Abstract
The implantation of micro-electrodes into visual cortex has been proposed as a means to restore vision to those blinded by a wide range of causes. The effectiveness of such an approach has been limited however, in part due to the inability to precisely and selectively stimulate specific types of neurons. Maintaining a stable interface has also proven challenging with foreign body responses and electrode degradation both contributing to a loss of performance over time. Recently, we proposed the use of magnetic stimulation from micro-coils as an alternative to conventional micro-electrodes. Micro-coils are potentially attractive in that that their fields are spatially asymmetric and thus can be harnessed to selectively target specific types of neurons, e.g. vertically-oriented pyramidal neurons, without simultaneously activating other types, e.g. horizontally-oriented passing axons. This confines activation to a more focal region and thus supports higher acuity vision. Magnetic fields pass readily through biological materials and thus are much less susceptible to encapsulation or other foreign body responses. Also, coils do not require metal-to-brain contact and thus remain more stable over time. Extensive in vitro and in vivo physiological testing verifies the enhanced performance of coils and ongoing development efforts may allow even higher levels of selectivity. In vivo implantation has been used to verify that micro-coils also drive behavioral responses. Taken together, the results to date suggest the possibility that coil-based visual prostheses may prove to be more effective and more stable than conventional devices.