Transcranial magnetic stimulation (TMS) is one of the most widely used noninvasive brain stimulation methods. It has been
utilized for both treatment and diagnosis of many neural diseases, such as neuropathic pain and loss of function caused by stroke.
Existing TMS tools cannot deliver focused electric field to targeted penetration depth even though many important neurological
disorders are originated from there. A breakthrough is needed to achieve noninvasive, focused brain stimulation. We demonstrated
using magnetic shield to achieve magnetic focusing without sacrificing significant amount of throughput. The shield is composed of
multiple layers of copper ring arrays, which utilize induced current to generate counter magnetic fields. We experimentally set up
a two-pole stimulator system to verify device simulation. A transient magnetic field probe was used for field measurements. The
focusing effect highly depends on the geometric design of shield. A tight focal spot with a diameter of smaller than 5 mm (plotted in
MATLAB contour map) can be achieved by using copper ring arrays. With properly designed array structures and ring locations,
the combined original and induced counter fields can produce a tightly focused field distribution with enhanced field strength at a
depth of 7.5 mm beyond the shield plane, which is sufficient to reach many deep and critical parts of a mouse brain.
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Theoretical and experimental studies of transcranial alternating current stimulation (tACS) beating signal in phantoms and mice brains
Brain simulation techniques have demonstrated undisputable therapeutic effects on neural
diseases. Invasive stimulation techniques like deep brain stimulation (DBS) and noninvasive
techniques like transcranial magnetic stimulation (TMS) have been approved by FDA as
treatments for many drug resist neural disorders and diseases. Developing noninvasive, deep, and
targeted brain stimulation techniques is currently one of the important tasks in brain researches.
Transcranial direct current stimulation (tDCS) and transcranial alternative current stimulation
(tACS) techniques have the advantages of low cost and portability. However, neither of them can
produce targeted stimulation due to lacking of electrical field focusing mechanism. Recently,
Grossman et al. reported using the down beating signals of two tACS signals to accomplish
focused stimulation. By sending two sine waves running at slightly different high frequencies
(~2kHz), they demonstrated that they can modulate a “localized” neuron group at the difference
frequency of the two sine waves and at the same time avoid excitation of neurons at other locations.
As a result, equivalent focusing effect was accomplished by such beating mechanism. In this work,
we show neither theoretically nor experimentally the beating mechanism can produce “focusing
effect” and the beating signal spread globally across the full brain. The localized modulation effect
likely happened right at the electrode contact sites when the electrode contact area is small and the
current is concentrated. We conclude that to accomplish noninvasive and focused stimulation at
current stage the only available tool is the focused TMS system we recently demonstrated.
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- Award ID(s):
- 1631820
- NSF-PAR ID:
- 10063437
- Date Published:
- Journal Name:
- Proc. SPIE 10662, Smart Biomedical and Physiological Sensor Technology XV, 106620D
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Transcranial magnetic stimulation (TMS) is widely used for noninvasive brain stimulation. However, existing TMS tools cannot deliver targeted neural stimulation to deep brain regions, even though many important neurological disorders originate from there. To design TMS tools capable of delivering deep and focused stimulation, we have developed both electric and magnetic field probes to evaluate and improve new designs and calibrate products. Previous works related to magnetic field measurement had no detailed description of probe design or optimization. In this work, we demonstrated a magnetic field probe made of a cylindrical inductor and an electrical field probe modified from Rogowski coil structure. Both have much smaller size and higher directivity than commercial dipole probes. Using probe, we can calibrate and monitor any new types of TMS coil or array design and verify measured results with the other probe. We mathematically analyze their characteristics and performance and obtained a two-dimensional vector plot of the induced electric field, which matched the measured results from the second type of probe. A commercial circular coil and a figure-8 coil, with relatively complex vector field distribution, were used as examples to demonstrate the high-resolution and accurate measurement capability of our probes.more » « less
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Abstract Transcranial direct‐current stimulation (tDCS) is a noninvasive method for modulating human brain activity. Although there are several hypotheses about the net effects of tDCS on brain function, the field's understanding remains incomplete and this is especially true for neural oscillatory activity during cognitive task performance. In this study, we examined whether different polarities of occipital tDCS differentially alter flanker task performance and the underlying neural dynamics. To this end, 48 healthy adults underwent 20 min of anodal, cathodal, or sham occipital tDCS, and then completed a visual flanker task during high‐density magnetoencephalography (MEG). The resulting oscillatory responses were imaged in the time‐frequency domain using beamforming, and the effects of tDCS on task‐related oscillations and spontaneous neural activity were assessed. The results indicated that anodal tDCS of the occipital cortices inhibited flanker task performance as measured by reaction time, elevated spontaneous activity in the theta (4–7 Hz) and alpha (9–14 Hz) bands in prefrontal and occipital cortices, respectively, and reduced task‐related theta oscillatory activity in prefrontal cortices during task performance. Cathodal tDCS of the occipital cortices did not significantly affect behavior or any of these neuronal parameters in any brain region. Lastly, the power of theta oscillations in the prefrontal cortices was inversely correlated with reaction time. In conclusion, anodal tDCS modulated task‐related oscillations and spontaneous activity across multiple cortical areas, both near the electrode and in distant sites that were putatively connected to the targeted regions.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/doi: 10.1117/12.2304947
