WHEN LISTENING TO MUSIC, YOUR BRAIN IS ‘MOVING’ EVEN IF YOU ARE NOT
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WHEN LISTENING TO MUSIC, YOUR BRAIN IS ‘MOVING’ EVEN IF YOU ARE NOT
ATLANTA, October 15, 2006 - Recent findings have uncovered that when listening to a rythmic sound, the motor region of our brain is active even if our body isn't. Research also shows, for the first time, activation of another area of our brain, the visual center, when temporarily blinded individuals recognize an object by touch.
Other new reports include an understanding of how neurons are arranged in the visual cortex, how evolution gave human vision an advantage by being sensitive to three primary colors instead of just two, and a new understanding of synesthesia, a rare condition of crossed senses, such as tasting sounds.
While you listen to music, the areas of your brain that enable your body to move are active, even if you are not. Recent research shows that you don't have to think about the music's rhythm or tap your feet to the beat to engage your brain's motor control areas.
"This finding goes against the traditional view that the brain's motor regions are involved only in executing body movement," says Robert Zatorre, PhD, of McGill University in Montreal.
Does this research help explain the irresistible urge to dance, or at the least, to tap your fingers, when music is played? "Research carried out in our laboratory and in others have already shown that both auditory and motor regions of our brain become engaged when we listen to a musical rhythm and concurrently tap our fingers with it," says Joyce Chen, who collaborated with Zatorre.
"More interestingly, we also know that when we listen to a musical rhythm and just think about, or imagine ourselves, tapping along with it, motor regions of our brain are also engaged," she adds.
Using functional magnetic resonance imaging (fMRI), Zatorre and Chen pinpointed the brain areas in which neurons became active when the human volunteers listened to music. The fMRI measures the changes in blood flow that occur when neurons are active.
The researchers used fMRI to monitor the volunteers' brains during three conditions. In the first, they asked the volunteers just to listen to the music. Next, the volunteers were instructed to anticipate, as they listened to the music, that they they would tap their fingers to the beat of the music. In the third condition, they actually tapped their fingers while listening to the music.
"The results revealed that the brain's motor regions were involved in all three conditions, surprisingly, even when the volunteers were listening to a sequence of sounds that had no explicit association to movement," Chen says.
"The sounds we used sounded like a wood block, that is, they had no pitch, so there was no melody per se. So we really think it's the complex time patterns involved in rhythm that engage the motor system."
Hearing is only one of the senses with previously unknown links in the functional architecture of the brain. Tactile inputs, associated with the sense of touch, exist in the visual brain areas of people who can see, says Amir Amedi, PhD of Harvard Medical School.
"Our findings are important because they reveal that even in people with normal vision, there must be non-visual inputs into the primary visual cortex of their brain, and these inputs can undergo rapid, dynamic changes in strength if a person's sight is lost," said Amedi.
Such research may help identify the brain mechanisms that can allow blind people to become extraordinarily adept in one or more of the other senses.
Using fMRI and transcranial magnetic stimulation in his research, Amedi discovered that the sense of touch encroached into the brain's visual centers in volunteers whose vision is normal but who were blindfolded for five days to simulate sudden, total blindness.
"Because humans rely heavily on visual information to understand the world, large portions of our brain appear specialized to process vision," Amedi says. "Therefore, it is not surprising that the brain of a blind person undergoes changes in order to adapt to the sensory loss by processing tactile, auditory or other sensory information from other senses."
However, Amedi's study showed for the first time that robust and significant activation occurs in the brain visual centers when temporarily blinded individuals recognize an object by touch. The activation of the brain's primary visual cortex, which was absent during the baseline fMRIs, declined dramatically soon after the volunteers removed their blindfolds. Two days later, it was negligible.
"The extremely rapid time-course of the brain's recruitment of the visual cortex for tactile processing suggests that the visual cortex of sighted people have tactile inputs that can be rapidly unmasked by sudden and complete visual loss," he added.
New research results show for the first time that pinwheel centers are the convergence site for the orientation domains in the visual cortex, and that that these singular points in the cortical map exist at the finest possible scale: individual cells. This finding suggests that cortical circuits can be built with tremendous precision.
Even when we are viewing a very simple scene -- for example, a paper clip on a piece of white paper -- the visual signals from our retina must be processed by roughly 30 visual areas, or zones, of the brain. Each area is responsible for interpreting certain defining attributes such as contours of specific orientations. In the visual cortex, neurons that respond to contours of specific orientations are arranged in well-organized orientation maps.
One of the best-studied features in orientation maps is known as a pinwheel, a small region in which all orientations are represented in segments that appear to come to a point. "A long-standing question is, 'How are neurons arranged in the pinwheel centers?'" says R.C. Reid, PhD, of Harvard Medical School.
Reid provided the answer by using two-photon calcium imaging, which determines the physiological response of hundreds of cells simultaneously as well as their precise location in the cortical circuit.
"By recording from hundreds to thousands of neurons at each pinwheel center, we demonstrated that pinwheel centers are remarkably well organized," he says.
"Neurons selective to different orientations are arranged in an orderly manner even in the very center," he adds. "There was virtually no mixing of cells with different orientation preferences even at the center. Thus, pinwheel centers truly represent singularities in the cortical map." This finding is suggesting extraordinary precision in the development of cortical circuits.
Over evolutionary time, color perception has given humans an advantage. "It is known that color facilitates object perception and improves memory of these objects," says Hans-Peter Frey, PhD, of the University of Osnabrueck in Germany.
Many humans and some non-human primates are trichromatic because their eyes contain three types of cones that are sensitive to red, green, and blue. These three cone types work together to convey information about all visible colors. Some humans and most primate species are dichromatic -- with cone types sensitive to only two of the three colors.
Although our visual system allows us to see colors, our ability to perceive movement, depth, perspective, the relative size of objects, shading, and gradations in texture all depend primarily on contrasts in light intensity rather than in color.
Supporting this concept, Frey describes an "ongoing controversy" in color perception: Did trichromacy evolve to enable humans and some non-human primate species to detect ripe fruit embedded in foliage?
To answer this question, he determined whether trichromatic human volunteers took advantage of their ability to discriminate red and green colors. He recorded eye movements of human subjects while they were looking at images of the Kibale rainforest in Uganda. The images showed foliage with embedded fruits. Previously, researchers observed non-human primates in the Kibale rainforest for several months and recorded the color of their food with a spectrometer, Frey explains. They found that ripe fruits could be discriminated from unripe fruits using the blue-yellow color channel, or the red-green color channel.
"This result suggests that there is no advantage of being trichromatic," he says.
In his studies of human volunteers, he did not evaluate the opponent color channels themselves, but instead the color contrast in these channels. He determined that the color contrast of the red-green visual inputs enables trichromatic individuals to detect fruit among foliage, because this color contrast is especially high in parts of the visual scene that contain fruit.
"Thus, dichromatic non-human primates should be less efficient in finding fruit by visual inspection," he points out. " This prediction could also apply to some of the dichromatic humans."
Imagine that when you think of Wednesday, you experience the color magenta. Or when you hear the name Susan, you taste cinnamon in your mouth. Or when you hear a Beethoven symphony, you smell gardenias.
"You may be one in a hundred otherwise normal people who experience the world this way," says David Eagleman, PhD, of the University of Texas, Houston Medical School, referring to the neurological condition called synesthesia.
"In synesthesia, stimulation of one sense triggers an experience in a different sense," says Eagleman. "For example, a voice or piece of music are not only heard but also seen, tasted, or felt as a touch. Synesthesia is a fusion of different sensory perceptions, and most synesthetes are unaware their experiences are in any way unusual." Synesthetic perceptions are involuntary, automatic, and generally consistent over time.
Eagleman will describe a new, large scale genetic study -- called a family linkage analysis -- to map the gene or genes that correlate for color synesthesias.
"Understanding the genetic basis of synesthesia yields insight into the way normal brains are wired," he explains. "And it demonstrates that more than one kind of brain, and one kind of mind, is possible."
