HOW BRAIN MAPS DEVELOP TO HELP US PERCEIVE THE WORLD
Driving to work
becomes routine -- but could you drive the entire way in reverse gear? Humans,
like many animals, are accustomed to seeing objects pass behind us as we go
forward. Moving backwards feels unnatural.
In a new study, scientists from The
Scripps Research Institute (TSRI) reveal that moving forward actually trains
the brain to perceive the world normally. The findings also show that the
relationship between neurons in the eye and the brain is more complicated than
previously thought -- in fact, the order in which we see things could help the
brain calibrate how we perceive time, as well as the objects around us.
"We were trying to understand
how that happens and the rules used during brain development," said the
study's senior author Hollis Cline, who is the Hahn Professor of Neuroscience
and member of the Dorris Neuroscience Center at TSRI.
This research, published this week
in the journal Proceedings of
the National Academy of Sciences could
have implications for treating sensory processing disorders such as autism.
Reversing the Map
The new study began when Masaki
Hiramoto, a staff scientist in Cline's lab, asked an important question:
"How does the visual system of the brain get better "tuned" over
time?"
Previous studies had shown that
people use the visual system to create an internal map of the world. The key to
creating this map is sensing the "optic flow" of objects as we walk
or drive forward. "It's natural because we've learned it," said
Cline.
To study how this system develops,
Hiramoto and Cline used transparent tadpoles to watch as nerve fibers, called
axons, developed between the retina and the brain. The scientists marked the
positions of the axons using fluorescent proteins.
The tadpoles were split into groups
and raised in small chambers. One group was shown a computer screen with bars
of light that moved past the tadpoles from front to back -- simulating a normal
optic flow as if the animal were moving forward. A second group saw the bars in
reverse -- simulating an unnatural backwards motion. Using the TSRI Dorris Neuroscience
Center microscopy facility, Hiramoto then captured high-resolution images of
these neurons as they grew over time.
The researchers found that
tadpoles' visual map developed normally when shown bars moving from front to
back. But tadpoles shown the bars in reverse order extended axons to the wrong
spots in their map. With those axons out of order, the brain would perceive
visual images as reversed or squished.
Rewriting the Rules
This discovery challenges a rule in
neuroscience that dates back to 1949. Until now, researchers knew it was
important that neighboring neurons fired at roughly the same time, but didn't
realize that the temporal sequence of firing was important.
"According to the old rule, if
there was a stimulus that went backwards, the map would be fine," said
Cline.
The new study adds the element of
order. The researchers showed that objects moving from front to back in the
visual field activated retinal cells in a specific sequence.
Cline and Hiramoto believe that
this sequence helps the brain perceive the passage of time. For example, if you
drive for a few minutes and pass a street sign, your brain will map its
position behind you. If you keep driving and you pass another street sign, your
brain will map out not only the street signs' positions relative to each other,
but their distance in time as well.
This link between time and space in
the visual system might also apply to hearing and the sense of touch. The
original question of how the visual system gets "tuned" over time
might be applicable across the entire brain.
The researchers believe this study
could have implications for patients with sensory and temporal processing
disorders, including autism and a mysterious disorder called Alice in
Wonderland syndrome, where a person perceives objects as disproportionately big
or small. Cline said the new study offers possibilities for retraining the
brain to map the world correctly, for instance after stroke.
Support for the work came from the
National Institutes of Health (EY011261 and DP1OD000458), the Nancy Lurie Marks
Family Foundation and an endowment from the Hahn Family Foundation.
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