Shelley I. Fried1, 2, Sang Baek Ryu2, Justin Tanner3, Bradley Greger3, Andrew Whalen4, John S. Pezaris2, Seung Woo Lee5
1 Boston VA Healthcare System, Boston, MA, USA
2 Department of Neurosurgery, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA
3 Department of Biological and Health Systems Engineering, Arizona State University, Tempe, AZ, USA
4 Department of Neurosurgery, Yale University
5 Department of Brain and Cognitive Sciences, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea
Purpose: We are exploring the use of implantable microcoils as an alternative to conventional microelectrodes in the development of a cortical visual prosthesis. The use of magnetic stimulation to drive cortical neurons is attractive because the induced fields from microcoils are spatially asymmetric and therefore, can be harnessed to selectively target or avoid specific neuronal sub-populations. For example, implanted coils can activate local pyramidal neurons while avoiding activation of passing axons, thus confining activation to a focal region around each coil. Also, because there is no direct contact between coils and cortical tissue, many of the concerns about stability of the interface associated with microelectrodes can be avoided.
Methods: A series of in vitro, in vivo, and behavioral experiments have been performed in both rodents and non-human primates (NHPs) to evaluate the viability of microcoils. The in vitro and in vivo experiments in mice consistently show responses from magnetic stimulation are better confined to focal regions around each coil vs. those from electrodes. Psychophysical behavioral experiments in NHPs use a detection task to evaluate efficacy for eliciting phosphenes and determining activation thresholds. Additional experiments are underway to evaluate the safety and stability of implanted microcoils. Parallel efforts have been performed to assess the stability of implantable electrodes for stimulation of cortex.
Results: Chronic implantation of micro-electrodes into S1 of rats revealed changes in threshold over the course of implantation with some variability depending upon which layer was stimulated. Acute testing of magnetic and electric stimulation (implanted into V1 of mice, layers 2/3 and 5) both elicited strong surface (ECoG) responses, although the area activated by micro-coils was confined focally within a ~300-µm in diameter region, and that of micro-electrodes was more spatially expansive, often extending more than 1-mm from the stimulation site. In vitro testing of human cortical tissue (resected during medically necessary neurosurgical procedures) revealed comparable sensitivity of individual neurons to those measured in mice. A limited amount of psychophysical testing in non-human primates provides encouraging preliminary indications that the animal can detect magnetic stimulation from micro-coils acutely implanted in V1, and further, that activation thresholds are considerably lower than those from experiments in anesthetized rodents. Results from chronic implantation experiments indicate that microcoil performance remains stable. Temperature measurements indicate that repeated microcoil stimulation can be performed safely.
Conclusions: Our results continue to support the viability of microcoils as an alternative to conventional micro-electrodes. The focal magnetic activation from micro-coils in rodent in vitro and in vivo experiments, built on a theoretical analysis of microcoil magnetic fields, supports the notion that distinct, focal phosphenes can be created that will summate to produce more spatially complex percepts. The generation of phosphenes during a limited number of psychophysical experiments with NHPs provides additional support although further testing is needed to verify that elicited percepts are indeed more focal with magnetic.
Funding: Research supported by the NIH (NINDS and NEI) and by the Dept. of Defense. William M. Wood Foundation, Bank of America Trustee (JSP).