Lateralization


Brain lateralization is a reoccurring phenomenon of interest that has been examined across various disciplines within the realm of neuroscience. Although there has been a lack of direct proof for absolute lateralization, there is increasing evidence to suggest lateral dominance in some neural processes. In order to examine the role of lateralization in music processing, most scientists have adopted a “bottom up” approach, taking the fundamental elements of music such as pitch and tone (most scientists recognize these as being the same and study them synonymously), and rhythm, which allowed for a methodologically consistent way to study and map out respective neural correlates. Note that this topic is still currently heavily debated with arguments for and against lateralization. For example, one recent study has shown that music processing as a whole is right-hemispheric dominant (Hoch & Tillmann, 2010), while another recent study has shown that music processing is right-hemispheric dominant in non-musicians and symmetrically distributed bilaterally in musicians (Ono, et al., 2011). Since the majority of the literature covers rhythm, pitch and tone, these will be the focus for the following subsections.


Rhythm


Rhythm and pitch processes have been shown to be hemispherically specialized. One group of neuroscientists examined rhythm laterality in brain-damaged patients (Prior et al., 1990). Patients with left cerebro-vascular accidents (LCVA) were significantly more impaired in correctly perceiving rhythmic changes when compared to patients with right cerebro-vascular accidents (RCVA). In addition, LCVA patients were more likely to experience greater difficulty in singing novel melodies. Using MEG, another study demonstrated that when musicians exchange non-verbal rhythmic cues as messages whilst producing music, there was a left-lateralized dominance in response to rhythmic incongruence (Vuust, et al., 2005).

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Fig.1. Adapted from Limb et al. (2006). Conjunction analysis of rhythm processing in musicians and non-musicians.

The authors likened this pre-attentive response of non-verbal communication to language. Using functional magnetic resonance imaging, a group of researchers found interesting distinctions and similarities between rhythm processing of musicians and non-musicians (Limb et al., 2006). According to their results, both musicians and non-musicians exhibited activation in bilateral superior temporal areas, left inferior parietal lobule, and right frontal operculum (Fig. 1). However, they did find a significantly greater left lateralization during rhythm perception in musicians compared to non-musicians in the left frontal operculum, superior temporal gyrus, and inferior parietal lobule (particularly within the perisylvian cortices). In their conclusion, as musical training progresses, the left perisylvian brain areas become recruited. Interestingly, the distinction between the neural correlates of musicians vs. non-musicians may be one of the factors that has led to confounding findings regarding brain laterality in rhythm processing (Shapiro et al., 1981).




Pitch and Tone



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Fig. 2. Adapted from Mathys et al. (2010). Top: Coronal section; Bot: Axial section. Relationship of the electrode position to the skull position (HG).
A pioneer study in establishing laterality in sound processing comes from the work of Shapiro et al. with brain damaged patients (Shapiro et al., 1981). They showed that of all of their subjects, those with a right anterior trauma performed significantly worse than all other groups in pitch processing, followed by patients with right central trauma. All the left brain damaged patients performed near perfect on the task. This suggests a right-lateralized dominance in pitch processing. A new line of evidence, working with infants aged three and six months of age, suggests laterality in tone processing (Homae et al., 2012). When presented with distinct, temporally successive tones, both subsets of infants displayed bilateral activity in the auditory areas, and lateralized activity in the right temporoparietal region. A recent innovative study used transcranial direct current stimulation (tDCS) to stimulate either the left or right Heschl’s gyrus (HG; transverse temporal gyrus; Fig. 2) (Mathys et al., 2010), which has previously been shown to play a critical role in pitch discrimination (Trasmo et al., 2005), as well as the overall left or right auditory cortex, whose key contributions have been previously shown as well (Hyde et al., 2008). They used tDCS in order to establish a claim of causality, since neuroimaging techniques cannot directly allow for this. Their results indicate that both the left and right HG are causally involved in pitch processing, but the right auditory cortex plays a more crucial role, which was demonstrated by its higher activation.












Memory and Music


There are several different disciplines of study in memory and music. The following subsections will attempt to cover the primary areas of investigation concerning this topic.


Episodic Vs. Semantic Memory and Music



Very little research has delved explicitly into the study of episodic and semantic memory, and music. Most of the literature on long-term memory encompasses verbal (e.g. language) and visual tasks. However, event-related potential studies, such as the one conducted by Besson and Schon (Besson & Schon, 2001), revealed a substantial distinction between music and language processes in long-term memory. In addition, some clinical studies on amnesic syndromes have revealed maintenance in long-term musical memory integrity, although other long-term memory stores were compromised in other domains (Sanchez, et al., 2004). This suggested that a separate form of information processing may exist for musical long-term memory in particular. The growing evidence obtained from experimental and clinical research opened a new window of investigation into the neural correlates of music and long-term memory.

A very influential study on this subject came from the work of Herve Platel and his colleagues (Platel et al., 2003; Platel, 2005). Not only did his esteemed team investigate the neural correlates for semantic and episodic memory of music, but they also found unique subsets of neural networks for both types of long-term musical memory. In order to study semantic memory for music, they addressed it in terms of familiarity (as that referring to “well-known” excerpts of music stored in memory) without the ability to relate it to its temporal or spatial context. For episodic memory, the participants must have been able to address the temporal and spatial context relevant with their experiences to the tune (either familiar or not). The authors adopted Tulving’s definition for semantic and episodic distinctions when setting up their procedural tasks; they employed positron emission tomography (PET) to collect their neuroimaging data.For episodic memory recall of music, they observed increased activation of the rostral-most part of the middle frontal areas, and the precuneus bilaterally, with a significantly higher activation in the right precuneus (Fig. 3). For semantic memory search on music, they found increased activations in the medial and orbital frontal regions bilaterally, the left hemisphere involving an extensive band of the middle temporal gyrus and extending into the inferior frontal gyrus, and isolated activation of the left angular gyrus (Fig. 4). Considering their data collectively, they confirmed a left hemisphere dominance in semantic memory processing of music, and a right hemisphere dominance in episodic memory processing of music.


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Fig. 3. Adapted from Platel et al. (2003). Brain activity of episodic tasks vs. semantic tasks.




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Fig. 4. Adapted from Platel et al. (2003). Brain activity of semantic tasks vs. episodic tasks.


For memory retrieval of newly composed music, an fMRI study showed an increase in activity of the right hippocampus, bilateral lateral temporal regions, left inferior frontal gyrus and left precuneus (Watanabe et al., 2008). Also, more recent research has looked into the neural representations of autobiographical memory and music (Janata, 2009). Their results showed high activity in the dorsal medial prefrontal cortex (MPFC) when participants were presented with popular music associated with their childhood. In addition, the MPFC exhibited simultaneous activity with the lateral prefrontal and posterior cortices (relating to tonality tracking and overall responsiveness to familiarity of the autobiographically related tune).


Procedural/Implicit Memory and Music



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Fig. 5. Adapted from Bangert et al. (2006). Auditory, motor, and conjunction summary.
Not surprisingly, there is significant evidence to support neural correlate distinctions between declarative and non-declarative memory. Related to musical processing, studies with Alzheimer’s patients have shown that although their explicit musical memory is impaired, their implicit musical memory was spared (Baird & Samson, 2009; Crystal et al., 1989). Current research with cerebellar patients have shown that the cerebellum is critical for procedural learning (Torriero et al., 2007) but it is not necessary for retrieving implicit knowledge stored in long-term memory (Tillmann et al., 2008).






Most of the work related to proceduraland implicit memory on this subject is done on audio-motor conjunction processing with fMRI (Bangert, et al., 2006) (Baumann, et al., 2007). The reason for adapting a conjunction paradigm for analyzing fMRI is to dissociate any external correlates of a putative cognitive auditory-sensorimotor link. Comparing the conjunction data between auditory and motor respective brain related activity, it was found that pianists (compared to non-musicians) revealed a left-hemispheric supramodal network activation, which comprises frontal, temporal, and parietal areas (including pre-motor, motor, and auditory cortex areas) (Fig. 5).









Learning, Working Memory, and Music


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Fig. 6. Adapted from Lahav et al. (2007). An fMRI comparison between trained and untrained musicians.
In order to study the neural correlates associated with learning and music, this next group of scientists employed an auditory-motor association task where the participants had to learn a pitch-to-key-press matching task (Chen et al., 2011). As the participants learned, there was an observed reduction of neural activity in the left dorsal premotor cortex (PMd) and ventral premotor cortex (PMv). Also, learning was associated with a reduction of neural activity in the right superior temporal gyrus (STG). The reduction referred to here is the result of the relative difference in activation between early and late training (overall, however, there was an increase in activity in these regions). Another earlier study’s observations were in accord with the results obtained from Chen’s team, but also found that compared to untrained participants, the trained participants had increased activation (note: activation here is not comparing early vs. late learning, but rather untrained vs. trained) in their pars opercularis, pars triangularis, premotor cortex, bilateral inferior parietal lobule (IPL), and left cerebellum (Lahav et al., 2007) (Fig. 6).




Very recent studies have shown a significant amount of plasticity in the human auditory cortex in response to musical training (Pantev & Herholz, 2011). One study used magnetoencephalography to study the neuronal representations involved in musical training (Herholz et al., 2011). Their results showed that long term musical training can enhance short term auditory learning processes within the auditory cortex, and this plasticity was more prevalent in the left hemisphere. In addition, it was found that musical training leads to significant changes in the adult hippocampus in terms of functional plasticity (Herdener et al., 2010).

With the turn of the new century, there has been an increased interest in working memory and music. By means of fMRI, a comparison study on working memory was made between musician and non-musicians (Schulze et al., 2011). During the working memory task, musicians (compared to non-musicians) demonstrated increased activity in the right pars orbitalis, lateral pre frontal-parietal network, right premotor cortex, right inferior precentral sulcus, and left intraparietal sulcus. A few months later, a PET study came out which examined rhythmic and melodic elements of music in working memory (Jerde et al., 2011). Compared to passive rhythm listening, working memory for rhythm activated the cerebellar hemispheres and vermis, right anterior insular cortex, and left anterior cingulate gyrus. For working memory on melody vs. passive melody listening, there was primarily activation in the right-hemisphere network offrontal, parietal, and temporal cortices.


Lyrical Vs. Instrumental Differences in Musical Processing


There has not been much research done explicitly on the topic of lyrical vs. instrumental processing. However, some researchers have come across some interesting findings. For example, one study examined the electrocardiography (EKG) and electroencephalography (EEG) responses of neonates in depressed and non-depressed mothers (Hernandez-Reif et al., 2006). In both groups of mothers, the neonates demonstrated different EKG and EEG responses to instrumental vs. lyrical music. Another study showed that musical emotion can be elicited more pronouncedly depending on the type of music (Brattico, et al., 2011). For example, relevant lyrics seem to be more influential in conveying sad musical emotions, while the role of acoustic cues seem to be more related to the experience of happiness in music. In addition to these findings, one paper demonstrated that vocal and instrumental musicians process pitch discriminations differently (Nikjeh et al., 2008). In fact, the research showed that vocal musicians who have also been trained with instruments show an overall advantage over vocal musicians or instrumental musicians alone, which in itself suggests superior processing when trained in more than one domain.

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