Last week, we took a group trip to Sainte Chapelle, a Gothic church in the heart of Paris. Though many visit the church to admire the magnificent collection of 13th century stained glass and extensive collection of Christian relics, we were there to hear an authentic Baroque music concert. Much of the hour-long concert featured a trio of harpsichord, violin, and recorder, but at one point, the recorder player took a solo and switched from playing the soprano version of the instrument to the higher-pitched sopranino. After the recorder player began his solo on the stage, he proceeded to wander throughout the entire chapel—through the audience, along the walls of the chapel, and at one point even directly behind the seating area.
When I said "heart of Paris," I meant it.
As the soloist walked around far behind me, I noticed that I was able to identify where the sound was coming from even though I was facing the opposite direction. I immediately remembered a lecture from neurobiology class that covered the neural mechanisms involved in sound localization. The process depends on the fact that our brain has the ability to compare inputs from both ears. When sound emanates from a source, it usually is not exactly the same distance away from each ear, so when the sound reaches both ears, the difference is calculated and converted into an angle to the direction of the sound (Grothe, 2003). Oftentimes, the difference between arrival at each ear can be as little as 10 microseconds (that’s 1/100,000th of a second)!. This calculation of interaural time difference (ITD) occurs in the medial superior olive, a collection of cells in the brainstem.
Basis of interaural time difference
I was curious, though, if the same process was taking place during the concert. I remembered that the ITD calculations are particularly effective for low frequency sounds, certainly lower than the high-pitched sopranino recorder (Devore and Delguette, 2010). Additionally, I imagined that the tall ceiling and compartmentalized roof, typical of Gothic architecture, would create echoes that further complicate the use of ITD to localize sound. I looked into the matter, and found an article that attempted to explore how the brain’s ability to localize sound is affected by reverberation (Devore and Delguette, 2010). The researchers thought that the lateral superior olive, which uses interaural level difference (ILD) to localize sound rather than ITD, would be preferentially involved in sounds distorted by reverberation. In order to test this hypothesis, the experimenters used a technique called in-vivo electrophysiology, which places a collection of small recording electrodes into the brain and is able to isolate activity from single brain cells in awake, behaving subjects. Once the electrodes were in place, the subjects (in this case, rabbits) were exposed to a variety of auditory stimuli. The stimuli varied in direction, pitch, and reverberant nature so that the researchers could determine which pathways were in use in different situations. They found that the brain relies more heavily on ILD than ITD in reverberant situations, particularly for high-frequency pitches.
Beautiful? Yes. Reverberant? Absolutely.
While researching sound localization, I stumbled across another article that I found to be particularly interesting. I have always been amazed by conductors’ abilities to identify a single individual who misses a note in a 70-person ensemble. Researchers found that the constant exposure to wide field multi source sound environments experienced by conductors actually changes the way that conductors’ brains process the information (Münte et al., 2001). Not surprisingly, conductors outperformed both non-musical controls and pianists in accuracy of sound localization, but interestingly, recordings of brain activity during the task revealed that all groups utilized identical regions of the brain. This finding led the researchers to suggest that the intensive training and exposure that conductors receive simply trains their brain to more effectively use the sound localization pathways that we all use everyday. Take home message: the phrase “mental exercise” has a good deal of validity to it.
– Max Farina
Works Cited:
Devore S, Delguette B (2010) Effects of reverberation on the directional sensitivity of auditory neurons across the tonotopic axis: influences of interaural time and level differences. Journal of Neuroscience 30: 7826-7837.
Grothe B (2003) New roles for synaptic inhibition in sound localization. Nature Reviews Neuroscience 4: 540-550.
Münte TF, Kohlmetz C, Nager W, Altenmüller E (2001) Neuroperception: Superior auditory spatial tuning in conductors. Nature 409.
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