The ragas of North Indian Classical Music (NICM) have been historically known to elicit emotions. Recently, Mathur et al. (2015) provided empirical support for these historical assumptions, that distinct ragas elicit distinct emotional responses. In this review, we discuss the findings of Mathur et al. (2015) in the context of the structure of NICM. Using, Mathur et al. (2015) as a demonstrative case-in-point, we argue that ragas of NICM can be viewed as uniquely designed stimulus tools for investigating the tonal and rhythmic influences on musical emotion.

Introduction
Music is the art of sound in time, organized to the principles of pitch, rhythm, and harmony (Limb and Braun, 2008). An important function of music is its capacity to communicate emotions (Tanner and Budd, 1985), a view that has been agreed upon by both music performers (Laukka, 2004) and music listeners (Juslin and Laukka, 2004). Indeed, almost all known forms of music have been recognized for their affective emotional qualities (Goldstein, 1980). However, the exact causal mechanisms by which musical sounds generate emotions are still unclear. Current models posit that specific acoustic factors embedded in a music signal exploit the physical environment, the cognitive and perceptual processing systems, and the structure of the auditory system, to generate emotional responses (Huron, 2006; Thompson and Schellenberg, 2006).

Though the link between music and emotion has been empirically established (Juslin and Sloboda, 2011), most findings lack generalizability across multi-cultural representations of music. Consequently, while music and emotion studies have standardized the use of Western Classical music as a staple source of stimuli, only a handful have incorporated genres of music native to other cultures. This not only precludes interpretations of universality in musical emotions from their findings, it also overlooks musical stimuli which might have advantages as tools for studying musical emotion (Thompson and Balkwill, 2010). The goal of this review is to make such a case for the unique experimental utility offered by North Indian Classical Music (NICM).

In particular, this review will highlight and expand upon the findings of Mathur et al. (2015), to demonstrate that NICM comprises of stimuli that not only permit the study of music and emotional response, but are also uniquely designed stimulus tools to investigate how specific psychophysical features like tonality and rhythm modulate musical emotion as separable factors.

North Indian Classical music (NICM), or Hindustani music, is an ancient musical form of India that emerged from a cultural synthesis of the Vedic chant tradition and traditional Persian music (Kaufmann, 1965). The central notion in this system of music are ragas, which are described as musical compositions capable of inducing specific moods or emotions. Past studies have investigated ragas and have shown that distinct ragas elicit distinct emotions (Balkwill and Thompson, 1999; Chordia et al., 2008; Wieczorkowska et al., 2010). In a study published recently, Mathur et al. (2015) exploited a novel feature of raga stimuli, namely that of different presentation modes, differing in tempo/rhythm but matched in tonal structure, to study music and emotion. They found that when the same raga was presented in distinct presentation modes participants reported elicited emotions with varying levels of arousal. They also found that specific tonal combinations emerged as reliable predictors of emotions that participants reported feeling. These findings indicated that the ragas of NICM not only served as interesting and useful acoustic stimuli that could be exploited to study emotion, but also that the structure of the ragas permitted a systematic, controlled investigation of the role of specific features, namely tonality and rhythm in modulating emotions felt by listeners.

KEY CONCEPT 1. Raga
Modal melodies comprising the canon of North Indian Classical Music. Each raga is constructed from five or more musical notes, organized into one ascending sequence, and one descending sequence of notes, which together comprise a single melodic framework. Performance of a raga is restricted within the note sequences of its ascending and descending halves, but is improvised in all other respects (e.g., timing between notes; sustain, attack of each note).

In this review we expand upon these findings, and make the case that NICM is tailor-made for disentangling tonal and temporal influences on musical emotion, and thus an invaluable stimulus tool worth bringing to the attention of researchers in all cultural contexts. Specifically, we will build evidence to support that NICM provides (1) a catalog of systematically varying emotion valence, best reflected in the Circle of Thaats (described below); and (2) a form of musical stimulus which has embedded in its very structure an experimentally controlled manipulation of rhythm and tempo keeping tonality constant, allowing for the disentanglement of tonal from rhythmic influences on emotion.