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 Motor control requires the coordination of spatiotemporally precise neural oscillations in the beta and gamma range within the primary motor cortex (M1). Recent studies have shown that motor performance can be differentially modulated based on the spectral target of noninvasive transcranial alternating current stimulation (tACS), with gamma-frequency tACS improving motor performance. However, the spectral specificity for eliciting such improvements remains unknown. Herein, we derived the peak movement-related gamma frequency in 25 healthy adults using magnetoencephalography and a motor control paradigm. These individualized peak gamma frequencies were then used for personalized sessions of tACS. All participants completed 4 sessions of high-definition (HD)-tACS (sham, low-, peak-, and high-gamma frequency) over M1 for 20 min during the performance of sequential movements of varying complexity (e.g. tapping adjacent fingers or nonadjacent fingers). Our primary findings demonstrated that individualized tACS dosing over M1 leads to enhanced motor performance/learning (i.e. greatest reduction in time to complete motor sequences) compared to nonspecific gamma-tACS in humans, which suggests that personalized neuromodulation may be advantageous to optimize behavioral outcomes.
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In recent years, there is an increasing interest in noninvasive treatments for neurological disorders like Alzheimer and Depression. Transcranial magnetic stimulation (TMS) is one of the most effective methods used for this purpose. The performance of TMS highly depends on the coils used for the generation of magnetic field and induced electric field particularly their designs affecting depth and focality tradeoff characteristics. Among a variety of proposed and used TMS coil designs, circular coils are commonly used both in research and medical and clinical applications. In current study, we focus on changing the outer and inner sizes (diameter) and winding turns of ring coils and try to reach deeper brain regions without significant field strength decay. The induced electric field and the decay rate of the generated field with depth were studied with finite element method calculations. The results of the performed simulations indicate that larger diameter coils have a larger equivalent field emission aperture and produce larger footprint of induced electric field initially. However, their emission solid angles are smaller and, as a result, the field divergence or the decay rates of the generated field with depth are smaller as well, which give them a good potential to perform better for deep brain stimulation compared with that of smaller coil.more » « less
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Brain damage or disruption to the primary visual cortex sometimes produces blindsight, a striking condition in which patients lose the ability to consciously detect visual information yet retain the ability to discriminate some attributes without awareness. Although there have been few demonstrations of somatosensory equivalents of blindsight, the lesions that produce “numbsense,” in which patients can make accurate guesses about tactile information without awareness, have been rare and localized to different regions of the brain. Despite transient loss of tactile awareness in the contralateral hand after transcranial magnetic stimulation (TMS) of the primary somatosensory cortex but not TMS of a control site, 12 participants (six female) reliably performed at above-chance levels on a localization task. These results demonstrating TMS-induced numbsense implicate a parallel somatosensory pathway that processes the location of touch in the absence of awareness and highlight the importance of primary sensory cortices for conscious perception.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/doi: 10.1117/12.2304947
