Laboratory
Equipment - July 2009
Vibration Isolation Pushes Brain Research Ahead
Negative-stiffness
vibration isolators help researchers to measure micron-level
patterns of neuronal activity and thus shed new light into
cardiac fibrillation and epilepsy.
By Jim McMahon
Isolating a laboratory's
sensitive microscopy equipment against low-frequency vibration
has become increasingly vital to maintaining imaging quality
and data integrity for neurobiology researches. Researchers
are discovering that conventional air tables and the more
recent active (electronic) vibration isolation systems are
not able to cancel out the lower frequency perturbations derived
from air conditioning systems, outside vehicular movements,
and ambulatory personnel.
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Propagating
Waves in Neocortical Slices
At the Georgetown University Medical Center in the Department
of Physiology and Biophysics, Professor Jian-Young Wu
and his colleagues visualize wave-like patterns in the
brain cortex using a new method called voltage-sensitive
dye imaging. The special dye used 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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Figure 1. Image of neuronal patterns
in the brain.
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"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," explains
Dr. Jian-Young Wu.
"Voltage-sensitive dye is a compound that stains neuronal
membrane and changes its color when the neuron is excited,"
Wu continues. "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 up to billions of neurons. Currently, we study
how oscillations and propagating waves can be generated by
small ensembles of neocortical neurons."
Spiraling Waves Shed Light
Wu's imaging team has uncovered spiraling wave patterns resembling
little hurricanes in the brain. "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," says Wu.
But like a hurricane, spiral waves can be a powerful force
for 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 would mean that epilepsy could be viewed not
just as miss-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. These rotating waves can kill a patient
instantly as the pumping function of the heart is disrupted
by the 5- to 10-Hz rotations that drive chaotic and abnormally
rapid cardiac contractions.
Wu believes
that propagating waves are a basic pattern of cortical neuronal
activity, and these wave patterns may play an important role
in initiating and organizing brain activity involving 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.
Schematic
of Negative-Stiffness Isolator
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Figure 2. Schematic of negative-stiffness
isolator
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Vibration Isolation
Because voltage-sensitive dye signals are small, usually a
change of 0.1% to 1.0% of the illumination intensity, Wu's
team has used a high-dynamic-range camera, photodiode array
to detect the signals of the cortical activity. The photodiode
array can resolve extremely small changes in light, usually
one part of ten thousands.
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,"
explains 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."
The Georgetown lab eventually tested and settled upon negative-stiffness
mechanism vibration isolation systems from Minus K Technology,
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
an 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 an
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% isolation efficiency at 2 Hz, 99% at
5 Hz, and 99.7% at 10 Hz.
NSM transmissibility is also improved over active systems.
Because they run on electricity, active systems 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, causing 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.
Continuing Research
Within the past 10 years, Wu's team has documented a variety
of waveforms 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 lab is also involved in developing new optical imaging
techniques "to visualize waves in the cortex in vivo
during sensory processes and in awake animals while behavioral
and cognitive tasks are performed," says Wu.
For more information, contact:
Dr. Jian-Young Wu, Ph.D., wuj@georgetown.edu, www.georgetown.edu
Steve Varma, Minus K Technology, Inc., 310-348-9656, sales@minusk.com,
www.minusk.com
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