Monday, February 25, 2008

Neural Overview

"Music is auditory cheesecake, an exquisite confection crafted to tickle the sensitive spots of at least six of our mental faculties." - Stephen Pinker

In many research studies of individuals with amusia, there appears to be a number of cortical regions that are involved in processing music. There is some agreement that the primary and secondary auditory cortices and the limbic system are responsible for the appreciation of music. However, recent studies have suggested that
lesions in other cortical areas, abnormality in cortical thickness and deficiency in neural connectivity and brain plasticity, may be other factors causing amusia. While various etiologies of amusia exist, here are some general findings that provide an insight to the brain mechanisms involved in music appreciation.

The right temporal gyrus plays an important role in computing components of frequency, or pitch relations. In particular, the right anterolateral part of Heschl’s gyrus (primary auditory cortex) is concerned with processing pitch information (Tramo et al. 2002). In addition, analysis of pitch change and manipulations of fine tunes are primarily accomplished by the right secondary auditory cortex (Zatorre & Berlin 2001). In melodic tunes, where multiple pitches are involved, the brain recruits contour (pitch directions) and interval (frequency ratios between successive notes) information from auditory input for analysis. It has been found that right superior temporal gyrus is involved in evaluating contour information, while both right and left temporal regions are involved for evaluating interval information (Ayotte, et al. 2000). These studies suggest the right temporal region as the basis for analyzing temporal components of music. Pitch relations also involve musical scales, which are sets of pitches. Because fundamental frequency of musical scale is unequally divided, processing of musical scale is beyond purely analyzing pitch contours and intervals. Although there is evidence to suggest that a specialized area for processing musical harmony and chords exists, the exact location remains undetermined. Listeners often assimilate to what is considered a harmony or chord by passively listening to music. Thus, it could be hypothesized that the ability to analyze harmony is not so much ‘innate’ (Bigand, 2003). Koelsch et al., suggests that through passive listening, children by age five, can recognize harmony to some degree. Given this evidence, congenital amusia may be attributed to the lack of brain plasticity in young children.

In terms of the temporal (rhythmic) relations, there are two components to be addressed: the ability for the brain to segment ongoing sequences of music into temporal events based on duration and to extract the basis of those temporal events to understand the underlying beat to music. In a rhythmic discrimination study performed on individuals with amusia, where they were asked to tap according to either grouping or individually segmenting temporal information, results revealed that the left temporal auditory cortex was responsible for temporal grouping and the right temporal auditory cortex was responsible for individually segmenting (Di Pietro et al, 2004, Wilson, et al, 2002). Thus, it can be hypothesized that one component of temporal relations to music could be spared in the existence of deficits in the other. Other studies have suggested that the cerebellum and basal ganglia may also be involved in processing and controlling perceptual timing (Janata & Grafton, 2003), as well as the participation of motor cortical areas in rhythm perception and production (Halsband, et al, 1993). Given these empirical data, the lack of involvement and networking between bilateral temporal cortices and neural motor centers may be contributing to both congenital and acquired amusia.

Memory is a pertinent aspect in studying the cause of amusia. Because musical events occur over time, a certain amount of memory is required in order to process both melodic and rhythmic aspects of music. The contribution of working memory is also crucial in order to integrate these components and to analyze them on-line. A lesion study, which focused on the working memory for pitch materials in individuals with amusia, has found involvement of the right auditory cortex and frontal cortical areas (Zatorre & Samson, 1991, Gaab, et al., 2003). In a neural imaging study, dorsolateral and inferior frontal areas were activated when working memory load for music was too high for the subject. Both studies suggest that there is a rich interconnection between right temporal gyrus and frontal cortical areas for working memory in music appreciation. Memory is also concerned with the recognition and internal representation of tunes, which helps to identify familiar songs and the ability to, sing a tune in your head. The general findings are that the recognition of a familiar tune is recruited by the activation of superior temporal region and left inferior temporal and frontal areas (Ayotte, et al. 2000), and the right auditory cortex is important for internal representation of tunes because imagery of music requires access to perceptual mechanisms (Zatorre & Halpern, 1993). Again, consolidation of these research findings suggests possible abnormality or lack of interconnectivity between a number of cortical regions in individuals with amusia.

Although the exact cause of amusia is still inconclusive, a number of promising findings reveal that lesions in and the absence of rich associations between the right temporal lobe and inferior frontal lobe, may cause an individual to inadequately process music. Continued research in the area of amusia has given rise to new theories that account for other possible causes of amusia. In particular, cortical thickness and reduced white matter have been suggested to cause this disorder, as well as the involvement of parahippocampal gyrus for emotional reaction to music. It is hoped that persistent dedication to amusia research will provide a more comprehensible idea as to the cause of amusia.


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