In what has to be one of the most pleasant brain studies on record, researchers asked subjects to listen to symphonies in order to probe one of the central talents of the brain--its ability to segment the continual stream of sensory information into perceptual chunks to extract meaning. Their studies revealed new details about how the brain circuitry that is key to such "event segmentation" functions, said the researchers.
Led by Vinod Menon of Stanford University School of Medicine, the researchers published their findings in the August 2, 2007, issue of the journal Neuron, published by Cell Press.
In their experiments, the researchers asked subjects to listen to symphonies of the English composer William Boyce. The symphonies were chosen because they are relatively short and comprise well-defined movements. The movement transitions were especially important, because the researchers used them as perceptual events that would enable the analysis of the brain circuitry involved in event segmentation. Movements in musical works are distinguished by such features as changes in tempo, tonality, rhythm, and pitch, and for their brief silences. Using such natural experiences as listening to music rather than artificial stimuli offered important experimental advantages, wrote the researchers.
"Studying event segmentation in real-world or "ecologically valid" stimuli is of particular interest for two reasons: first, such an investigation can reveal perceptual grouping processes that occur under natural conditions; second, there is growing evidence suggesting that neuronal populations behave differently under natural conditions than they do under controlled experimental conditions," they wrote. "For instance, responses of neurons to simple, controlled stimuli are often not predictive of how they respond to more complex, natural stimuli. Currently, the brain systems underlying the segmentation of ecologically valid stimuli, particularly in the auditory domain, are poorly understood."
What's more, wrote the researchers, "Studying such segmentation processes in music may be a useful window into similar processes in other domains, such as spoken and signed language, visual perception, and tactile perception."
As the subjects listened to the symphonies, the researchers scanned their brains using functional magnetic resonance imaging. This widely used technique involves using harmless magnetic fields and radio waves to measure blood flow in brain regions, which reveals brain activity in those regions.
The researchers' analysis of the ten seconds surrounding movement transitions revealed that two distinct networks of brain regions were involved in perceiving the transitions. One was in the ventral region of the prefrontal cortex and one in the dorsal region. The researchers' analysis revealed that activity in the ventral region preceded and caused subsequent activity in the dorsal region. Significantly, these active regions were in the right hemisphere of the brain, which is believed to be involved in processing music.
"Our finding of distinct dorsal and ventral networks in event segmentation provides a novel perspective on auditory scene analysis and significantly expands on our current knowledge of event segmentation processes in the brain," they wrote.
"Although the right hemisphere has been implicated in music processing, its precise role in music perception remains poorly understood, and lateralization of activation for music processing has never been adequately tested. Our findings suggest that the right hemisphere plays a dominant role in the perceptual segmentation of salient, coarse-grained event boundaries in music."
The researchers said their findings suggest that the two regions active in detecting movement transitions "form a tightly coupled network that plays an important role in directing and maintaining attention during the movement transitions and in the perceptual updating that ensues."
They said that the ventral network appears to be involved in the "detection of salient events based on the sensory features of the stimulus stream." This network acts as a "circuit breaker" to activate the dorsal network, which "turns the spotlight of attention to the event boundary."