How to Measure Neuron Activity with Vibration Isolators

In the Department of Physiology and Biophysics at Georgetown University, Researchers utilize negative-stiffness vibration isolators to help evaluate micron-level patterns of neuronal activity in the mammalian neocortex.

The research is revealing new information about motor processing and brain sensory functions in the case of epilepsy and cardiac fibrillation.

Isolating sensitive microscopy equipment from a laboratory against low-frequency vibration has become ever more crucial in the maintenance of data integrity and imaging quality for researchers in neurobiology.

Laboratory researchers are increasingly finding that traditional air tables, and the more current active (electronic) vibration isolation systems, are unable to properly cancel out the lower frequency perturbations as a result of air conditioning systems, outside of ambulatory personnel and vehicular movements.

This was the case at Georgetown University’s Medical Center in the Department of Physiology and Biophysics, where Professor Jian-Young Wu, Ph.D. has been performing research on neuronal activity waves in the neocortex of the brain.

Propagating Waves in Neocortical Slices

Wu and his colleagues envision patterns similar to waves in the brain cortex by employing a new technique known as voltage-sensitive dye imaging. A special dye is used that binds to the neuron membrane and changes color when the electrical potential on the membrane of active neurons changes.

The neuronal sample is taken from slices of rat neocortex. The neocortex is the outer layer of the cerebral hemispheres in the mammalian brain. It contains six layers and is involved in higher functions, for example, the generation of motor commands, sensory perception, and language in humans.

The neurons of the neocortex are organized in vertical structures known as neocortical columns. These patches of the neocortex have a depth of 2 mm and a diameter of around 0.5 mm. Every column normally reacts to a sensory stimulus, signifying a particular part of the body, or a region of vision or sound.

It is suggested that the human neocortex contains an estimated half-million of these columns and each of these comprise around 60,000 neurons. The neocortex can be imagined as a complex web containing billions to trillions of neurons and hundreds of trillions of interconnections.

Individual neurons are too basic to possess intelligence, but the behavior of the billions of collective interneuronal interactions happening every second can possess great intelligence.

Scientists have historically researched brain activity by putting some electrodes in the brain and analyzing the electrical signals of the neurons near the electrodes.

This technique is effective when aiming to understand interactions between individual neurons and the function of the cortex, but it is not appropriate for investigating the emerging characteristics of the nervous system.

It is like viewing a few pixels on a television screen and trying to figure out the story,” describes Dr. JianYoung 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,” explains Dr. 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 up to billions of neurons.”

Dr. Wu continues, “Currently, we study how oscillations and propagating waves can be generated by small ensembles of neocortical neurons.” Observing the spatiotemporal patterns of neuronal population in the cortex is significantly different from reporting individual neurons.

The cortical activity, in this case, is looked upon as ‘population activity’, which can be more complicated than the linear addition of a particular neuron’s activity. Optical recording methods and voltage-sensitive dye provide the neuroscientist with a novel tool to understand how the brain cortex functions.

Spiraling Waves in the Brain Shed Light on Cardiac Fibrillation and Epilepsy

Spiraling wave patterns, which look like small hurricanes in the brain, have been revealed by Wu’s imaging team. He suggests that this spiral pattern resembling a hurricane is an emergent network behavior.

A metrological hurricane is an emergent behavior of a large volume of air molecules,” explains Dr. Wu. “If you were to dissect a hurricane into individual air molecules you would not find any special process that generates a hurricane.”

Dr. Wu continues, “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.” Similar to a hurricane, spiral waves can be a powerful tool for managing the functions of a neuronal population.

Spirals created in a small area can deliver a powerful storm that takes over large, healthy brain areas and begins a seizure attack. This theory suggests that epilepsy is not only a mis-wiring in the brain but also an unusual wave pattern that takes over normal tissue.

During cardiac fibrillation, spiral waves develop in the heart generating scroll and rotating waves in two and three dimensions.

As one of the most common life-endangering situations, these rotating waves can instantly kill the patient because the heart’s pumping function is disturbed by the 5 to 10 Hz rotations. This results in unusually rapid and chaotic cardiac contractions.

Wu thinks that propagating waves are a simple pattern of cortical neuronal activity and that these wave patterns may have an essential role in organizing and initiating brain activity comprising of millions to billions of neurons.

Researching the spatiotemporal patterns of neuronal population activity may give additional information about pathological disorders and normal brain functions.

This research can assist scientists in comprehending the abnormal waves that develop in the brains of epilepsy patients.

Vibration Isolation

As voltage-sensitive dye signals are small, normally with a variation of 0.1 to 1% of the illumination intensity, Wu’s team has utilized a high-dynamic-range camera and photodiode array to perceive the voltage-sensitive dye signals of the cortical activity.

The photodiode array can resolve incredibly small variations in light, normally one part of ten thousand. To compare, human eyes and normal digital cameras resolve light changes of one part to a hundred. The detection of such tiny signals necessitates extreme isolation of vibration.

The laboratory had to face the challenges of low-frequency vibrations, where vibrations as low as 1 Hz were reducing the integrity of the data and images. These were due to wind blowing against the building, people walking past, and air conditioning equipment.

At first, we used high quality air tables, but they were not adequate for isolating low frequency vibrations,” Dr. Wu states.

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 laboratory eventually tested and chose to use the negative-stiffness mechanism vibration isolation systems from Minus K Technology. This allowed the laboratory to reduce vibration isolation to a level of 1 Hz. This correctly removed any vibration noise challenges that were reducing the quality of data and image readings.

Negative-stiffness mechanism (NSM) isolators provide the flexibility of customized tailoring resonant frequencies both horizontally and vertically. They utilize an entirely mechanical design in low-frequency vibration isolation.

Vertical-motion isolation is offered by a stiff spring that supports a weight load, in combination with an NSM. The net vertical stiffness is made very low without influencing the static load-supporting ability of the spring. Beam-columns fixed in series with the vertical-motion isolator create horizontal-motion isolation.

The horizontal stiffness of the beam-columns is decreased by the ‘beam-column’ effect. A beam-column performs as a spring mixed with an NSM.

The outcome is a closely packed passive isolator able to perform with very low horizontal and vertical natural frequencies and significantly high internal structural frequencies.

Transmissibility with negative-stiffness is significantly enhanced over air systems, which can make vibration isolation issues worse because they have a resonant frequency that is the same as floor vibrations.

Transmissibility is a measure of the vibrations that transmit through the isolator corresponding to the input vibrations.

When adjusted to 0.5 Hz, the NSM isolators meet a 93% isolation efficiency at 2 Hz; 99% at 5 Hz; and 99.7% at 10 Hz.

Active systems can be negatively affected by issues of power modulations and electronic dysfunction because they are powered by electricity, which can disturb scanning.

They also have a restricted dynamic range, which is simple to exceed and causes the isolator to go into positive feedback and create noise beneath the equipment.

Active isolation systems essentially have no resonance and their transmissibility does not roll off as quickly as negative-stiffness isolators.

Continuing Research

In the last ten years, Dr.Wu’s team has recorded various waveforms (for example, spirals and plane wave) in brain slices throughout artificially induced oscillations.

Through the use of mathematical models and neocortical slices, they are researching the initiation of the waves and the elements that determine their propagating velocity and direction.

The laboratory is also taking part in the production of novel optical imaging methods that are reliant upon the negative stiffness vibration isolation systems, for example, imaging propagating waves in vivo, in intact brains.

This is technically difficult,” describes Dr. Wu. “Other imaging methods (MRI, PET or MEG) provide inadequate spatiotemporal resolution. Large scale neuronal activity is a hallmark of a living brain.”

We are improving the optical imaging techniques with a hope to visualize the waves in the cortex in vivo during sensory processes and in awake animals while behavioral and cognitive tasks are performed.

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This information has been sourced, reviewed, and adapted from materials provided by Minus K Technology.

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