Medical
Design - June 2009
Industry
News
'Dyeing' to figure out how the brain works
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Voltage-sensitive dye and optical recording techniques
are giving neuroscientists at Georgetown University's
Department of Physiology and Biophysics a new means
for figuring out how the brain works.
Professor Jian-Young Wu and his colleagues are conducting
research on waves of neuronal activity. They visualize
wave-like patterns in the brain cortex using a new method
called voltage-sensitive dye imaging. The dye binds
to the membrane of neurons and changes color when electrical
potential changes on the membrane of active neurons.
The neuronal sample is derived from slices of rat neocortex.
The neocortex is the outer layer of the cerebral hemispheres
in the brain of mammals. Made up of six layers, it is
involved in higher functions such as sensory perception,
generation of motor commands and, in humans, language.
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Images of neuronal brain patterns. |
The neurons of the neocortex are arranged in vertical structures
called neocortical columns. These are patches of the neocortex
with a diameter of about 0.5 mm and a depth of 2 mm. Each column
typically responds to a sensory stimulus representing a certain
body part, or region of sound or vision, hi the human neocortex,
there are believed to be about a half-million of these columns,
each of which contains approximately 60,000 neurons.
The neocortex can be viewed as a huge web, consisting of billions
to trillions of neurons and hundreds of trillions of interconnections.
While individual neurons are too simple to have intelligence,
the collective behavior of the billions of interneuronal interactions
occurring each second can be highly intelligent.
Traditionally, scientists have studied brain activity by placing
a few electrodes in the brain and measuring the electrical signals
of the neurons close to the electrodes. This method is workable
for understanding the function of the cortex and interactions
between individual neurons, but it is not suitable for studying
the emerging properties of the nervous system.
"It is like viewing a few pixels on a television screen
and trying to figure out the story," explains Wu. "Now,
with optical methods and voltage-sensitive dyes, we can visualize
the activation in a large area of the neocortex when the brain
is processing sensory information, similar to watching the whole
television screen."
"Voltage-sensitive dye is a compound that stains neuronal
membrane and changes its color when the neuron is excited,"
continues Wu. "This allows us to visualize population neuronal
activity dynamically in the cortex. We study how individual
neurons in the neocortex interact to generate population neuronal
activities that underlie sensory and motor processing functions.
Population activities are composed of the coordinated activity
of billions of neurons. Currently, we study how oscillations
and propagating waves can be generated by small ensembles of
neocortical neurons."
Viewing the spatiotemporal patterns of neuronal population in
the cortex is markedly different from recording individual neurons.
Here the cortical activity is viewed as "population activity,"
which can be more complex than the linear addition of an individual
neuron's activity. Voltage-sensitive dye and optical recording
techniques give the neuroscientist new tools for figuring out
how brain cortex works.
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Negative-stiffness mechanism
vibration isolation systems from Minus K Technology
enabled the lab to get vibration isolation down to a
level of 1 Hz.
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Wu's imaging team has uncovered spiraling wave patterns resembling
little hurricanes in the brain. He believes that this hurricane-like
spiral pattern is an emergent behavior of the network.
"A metrological hurricane is an emergent behavior of
a large volume of air molecules," says Wu. "If you
were to dissect a hurricane into individual air molecules
you would not find any special process that generates a hurricane.
Similarly, in the nervous system, spiral waves are an emergent
process of the neuronal population and there might be no special
cellular process attributed to spirals."
However, like a hurricane, spiral waves can be a powerful
force. Their power is seen when it comes to organizing the
activity of a neuronal population. Spirals generated in a
small area can send out a powerful storm that invades large,
normal brain areas and starts a seizure attack. This hypothesis
means that epilepsy could be viewed not just as "mis-wiring"
in the brain, but as an abnormal wave pattern that invades
normal tissue.
Similarly, during cardiac fibrillation, spiral waves form
in the heart emitting rotating and scroll waves in two and
three dimensions. As a leading life-threatening situation,
these rotating waves can kill the patient instantly as the
pumping function of the heart is disrupted by the 5 to l0
Hz rotations, which drives chaotic and abnormally rapid cardiac
contractions.
Wu believes that propagating waves are a basic pattern of
cortical neuronal activity, and that these wave patterns may
play an important role in initiating and organizing brain
activity involving millions to billions of neurons. Studying
the spatiotemporal patterns of neuronal population activity
may provide insight into normal brain functions and pathological
disorders. This research has the potential to help scientists
understand abnormal waves that are generated in the brains
of patients with epilepsy.
Since voltage-sensitive dye signals are small - a change of
0.1 to 1.0 percent of the illumination intensity - Wu's team
uses a high-dynamic-range camera, photodiode array to detect
the voltage-sensitive dye signals of the cortical activity.
The photodiode array can resolve extremely small changes in
light, usually one part, of ten thousands. (Human eyes and
ordinary digital cameras register light changes of one part
to a hundred). Detecting such small signals requires an extreme
isolation of vibration. The lab had to contend with low frequency
vibrations from air conditioning equipment, people walking,
and wind blowing against the building. Vibrations as low as
1 Hz were inhibiting the integrity of the images and data.
"At first, we used high quality air tables, but they
were not adequate for isolating low frequency vibrations,"
says Wu. "We tried putting a second air table on top
of the first one, but that still did not give us the isolation
we needed. Then we tried an active, electronic system, but
we were still spending much time fighting with floor vibrations.
We were dealing with an unresolved vibration problem for many
years."
The Georgetown lab eventually tested, and settled upon negative-stiffness
mechanism vibration isolation systems from Minus K Technology,
Inglewood, CA, (www.minusk.com) which enabled the lab to get
vibration isolation down to a level of 1 Hz. This effectively
cancelled out any vibration noise difficulties that were inhibiting
image and data readings.
Negative-stiffness mechanism (NSM) isolators have the flexibility
of custom tailoring resonant frequencies vertically and horizontally.
They employ a completely mechanical concept in low-frequency
vibration isolation. Vertical-motion isolation is provided
by a stiff spring that supports a weight load, combined with
a NSM. The net vertical stiffness is made very low without
affecting the static load-supporting capability of the spring.
Beam-columns connected in series with the vertical-motion
isolator provide horizontal-motion isolation. The horizontal
stiffness of the beam-columns is reduced by the "beam-column"
effect. A beam-column behaves as a spring combined with a
NSM. The result is a compact passive isolator capable of low
vertical and horizontal natural frequencies and high internal
structural frequencies.
Transmissibility with negative stiffness is substantially
improved over air systems, which can make vibration isolation
problems worse since they have a resonant frequency that can
match that of floor vibrations. Transmissibility is a measure
of the vibrations that transmit through the isolator relative
to the input vibrations. The NSM isolators, when adjusted
to 0.5 Hz, achieve 93 percent isolation efficiency at 2 Hz;
99 percent at 5 Hz; and 99.7 percent at l0 Hz.
NSM transmissibility is also improved over active systems.
Active systems run on electricity and can be negatively influenced
by problems of electronic dysfunction and power modulations,
which can interrupt scanning. They also have a limited dynamic
range that is easy to exceed. This causes the isolator to
go into positive feedback and generate noise underneath the
equipment. Although active isolation systems have fundamentally
no resonance, their transmissibility does not roll off as
fast as negative-stiffness isolators.
Within the past 10 years, Wu's team has documented a variety
of waveforms (e.g., plane wave and spirals) in brain slices
during artificially induced oscillations. Using neocortical
slices and mathematic models they are studying the initiation
of the waves and the factors that control their propagating
direction and velocity.
The negative-stiffness vibration isolation system are also
used by the lab in the development of optical imaging techniques
such as imaging propagating waves in vivo, within intact brains.
"This is technically difficult," says Wu. "Other
imaging methods (MRI, PET or MEG) provide inadequate spatiotemporal
resolution. Large scale neuronal activity is a hallmark of
a living brain. We hope to visualize the waves in the cortex
in vivo during sensory processes and in animals while behavioral
and cognitive tasks are performed."
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