For over a century, the traditional method of stimulating neural activity has been based on electrical methods, which has undergone few modifications over the years and remains the gold standard to date. We report a technological breakthrough in neural stimulation that uses low intensity infrared laser light to elicit compound nerve and muscle potentials instead of electrical energy. We show that infrared laser light at relative valleys of tissue absorption can be used to consistently and reproducibly stimulate peripheral nerves in frogs and rats in vivo with no appreciable tissue damage using radiant exposures well below ablation threshold. Results demonstrate optical stimulation can circumvent many of the limitations of electrical stimulation, including lack of spatial specificity and electrical artifacts that limit data analysis and make simultaneous excitation and recording from adjacent nerve fibers difficult.
Optically induced potentials are spatially precise, highly controlled and artifact-free. Several experiments were performed using the sciatic nerve in vivo to verify the physiologic validity of optical stimulation. Application of a depolarizing neuromuscular blocker resulted in a measurable CNAP and loss of CMAP confirming normal propagation of impulses from nerve to muscle upon optical stimulation. The similarity in the shape and timing of the signals from optical and electrical stimulus show conduction velocities are equal. These traces imply the measured action potentials are identical regardless of excitation mechanism.
Histological analysis of 3-5 day survival studies shows no discernable tissue damage with chronic stimulation. Thus, optical stimulation presents an innovative approach to contact-free neural activation that has major implications to clinical neural stimulation; including higher spatial resolution in functional mapping, partial resection in blocked nerves, and the potential of implantable neuroprosthetics with compact laser diodes.
Note: This abstract submitted by Dr. Konrad, MD, Professor of Neurosurgery and Director of Functional Neurosurgery at Vanderbilt University Medical Center was selected for inclusion in the 2004 Press Book compiled by the Society of Neuroscience SfN) and distributed in advance of their annual conference. The SfN includes in the Press Book only those abstracts representing scientific work of great significance. The abstract, “Optical Stimulation of Neural Tissue,” was one of 600 preferred from among over 16,000 submissions.