Brain complexity: Responses to silence (Introduction)

by David Turell , Sunday, June 19, 2016, 14:15 (3079 days ago) @ David Turell

The world has become very noisy, but the brain has built-in reactions to silence: - http://nautil.us/issue/16/nothingness/this-is-your-brain-on-silence - "In the mid 20th century, epidemiologists discovered correlations between high blood pressure and chronic noise sources like highways and airports. Later research seemed to link noise to increased rates of sleep loss, heart disease, and tinnitus. (It's this line of research that hatched the 1960s-era notion of “noise pollution,” a name that implicitly refashions transitory noises as toxic and long-lasting.) - "Studies of human physiology help explain how an invisible phenomenon can have such a pronounced physical effect. Sound waves vibrate the bones of the ear, which transmit movement to the snail-shaped cochlea. The cochlea converts physical vibrations into electrical signals that the brain receives. The body reacts immediately and powerfully to these signals, even in the middle of deep sleep. Neurophysiological research suggests that noises first activate the amygdalae, clusters of neurons located in the temporal lobes of the brain, associated with memory formation and emotion. The activation prompts an immediate release of stress hormones like cortisol. People who live in consistently loud environments often experience chronically elevated levels of stress hormones. - *** - "Bernardi observed physiological metrics for two dozen test subjects while they listened to six musical tracks. He found that the impacts of music could be read directly in the bloodstream, via changes in blood pressure, carbon dioxide, and circulation in the brain. (Bernardi and his son are both amateur musicians, and they wanted to explore a shared interest.) “During almost all sorts of music, there was a physiological change compatible with a condition of arousal,” he explains. - "This effect made sense, given that active listening requires alertness and attention. But the more striking finding appeared between musical tracks. Bernardi and his colleagues discovered that randomly inserted stretches of silence also had a drastic effect, but in the opposite direction. In fact, two-minute silent pauses proved far more relaxing than either “relaxing” music or a longer silence played before the experiment started. - *** - " One of his key findings—that silence is heightened by contrasts—is reinforced by neurological research. In 2010, Michael Wehr, who studies sensory processing in the brain at the University of Oregon, observed the brains of mice during short bursts of sound. The onset of a sound prompts a specialized network of neurons in the auditory cortex to light up. But when sounds continue in a relatively constant manner, the neurons largely stop reacting. “What the neurons really do is signal whenever there's a change,” Wehr says. - *** - "The sudden onset of silence is a type of change too, and this fact led Wehr to a surprise. Before his 2010 study, scientists knew that the brain reacts to the start of silences. (This ability helps us react to dangers, for example, or distinguish words in a sentence.) But Wehr's research extended those findings by showing that, remarkably, the auditory cortex has a separate network of neurons that fire when silence begins. “When a sound suddenly stops, that's an event just as surely as when a sound starts.” - *** - "As it turned out, even though all the sounds had short-term neurological effects, not one of them had a lasting impact. Yet to her great surprise, Kirste found that two hours of silence per day prompted cell development in the hippocampus, the brain region related to the formation of memory, involving the senses. This was deeply puzzling: The total absence of input was having a more pronounced effect than any sort of input tested. - "Here's how Kirste made sense of the results. She knew that “environmental enrichment,” like the introduction of toys or fellow mice, encouraged the development of neurons because they challenged the brains of mice. Perhaps the total absence of sound may have been so artificial, she reasoned—so alarming, even—that it prompted a higher level of sensitivity or alertness in the mice. Neurogenesis could be an adaptive response to uncanny quiet. - *** - "In 2001, Raichle and his colleagues published a seminal paper that defined a “default mode” of brain function—situated in the prefrontal cortex, active in cognitive actions—implying a “resting” brain is perpetually active, gathering and evaluating information. Focused attention, in fact, curtails this scanning activity. The default mode, Raichle and company argued, has “rather obvious evolutionary significance.” Detecting predators, for example, should happen automatically, and not require additional intention and energy. (my bold) - *** - "During this time when the brain rests quietly, wrote Moran and colleagues, our brains integrate external and internal information into “a conscious workspace.” - "Freedom from noise and goal-directed tasks, it appears, unites the quiet without and within, allowing our conscious workspace to do its thing, to weave ourselves into the world, to discover where we fit in. That's the power of silence." - Comment: Two thoughts. This article shows the intimacy between the brain and bodily reactions. No surprise. But note the bolded paragraph. Our ancient hunter-gatherer reaction are still present. Our brain is built to help us.