Instrumental music and language are both syntactic systems, employing complex, hierarchically-structured sequences built using implicit structural norms. This organization allows listeners to understand the role of individual words or tones in the context of an unfolding sentence or melody. Previous studies suggest that the brain mechanisms of syntactic processing may be partly shared between music and language. However, functional neuroimaging evidence for anatomical overlap of brain activity involved in linguistic and musical syntactic processing has been lacking. In the present study we used functional magnetic resonance imaging (fMRI) in conjunction with an interference paradigm based on sung sentences. We show that the processing demands of musical syntax (harmony) and language syntax interact in Broca’s area in the left inferior frontal gyrus (without leading to music and language main effects). A language main effect in Broca’s area only emerged in the complex music harmony condition, suggesting that (with our stimuli and tasks) a language effect only becomes visible under conditions of increased demands on shared neural resources. In contrast to previous studies, our design allows us to rule out that the observed neural interaction is due to: (1) general attention mechanisms, as a psychoacoustic auditory anomaly behaved unlike the harmonic manipulation, (2) error processing, as the language and the music stimuli contained no structural errors. The current results thus suggest that two different cognitive domains—music and language—might draw on the same high level syntactic integration resources in Broca’s area.

Funding: This study was financially supported by a Dutch science organization (Nederlandse Organisatie voor Wetenschappelijk Onderzoek: www.nwo.nl/ ) Spinoza Prize awarded to PH and a PhD grant from the Max Planck Society ( www.mpg.de ) to RK. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2015 Kunert et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Introduction

Music and language are uniquely human abilities which, despite their obvious differences, appear to share more than just a common population of users. Specifically, it has been proposed that one overlapping aspect is found in syntactic processing [1]. Syntactic processing—whether in language or in music—involves the integration of discrete elements (e.g., words, tones/chords) into higher order structures (e.g., sentences in language and harmonic sequences in music) according to a set of combinatorial principles that are implicitly understood by members of a culture [1]. Using functional magnetic resonance imaging (fMRI), the present study aimed to find neural evidence for shared syntactic integration resources recruited by both music and language.

In the present study we defined music syntax processing as harmonic structure processing, in line with many previous studies (e.g., [2,3]). Harmony in Western tonal music refers to the organization of pitches in terms of scales, chords, and keys. The basic ‘pitch material’ of Western tonal/harmonic music (henceforth, tonal music) consists of 12 pitches per octave, each representing one of 12 octave-equivalent ‘pitch classes’ (e.g., all the C-notes on a piano keyboard). When playing in a musical ‘key’, a subset of 7 out of 12 pitch classes (in-key tones) is emphasized. Therefore, once a listener has derived a sense of key, e.g., C-major, from a musical piece (for a computational model see [4]) she or he expects certain tones—for example in-key tones such as C—more strongly than others—out-of-key tones such as C# [5,6]. Thus, in tonal music, incoming tones are evaluated in terms of a harmonic framework into which they are continuously integrated.

Do musical and linguistic syntactic processing overlap in the brain? On the one hand, it is known that sensitivity to linguistic syntax and to tonal harmony can dissociate after brain damage, suggesting independence of these two domains (e.g., [7]). On the other hand, there is evidence that linguistic syntactic processing and tonal harmonic processing involve similar brain responses [2,8–10](for a review see [11]). To resolve this paradox, the ‘Shared syntactic integration resource hypothesis’ or SSIRH [1] posited a distinction between domain-specific representations in long-term memory (e.g., stored knowledge of words and their syntactic features, and of chords and their harmonic features) and shared neural resources which act upon these representations as part of structural processing. This “dual-system” model considers syntactic processing to involve the interaction (via long-distance neural connections) of “resource networks” (hypothesized in frontal brain regions) and “representation networks” (hypothesized in temporal brain regions). Patel [1] posited that resource networks are recruited when structural integration of incoming elements in a sequence is costly; that is, when it involves the rapid and selective activation of low-activation items in representation networks. Cognitive theories of syntactic processing in language (dependency locality theory; [12]) and of tonal harmonic processing in music (tonal pitch space theory; [13]) were used to specify the notion of processing cost. In both models, incoming elements incur large processing (activation) costs when they need to be mentally connected to existing elements from which they are “distant” in a cognitive sense (e.g., in music, distant in tonal pitch space rather than in terms of physical distance in Hz; in language, distant in terms of the number of intervening words between a syntactic head and the to-be-integrated word). According to the SSIRH, in such circumstances, activity in frontal brain regions increases in order to rapidly activate specific low-activation representations in temporal regions via reentrant connections. Put another way, music and language share limited neural resources in frontal brain regions for the activation of stored structural information in temporal brain regions (for a similar model specific to language see [14,15]).

The SSIRH predicts that since neural resources for structural integration are limited, simultaneous costly integrations in harmony and language should lead to interference. Testing this prediction requires experiments which present music and language simultaneously, and which align points of difficult structural integration in the two domains. This prediction has been supported in several studies which presented chord sequences and sentences (two using ERPs [16,17] and two using behavioral methods [18,19]) or melodies and sentences (one using ERPs [20] and one using behavioral methods [3]), see [21] for an overview. For example, the behavioral study of Fedorenko et al. [3] (which informed the design of the current neural study) manipulated linguistic syntactic integration difficulty via the distance between dependent words. These researchers manipulated the structure of embedded relative clauses as shown below (italicized):

(a). The boy that helped the girl got an “A” on the test. (b). The boy that the girl helped got an “A” on the test.

The sentences were sung to melodies (one note per word) which did or did not contain an out-of-key note on the last word of the relative clause: ‘girl’ in (a), ‘helped’ in (b). According to dependency locality theory [12], this word is associated with a distant structural integration in (b) (between ‘helped’ and ‘that’) but not in (a). A control condition was included for an attention-getting but non-harmonically deviant musical event: a 10 dB increase in volume on the last word of the relative clause. After each sentence, participants were asked a comprehension question, and accuracy was assumed to reflect processing difficulty. The results revealed an interaction between musical and linguistic processing: comprehension accuracy was lower for sentences with distant versus local syntactic integrations (as expected), but crucially, this difference was larger when melodies contained an out-of-key note. The control condition (loud note) did not produce this effect: the difference between the two sentence types was of the same size as that in the conditions which did not contain an out-of-key note.

However, the brain areas underlying such interaction effects are unclear. Overall, a great number of brain lesion, electrophysiological and hemodynamic brain imaging studies converge in highlighting one key region for syntax processing in either music or language when studied separately: Broca’s area [9,22–26]. Thus this region may be the locus of the interaction effect, either in the left hemisphere and/or in the right hemisphere homologue of this area [9,22,24,27,28].

In searching for interactions between language and music in Broca’s area, the current study was mindful of a confound identified by Rogalsky et al. [29]. Many previous experiments using brain measures have operationalized syntactically challenging processing in language as syntactic violation processing [16,17,20,30]. Therefore, general error processing may be shared between music and language, rather than syntactic processing. We used a language manipulation and a music manipulation which did not involve syntactic violations.

Motivated by the hypothesis that Broca’s area was a neural site of interaction between linguistic and musical syntactic processing, the present study specifically focused on the activation pattern of Broca’s area and its right hemisphere homologue in response to structural manipulations of music and language. Participants heard songs containing either a syntactically easy construction containing only a local dependency (SR: subject-extracted relative clause) or a difficult construction containing a non-local dependency (OR: object-extracted relative clause; see [31]). Sentences were sung a cappella and the critical word which disambiguated between these two linguistic options was either sung on a regular tone (in-key tone which is easy to integrate in the prevailing harmonic context) or on an irregular tone (out-of-key tone which is not easy to integrate harmonically). Thus, the time point of integration difficulty in music was aligned with the one in language.

Note that neither integration difficulty involved errors. Both types of sentences used in the current study were fully grammatical, and differed in syntactic complexity. Similarly, the use of an out-of-key tone in some of the musical melodies increased their complexity in terms of tonal-harmonic structure [32], but such tones would not be considered ‘errors’ because they are common stylistic elements in tonal melodies. For example, the melodies of Schubert’s lieder often contain out-of-key notes, which are considered to play an important role in the pattern of tension and resolution within the melodies [33].

As noted above, a previous behavioral study in English using a similar design showed an interaction between linguistic and musical conditions in terms of sentence comprehension [3]. As in that study, we included a control condition involving a non-syntactic auditory anomaly—presenting the critical tone in-key but 10dB SPL louder—in order to rule out the possibility that any acoustic irregularity would elicit the predicted interaction. (This loudness increment was identical to that used in [3].)

It was hypothesized that Broca’s area would be sensitive to the increased processing difficulty of a concurrent syntactic integration challenge in both music and language. Furthermore, this brain area is not predicted to be sensitive to the interaction between language syntax and a perceptually salient loudness increase at the critical sentence position, as the latter is not syntactic in nature but instead merely acoustic.