just one specific case of entrainment. The history of the theory of entrainment can be traced to classical mechanics when Christiaan Huygens first noticed in 1666 that two pendulum clocks set on the same flexible surface would eventually become synchronized (Rosenblum and Pikovsky, 2003). This phenomenon has since been observed between all kinds of devices and organisms exhibiting rhythmic behavior as periodic oscillation. According to Clayton et al. (2005), at least two autonomous oscillating systems must be present in order to distinguish entrainment from other concepts. Autonomy means that the oscillators should be able to oscillate, that is, move about an equilibrium position, on their own even when not interacting. In the human body, many physiological processes can be conceptualized as oscillating systems, including cardiac activity, respiration, locomotion, female menstrual cycles, and the periodic firing of neurons. The condition of autonomy is important to differentiate entrainment from resonance, the latter of which is differentiated from the former by its effects being confined to an increase in an object’s natural frequency amplitude following exposure to another object with an similar frequency. Moreover, the oscillations of a resonating system cease when the active influence of the original impulse emitting system is removed. However, not all interacting oscillators will entrain, because they need to be relatively close in terms of periodicity for the phenomenon to occur and even then, strict phase and frequency synchronization is not necessarily observed. Entrainment can also occur only at the phase level, that is, phase entrainment or phase-locking; although the frequency may not be modified, we can still talk about frequency coupling or cross-frequency coupling. In cross-frequency coupling, the amplitude of a given frequency (e.g. gamma energy at the cortical level) is coupled to the phase of a lower frequency (e.g. alpha or theta; Canolty and Knight, 2010; Fries, 2005).
エントレインメントとは、2つの物理的・生物的システムが相互に作用することによって同期することである。しかし、エントレインメントという言葉は、2つの振動子の周期と位相が完全に同期している場合だけでなく、周期的な出力を出すことができる2つ以上のシステム間で観察されるその状態への傾向も表現する。したがって、完全同期とは、エントレインメント性の一つの具体的なケースに過ぎない。エントレインメント現象の理論の歴史は、1666年にChristiaan Huygensが、同じ柔軟な表面に置かれた2つの振り子時計が最終的に同期することに初めて気づいた古典力学にまでさかのぼることができる(Rosenblum and Pikovsky, 2003)。以来、この現象は、周期的な振動としてリズミカルな振る舞いをするあらゆる装置や生物の間で観察されるようになった。Claytonら(2005)によれば、エントレインメントを他の概念と区別するためには、少なくとも2つの自律的な発振系が存在する必要があるとされている。自律性とは、発振器が相互作用していない状態でも、単独で発振、つまり平衡位置を中心に移動できることである。人体では、心臓の活動、呼吸、運動、女性の月経周期、神経細胞の周期的な発光など、多くの生理的プロセスが振動系として概念化できる。この自律性という条件は、エントレインメントと共鳴を区別するために重要であり、後者は、ある物体が同じような周波数を持つ別の物体にさらされたときに、その物体の固有周波数の振幅が増加するという効果によって、前者から区別される。また、共振系の振動は、元のインパルス発信系の積極的な影響を取り除くと停止する。しかし、相互作用する発振器がすべてエントレインメントするわけではなく、周期性が比較的近いとエントレインメント現象が起こり、それでも厳密な位相や周波数のエントレインメントは必ずしも観察されない。また、位相レベルでのみエントレインメントが起こることもあり、これは位相エントレインメントまたは位相ロックと呼ばれます。周波数が変更されない場合でも、周波数カップリングまたはクロス周波数カップリングと呼ぶことができる。クロス周波数カップリングでは、ある周波数(例えば、皮質レベルのガンマエネルギー)の振幅が、より低い周波数(例えば、アルファまたはシータ;Canolty and Knight, 2010; Fries, 2005)の位相にカップリングされる。

1.2. Entrainment with music
1.2. 音楽によるエントレインメント

Music is a form of art that develops in time. This development is organized into several periodicities for most musical styles. Thus, music constitutes an acoustic signal that often contains different periodicities and therefore can have the properties of an oscillating system at several frequencies. The idea that music can be regarded as an entraining oscillator comes from ethnomusicology, where the effect of especially repetitive music on ritual dancing, including the synchronization of movements and communication between members of a group, has often been studied (Merker et al., 2009). The most obvious situation of entrainment with music is therefore the adaptation of movements and the production of musical rhythm with the body, such as hand clapping or swaying. If a musician is playing and producing rhythmical sequences, the musician can both adapt the music to his or her bodily rhythms, for example, to breathing cycles, and, in turn, his or her bodily rhythms can adapt to a chosen musical rhythm, for example, by accelerating breathing. In this case, proper entrainment can take place, since the interaction can work mutually between the musician’s body and the music. In the case of music listening, the bodily rhythms of the listener can entrain to the music, but the rhythms of the music are not necessarily influenced. Regarding the definition of entrainment, in this condition, the two oscillators interact only in one direction, that is, asymmetrically. However, this phenomenon is still considered a form of entrainment and is described as such in the literature (Large, 2008), since the rhythms of the body of the listener can adapt their phase and/or periodicities to those of the musical rhythms.
音楽は時間の中で展開する芸術の一形態である。この展開は、ほとんどの音楽スタイルにおいて、いくつかの周期性に整理されている。したがって、音楽は、多くの場合、異なる周期性を含む音響信号であり、いくつかの周波数で振動系の特性を持つことができる。音楽がエントレイン性オシレーターであるという考え方は、民族音楽学に由来しており、特に繰り返しの多い音楽が、集団のメンバー間の動作の同期やコミュニケーションを含む儀式的なダンスに与える影響について、しばしば研究されている(Merker et al.) したがって、音楽とのエントレインメントが最もわかりやすい状況は、手拍子や揺れなどの身体による動きの適応と音楽的リズムの生成である。音楽家が演奏してリズムを刻む場合、音楽家は自分の身体のリズム(例えば呼吸サイクル)に音楽を合わせることができ、さらに自分の身体のリズムも、例えば呼吸を加速させるなどして、選択した音楽のリズムに合わせることができる。このとき、音楽と身体の相互作用が働くので、適切なエントレインメントが行われる。音楽を聴く場合、リスナーの身体のリズムは音楽にエントレインメントするが、音楽のリズムは必ずしも影響されない。この場合、エントレインメントの定義としては、2つのオシレーターは一方向にのみ、つまり非対称に相互作用することになる。しかし、リスナーの身体のリズムが音楽のリズムに位相や周期を合わせることができるため、この現象は依然としてエントレインメントの一形態と考えられ、文献上もそのように記述されている(Large, 2008)。

1.3. Investigating rhythmic entrainment as an affect induction mechanism
1.3. 効果発現メカニズムとしてのリズミック・エントレインメントの検討

Rhythmic entrainment has only recently been recognized to play a potential role in affect induction in music listening (Juslin, 2013; Juslin et al., 2010; Scherer and Coutinho, 2013; Scherer and Zentner, 2001; Trost and Vuilleumier, 2013). The BRECVEMA (brain stem reflexes, rhythmic entrainment, evaluative conditioning, emotional contagion, visual imagery, episodic memory, musical expectancy, aesthetic judgment) framework consists of eight psychological mechanisms that explain how music induces emotions in listeners, citing rhythmic entrainment as one of these mechanisms (Juslin, 2013; Juslin et al., 2010). In this context, the authors claim that “the powerful, external rhythm of the music interacts with an internal body rhythm of the listener such as heart rate, such that the latter rhythm adjusts towards and eventually ‘locks in’ to a common periodicity” (Juslin et al., 2010, p. 621). This adjusted rhythm is then proposed to trigger an emotional response much in the same way that smiling could trigger amusement via proprioceptive feedback mechanisms (Strack et al., 1988). However, this definition does not clearly state which kind of entrainment the authors are referring to. Their main focus seems to be on entrainment of physiological processes, but they also appear to suggest a mixed concept in which processes of entrainment at the motor and the social levels are elicited via physiological entrainment. A similar process was also proposed by Scherer and colleagues (Scherer and Coutinho, 2013; Scherer and Zentner, 2001) through their peripheral “proprioceptive feedback” route of emotion production (Scherer and Zentner, 2001), later renamed “entrainment and proprioceptive feedback” route in a revised version of the framework (Scherer and Coutinho, 2013). The close link between rhythm and emotions is also illustrated by the central role of prediction and anticipation. By its very nature, rhythmic entrainment generates expectations, explicitly or implicitly, and it is known that dopamine release is indeed related to musical expectations (Salimpoor et al., 2011). It could be proposed, then, that entrainment is experienced as a desirable and pleasant state, which would explain our attraction toward activities that require entrainment or why we choose to organize some everyday activities and behavior in a rhythmical way. Musical expectancies are punctual anticipations of musical events to take place. It has been suggested that the confirmation or the disappointment of these anticipated musical events produces an emotion (Meyer, 1956). Rhythmic entrainment is also based on temporal expectancies in time and is therefore a specific kind of musical expectancy. These expectancies can appear in different musical contexts, from the listener’s anticipation of the next drumbeat, to the first violinist’s expectation of the director’s subtle hand signal. When considering the potential links between rhythmical entrainment and affect, a crucial question is, what kinds of affect rhythmic entrainment could induce. Indeed, affect can be understood as an overarching term that encompasses experiences such as preferences, attitudes, affective dispositions, interpersonal stances, moods, and emotions (Scherer, 2005). According to authors like Russell (Russell, 1980, 2003), all affective experiences may be described in terms of two basic dimensions: “valence” (or “pleasure”), ranging from negative to positive (or unpleasant to pleasant), and “arousal” (or “activation”), ranging from calm to excited (or deactivated to activated). In the context of their framework, Juslin and colleagues (2010) do not specify other affects beyond “general arousal” or “pleasant feelings of communion.” However, given the evolutionary origins of synchronous activities (Merker et al., 2009) and their suggested function for the community, it seems reasonable to limit the affective experiences induced by rhythmic entrainment to those that have positive valence, strengthen the feeling of unity with others and promote prosocial behaviors within a group, such as communication, cooperation and coordination (Koelsch, 2010). More specifically, using the emotional dimensions of the Geneva Emotional Music Scale (Zentner et al., 2008), which was specifically developed to measure emotions induced by music, this could include experiences of wonder, transcendence, tenderness, or joyful activation. In the following section, we will examine the different ways in which entrainment may induce emotions in listeners by expanding upon a framework developed by Trost and Vuilleumier (2013) where entrainment is proposed to occur at four different levels, even simultaneously.
リズム・エントレインメントが、音楽聴取におけるaffect誘導に潜在的な役割を果たすと認識されたのはごく最近のことである (Juslin, 2013; Juslin et al., 2010; Scherer and Coutinho, 2013; Scherer and Zentner, 2001; Trost and Vuilleumier, 2013)。BRECVEMA (brain stem reflexes, rhythmic entrainment, evaluative conditioning, emotional contagion, visual imagery, episodic memory, musical expectancy, aesthetic judgment) という枠組みは、音楽がリスナーに感情を誘発する仕組みを説明する8つの心理メカニズムからなり、その一つとしてリズム・エントレインメントを挙げている (Juslin, 2013; Juslin et al., 2010). この文脈では、「音楽の力強い外的リズムが、心拍数などのリスナーの内的身体リズムと相互作用し、後者のリズムが共通の周期性に向かって調整され、最終的に『ロックイン』する」(Juslinら、2010、p. 621)と主張している。この調整されたリズムは、微笑みが自己受容フィードバック機構を介して娯楽を引き起こすのと同じように、情動反応を引き起こすことが提案されている(Strack et al.、1988)。しかし、この定義では、著者らがどのようなエントレインメントに言及しているのかが明確に示されていない。彼らの主眼は生理的なエントレインメントにあるようだが、生理的なエントレインメントを介して運動レベルや社会レベルのエントレインメント過程が引き出されるという混合概念も示唆しているようである。同様のプロセスは、Scherrら(Scherr and Coutinho, 2013; Scherer and Zentner, 2001)も、感情生成の末梢「固有受容フィードバック」経路を通じて提案しており(Scherr and Zentner, 2001)、後に枠組みの改訂版で「エントレインメントと固有受容フィードバック」経路と改名されている(Scherr and Coutinho, 2013)。リズムと感情の密接な関係は、予測・予感の中心的な役割によっても示されている。リズム・エントレインメントは、その性質上、明示的または暗黙的に期待を生み出す。そして、ドーパミンの放出は、実際に音楽の期待と関連していることが知られている(Salimpoor et al.、2011)。このことは、エントレインメントを必要とする活動や、日常の活動や行動をリズミカルに構成することに魅力を感じる理由を説明することができる。音楽的予期とは、音楽的な出来事が起こることを時間通りに予期することである。この期待された音楽的事象の肯定や失望が感情を生み出すことが示唆されている(Meyer, 1956)。リズミック・エントレインメントもまた時間的な期待に基づくものであり、したがって音楽的期待の一種である。このような期待は、次のドラムビートを予期するリスナーから、監督の微妙なハンドシグナルを予期する第一バイオリニストまで、様々な音楽の文脈で現れる。リズムエントレインメントとaffect の関係を考える上で重要なことは、リズム・エントレインメントがどのようなaffect を引き起こすかということである。実際、affect は、嗜好、態度、感情、対人スタンス、気分、感情などの経験を包括する言葉として理解することができる(Scherer, 2005)。ラッセル(Russell, 1980, 2003)のような著者によれば、すべてのaffective experienceは2つの基本的な次元で記述することができる。ラッセル(Russell, 1980, 2003)によれば、すべての効果的経験は、ネガティブからポジティブ(または不快から快)までの「価数」(または「快」)と、冷静から興奮(または不活性から活性化)までの「覚醒」(または「活性化」)の2つの基本次元で記述される。Juslinら(2010)は、この枠組みの中で、”一般的な覚醒 “や “交感の快感 “以外の効果を明示していない。しかし、同期活動の進化的起源(Merker et al., 2009)やコミュニティに対する機能が示唆されていることから、リズム・エントレインメントが引き起こす効果的な経験は、正の価を持ち、他者との一体感を強め、コミュニケーション、協力、協調といったグループ内の向社会的行動を促進するものに限定することが妥当であると思われる(Koelsch, 2010)。具体的には、音楽によって誘発される感情を測定するために特別に開発されたGeneva Emotional Music Scale (Zentner et al., 2008)の感情の次元を用いると、驚き、超越、優しさ、喜びの活性化などの体験が含まれる。以下では、Trost and Vuilleumier(2013)が開発した、4つのレベルで同時に発生するとされるエントレインメントのフレームワークを基に、エントレインメントがリスナーにどのような感情を誘発するのかについて検討する。

2. Levels of entrainment and their role in affect induction
2. エントレインメントのレベルとffect 誘導におけるその役割

Trost and Vuilleumier (2013) have pointed out that in humans, entrainment behavior can be observed and distinguished on four levels: perceptual, autonomic physiological, motor, and social (see Fig. 1). These levels may in fact become integrated and contribute to the emergence of a subjective feeling of entrainment (Labbé and Grandjean, 2014). Although rhythmic entrainment phenomena can be seen as a hierarchical process, the exact form in which these different levels coexist or interact is still being researched. Furthermore, there are many aspects of the link between rhythmic entrainment and affect induction that remain unclear. Using the framework presented by Trost and Vuilleumier (2013) as our basis, we will review the different levels, provide empirical evidence for them from the literature in music psychology and auditory neuroscience, discuss the underlying brain mechanisms involved, and the link with affective experiences. The present review thus aims to describe the different levels in more detail and to present rhythmic entrainment in a more comprehensive manner. First, however, we will discuss the principle of neural entrainment and distinguish it from the other four levels. We make this distinction because neural entrainment is proposed as a general principle of neural communication (Fries, 2005) that is unspecific to music or affect (see Fig. 1). Because the main focus of this review is the link between entrainment processes and affect, however, we will mention neural entrainment only briefly. For more information concerning the specific field of neural entrainment and synchronization, we refer the reader to Buzsáki (2006).
Trost and Vuilleumier(2013)は、ヒトの場合、エントレインメント行動は知覚的、自律生理的、運動的、社会的の4つのレベルで観察・区別できると指摘している(Fig.1参照)。これらのレベルは、実際には統合され、エントレインメントの主観的な感覚の出現に寄与している可能性がある(Labbé and Grandjean, 2014)。リズム・エントレインメント現象は階層的なプロセスとして捉えることができるが、これらの多様なレベルがどのような形で共存・相互作用するかはまだ研究されていない。さらに、リズム・エントレインメントとffect誘導の関連性についても不明な点が多い。そこで、Trost and Vuilleumier (2013)が提示したフレームワークを基に、各レベルを概観し、音楽心理学や聴覚神経科学の文献からその実証的証拠を示し、関連する脳内メカニズムや効果体験との関連性を議論する。このように、本総説では、各レベルをより詳細に説明し、より包括的にリズム・エントレインメントを紹介することを目的としている。しかし、最初に神経エントレインメントの原理を説明し、他の4つのレベルと区別する。このように区別するのは、神経エントレインメントが音楽やffectに特定されない神経伝達の一般原理として提案されているからである(Fries, 2005)(図1参照)。しかし、このレビューの主眼はエントレインメントプロセスとffectの関連性であるため、神経エントレインメントについては簡単にしか触れない。神経エントレインメントおよび同期という特殊な分野については、Buzsáki (2006)を参照されたい。

2.2. Perceptual entrainment

Perceptual entrainment is defined as the process whereby auditory signals are integrated into a percept of the periodicities in an auditory input. Perceptual entrainment can be regarded as the process that is necessary to perceive periodic information, such as the musical beat Being entrained on the perceptual level creates a conscious or unconscious representation of a periodic rhythmical pattern, which should be distinguished from neural entrainment with regard to the level of processing, even if perceptual representations are rooted in neuronal entrainment. Perceptual entrainment requires an integration of sensory information and top-down influences (e.g. expectations) to form a cognitive representation of the auditory signal, which can be either accessible or inaccessible to consciousness. For perceptual entrainment, a kind of temporal pattern recognition has to take place in order to extract the periodic information contained in the auditory signal, which can happen very fast, as soon as the first couple of beats of the music are heard (Krumhansl, 2000). It is a general principle that when events happen in time with certain periodicities, temporal expectancies are engendered about when the next events are going to happen (for a review see Nobre and Rohenkohl, 2014). The same mechanism is at work in musical rhythms, and thus temporal expectancies are also at the basis of perceptual entrainment. In perceptual entrainment temporal expectancies play an important role as attention is permanently oriented and reoriented in time along the rhythmical structure and in particular along the musical beat. The beat in music, which is sometimes also called the pulse, is a psychological construct that is perceived because of repeatedly recur ring musical events in a specific time interval. The beat can be understood as the basic unit of measurement of evenly spaced events while the meter “involves our initial perception as well as subsequent anticipation of a series of beats that we abstract from the rhythmic surface of the music as it unfolds in time” (London, 2004, p. 4). The meter in music refers to the temporal organization of beats; it assigns a hierarchy between the beats belonging to one metrical unit by defining a metric for these beats (Lerdahl and Jackendoff, 1983). This defined metrical hierarchy allows the distinction between beats that occupy different levels in the hierarchy, which are perceived as being more or less salient. More salient beats are higher in the hierarchy and are often called “strong beats” (i.e. the first beat of a 4/4 time signature), and less salient beats, at lower levels of the hierarchy are called “weak beats (i.e. off-beat positions, like the second and fourth beat of a 4/4 time signature). A psychological theory that tries to explain the perception of the musical meter in terms of entrainment processes is the Theory of Dynamic Attending (Jones and Boltz, 1989). This theory suggests that the periodicities in a musical rhythm can only be perceived because of entrainment of attentional processes to the different periodicities of the musical meter (Jones and Boltz, 1989). Therefore, the recognition of a periodic pattern and even a metrical hierarchy between the beats presented in metrical music requires cognitive processing and thus constitutes a form of perceptual entrainment, even when it is implicit (Bolger et al., 2013; Grube and Griffiths, 2009). This implicit percep tual entrainment of attentional processes can be tested in reaction time paradigms. In keeping with the theory of dynamic attending, selective attention is increased in moments of the musical beat, and even more during metrically salient beats, which can lead to increased accuracy and faster processing during these moments. Accordingly, it has been shown that reaction times to visual and auditory targets are signifi cantly reduced when these targets are presented during metrically strong positions with both simple metrical sequences and classical music excerpts (Bolger et al., 2014, 2013; Trost et al., 2014). Perceptual entrainment effects can also be explained by predictive coding and predictive timing theories of perception that suggest that our predictions are the internal representation of the stimulus (Arnal and Giraud, 2012; Friston, 2005; Giraud and Poeppel, 2012), which is consistent with models of meter perception and particularly with the Theory of Dynamic Attending. These theories suggest that, when a pattern of beats is heard one forms a metrical representation of the rhythm allowing one to anticipate future beats. This representation is then constantly updated as the physical input (rhythm) is compared with the internal prediction (meter), which is corrected as needed, the difference between the two (the “prediction error”) being the only thing that is actually processed by the brain for maximum efficiency (Vuust and Witek, 2014). One example of perceptual entrainment is the phenomenon of subjective accenting. Subjective accenting demonstrates that the perception of rhythm is, to some extent, subjectively driven (Bolton, 1894). This phenomenon can be observed when a person is listening to identical isochronous clicks, but perceives a rhythmical pattern or grouping of these clicks instead of identical events, as in the tick tock of a clock, for example. This can even be reflected in greater amplitudes of event related potentials to subjectively “strong” beats in a series of equally spaced and equally loud tones (Brochard et al., 2003; Potter et al., 2009). Subjective accenting could be explained by resonance theories of rhythm perception (Large, 2008; Large and Jones, 1999; Large and Kolen, 1994), which propose that both the beat and the metrical structure of a piece of music (or any auditory stimulus) emerge in the mind of listeners as internal oscillators with a similar frequency resonate and entrain to the frequencies in the auditory signal. As these oscillators resonate, their amplitudes increase and it is the sum of these amplitudes that then gives rise to an impression of a hierarchical structure. When the auditory stream is an isochronous sequence, it has been proposed that internal oscillators at frequencies close to that of the isochronous sequence but also close to subharmonics of this frequency resonate and entrain, causing a hierarchical structure to be perceived (Bååth, 2015). We also know that a strong sense of both the beat and the meter can be induced through temporal accenting, that is, not necessarily by playing a note louder or softer or even changing its pitch, but simply by manipulating the length of the silences preceding and following a note (London, 2004). In one study, Grube and Griffiths (2009) manipulated both the strength of metrical sequences and the clarity of their endings. Participants reported feeling the pulse most strongly on sequences with more notes during the downbeat (strong sequences), and they accented the last downbeat of a sequence (compact ending) as though it were still playing during the rests until the end. Perceptual entrainment could also be regarded as the underlying mechanism of altered states of consciousness like trance that is induced via listening to repetitive sounds. These trance states can be compared to the experience of flow and are usually evoked to gain insight and are therefore states of high concentration (Hove et al., 2016). A number of studies, using a range of techniques from intracranial and cortical EEG to MEG, have shown evidence for perceptual entrainment and demonstrated how neuronal activity can become entrained to cognitively perceived periodicities in simple rhythms (Fujioka et al., 2014, 2013, 2010; Fujioka, Trainor et al., 2009, 2012; Nozaradan et al., 2011, 2012, 2015; Snyder and Large, 2005; Will and Berg, 2007). Empirical research in newborn babies in which the researchers used EEG suggests that detecting regularities in auditory rhythmic sequences seems to be innate or learned during the pregnancy (Winkler et al., 2009). This may indeed be the case since the fetus is already exposed to rhythmic auditory stimulations in the womb primarily in the form of the mother’s heartbeat. Perceptual rhythmic entrainment is therefore already trained at this early developmental stage, for example, in the context of rhythmic motor actions produced by the mother.