Hearing abilities also oscillate and depend on the exact timing of one’s brain rhythms. This discovery that sound, brain, and behavior are so intimately coupled will help us to learn more about listening abilities in hearing loss and perhaps people who stutter.
Our world is full of cyclic phenomena; some humans are 'morning people' and some are 'night owls', with different levels of alertness, even over the course of a day. Maybe you yourself are more alert in the morning, others more in the afternoon. Our bodily functions oscillate with environmental rhythms, like light and dark, and this in turn seems to influence our perception and behaviors. We're all beholden to our circadian rhythms, which in turn are slaves to environmental light–dark cycles.
A hard-to-prove idea in neuroscience has been that such couplings between rhythms in the environment, rhythms in the brain, and our behaviors are also present at much finer time scales. This idea holds fascinating implications for the way humans process speech and music: Imagine the melodic contour of a human voice or your favorite piece of music going up and down. If your brain becomes coupled to these melodic changes, then you might also be better prepared to expect fleeting but important sounds occurring in what the voice is saying, for example, a “d” versus a “t”.
The simple “fleeting sound” researchers used in their experiment was a very short and very hard-to-detect silent gap (about one one-hundredth of a second) embedded in a simplified version of a melodic contour, which slowly and cyclically changed its pitch at a rate of three cycles per second (3 Hz).
As the listener'' brain rhythms synchronize with the acoustic stimulus, hearing abilities "oscillate". Credit: Sebastian Willnow
To be able to track each listener’s brain activity on a millisecond basis, Henry and Obleser recorded the electroencephalographic signal from listeners’ scalps. First, the authors demonstrated that every listener’s brain was “dragged along” (this is what entrainment, a French word, literally means) by the slow cyclic changes in melody; listeners’ neural activity waxed and waned. Second, the listeners’ ability to discover the fleeting gaps hidden in the melodic changes was by no means constant over time.
Instead, it also “oscillated” and was governed by the brain’s waxing and waning. The researchers could predict from a listener’s slow brain wave whether or not an upcoming gap would be detected or would slip under the radar.
Why is that? “The slow waxings and wanings of brain activity are called neural oscillations. They regulate our ability to process incoming information”, Molly Henry explains. Jonas Obleser adds that “from these findings, an important conclusion emerges: All acoustic fluctuations we encounter appear to shape our brain’s activity. Apparently, our brain uses these rhythmic fluctuations to be prepared best for processing important upcoming information.”
Citation: Henry, M.J.&Obleser, J., 'Frequency modulation entrains slow neural oscillations and op-timizes human listening behavior', Proceedings of the National Academy of Sciences, Online November 12, 2012.
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