Via This is your brain on silence :

While it’s clear that external silence can have tangible benefits, scientists are discovering that under the hoods of our skulls “there isn’t really such a thing as silence,” says Robert Zatorre, an expert on the neurology of sound. “In the absence of sound, the brain often tends to produce internal representations of sound.”

Imagine, for example, you’re listening to Simon and Garfunkel’s “The Sound of Silence,” when the radio abruptly cuts out. Neurologists have found that if you know the song well, your brain’s auditory cortex remains active, as if the music is still playing. “What you’re ‘hearing’ is not being generated by the outside world,” says David Kraemer, who’s conducted these types of experiments in his Dartmouth College laboratory. “You’re retrieving a memory.” Sounds aren’t always responsible for sensations—sometimes our subjective sensations are responsible for the illusion of sound.

This is a reminder of the brain’s imaginative power: On the blank sensory slate of silence, the mind can conduct its own symphonies. But it’s also a reminder that even in the absence of a sensory input like sound, the brain remains active and dynamic.

In 1997, a team of neuroscientists at Washington University was collecting brain scan data from test subjects during various mental tasks, like arithmetic and word games. One of the scientists, Gordon Shulman, noticed that although intense cognition caused spikes in some parts of the brain, as you’d expect, it was also causing declines in the activity of other parts of the brain. There seemed to be a type of background brain activity that was most visible, paradoxically, when the test subject was in a quiet room, doing absolutely nothing.