By using a novel technique to
test brain waves, researchers are discovering how the brain processes external
stimuli that do and don't reach our awareness. The researchers used both
electroencephalography (EEG) and the event-related optical signal (EROS),
developed in the Cognitive Neuroimaging Laboratory. While EEG records the
electrical activity along the scalp, EROS uses infrared light passed through
optical fibers to measure changes in optical properties in the active areas of
the cerebral cortex. Because of the hard skull between the EEG sensors and the
brain, it can be difficult to find exactly where signals are produced. EROS,
which examines how light is scattered, can noninvasively pinpoint activity
within the brain. EROS is based on near-infrared light and exploits the fact
that when neurons are active, they swell a little, becoming slightly more
transparent to light.
This allowed the researchers to
not only measure activity in the brain, but also allowed them to map where the
alpha oscillations were originating. Their discovery: the alpha waves are
produced in the cuneus, located in the part of the brain that processes visual
information. The alpha can inhibit what is processed visually, making it hard
for you to see something unexpected. By focusing your attention and
concentrating more fully on what you are experiencing, however, the executive
function of the brain can come into play and provide ‘top-down’ control --
putting a brake on the alpha waves, thus allowing you to see things that you
might have missed in a more relaxed state. They found that the same brain
regions known to control our attention are involved in suppressing the alpha
waves and improving our ability to detect hard-to-see targets.
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