Micro-coils confine activation to single cortical columns in V1

Abstract

Purpose : The use of implantable electrodes to artificially stimulate the visual cortex has long been considered a potential treatment for restoring vision to the blind. Unfortunately, the stability of electrode-based cortical prostheses has been limited, in part due to the complex biological and chemical reactions that can diminish the effectiveness of individual electrodes over time. Further, conventional electrodes readily activate the passing axons of distant neurons, thereby reducing the potential acuity that can be achieved from implanted devices. Magnetic stimulation from micro-coils is a potentially attractive alternative to conventional electrodes because the spatially asymmetric fields that arise from coils can be oriented to avoid the unwanted activation of passing axons and further, because the use of induction to activate neurons provides for a more stable interface over time. Here, we compare the spatial spread of activation from coils vs. that from conventional electrodes; testing is performed in both mouse and non-human primate (NHP).

Methods : Computer simulations followed by in vitro physiological experiments allowed the effectiveness of several different coil designs to be compared; the optimum designs were selected for additional in vivo testing. A custom array of recording electrodes was developed for positioning on the cortical surface (ECoG); the array consisted of concentric rings that allowed the spread of activation to be evaluated. In some cases, a smaller, rectangular array of recording electrodes was used instead. Responses to stimulation from coils and electrodes were compared in both mouse and NHP.

Results : Craniotomies were performed followed by placement of a recording array onto V1 in 12 mice and 3 NHPs. A micro-coil (or electrode) was implanted through a hole in the center of the array and stimulation delivered. Responses to magnetic stimulation from the micro-coil were spatially confined to a narrow region, approximately 300 µm in diameter. In contrast, responses to stimulation from the electrode were spatially expansive, often extending more than 1 mm from the site of stimulation.

Conclusions : The ability to focally confine activation with implantable micro-coils is attractive in that it supports improved acuity of artificial vision. Future psychophysical testing will help to inform whether improved neural activity patterns do indeed correlate to improved quality of elicited percepts.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.