Magnetic Stimulation of Visual Cortex Creates More Focal Neural Activity
Key findings
- Massachusetts General Hospital researchers evaluated how the visual cortex of an anesthetized animal model responded to electric stimulation with an implanted micro-electrode versus magnetic stimulation with an implanted microcoil
- Relatively large regions of the cortical surface were activated in response to stimulation from micro-electrodes
- Neural activity arising from microcoils was much more spatially confined than that from micro-electrodes
- The ability of microcoils to confine activation might make them better candidates for visual prostheses and for devices that target other sensory cortices
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Electric stimulation is being evaluated for treatment of impaired function in a wide range of applications, including restoring sight to the blind. For effective stimulation of the sensory cortex, it's considered essential to create spatially precise neural activity, but electrodes typically activate all nearby neurons and neuronal processes.
Previous in vitro work by researchers at Massachusetts General Hospital suggested that magnetic stimulation induced by implantable microcoils is more focal than electric stimulation, as reported in Science Advances. Sang Baek Ryu, PhD, postdoctoral fellow, and research-scientists Shelley I. Fried, PhD, and Seung Woo Lee, PhD, of the Mass General Department of Neurosurgery, have now demonstrated similar results in vivo. The new report appears in the Journal of Neural Engineering.
Study Methods
The researchers used a custom-made, 128-channel electrocorticography array to capture responses from the cortical surfaces of 11 anesthetized mice. A hole in the center of the array allowed an electrode to be inserted, then replaced with a coil, so responses to both types of stimulation were compared directly.
Electric Stimulation
Following electric stimulation of the primary visual cortex, many neural channels exhibited strong activation that extended well beyond the stimulation site (the center of the array). Peak responses extended ~2 mm from the site and covered ~8 mm2 of surface area, almost the entire extent of the visual cortex.
In the cortex, vertically oriented pyramidal neurons are generally the targets of stimulation. The extensive spread almost certainly indicates undesired activation of horizontally oriented passing axons as well.
Magnetic Stimulation
Responses to magnetic coils typically occurred 275–448 micrometers from the stimulation site, much less than the ~2 mm spread from electrodes. Similarly, the surface area activated by coils was only ~1 mm2.
The results were consistent for stimulation of cortical layer 2 and 3, and layer 5 and across a range of stimulus strengths.
Next-Generation Visual Implants
It appears that with conventional cortical prostheses, electrodes may need to be separated by 2 mm or more to avoid overlapping visual sensations. The ability of coils to confine activation might make them better candidates for visual prostheses and for devices that target other sensory cortices.
Coils also make it unnecessary to deliver electric charge directly into the brain, which reduces numerous concerns about safety and stability of performance over time. Finally, magnetic fields pass readily through biological materials, so coils are less susceptible than electrodes to changes in the impedance of the tissue around the implant. Theoretically this is another way to help patterns of activation remain stable.
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