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Laser Stimulation of Auditory Neurons: Effect of Shorter Pulse Duration and Penetration Depth

Agnella D. Izzo ^* , Joseph T. Walsh, Jr. , Heather Ralph , Jim Webb , Mark Bendett , Jonathon Wells  and Claus-Peter Richter ^*  

^* Department of Otolaryngology, Northwestern University, Chicago, Illinois 60611; Department of Biomedical Engineering, Northwestern University, Evanston, Illinois 60208; Aculight Corp., Bothell, Washington 98021; and Auditory Physiology Laboratory (The Hugh Knowles Center), Department of Communication Sciences and Disorders, Northwestern University, Evanston, Illinois 60208

Correspondence: Address reprint requests to Agnella D. Izzo, Tel.: 312-503-4027; E-mail: a-izzo{at}northwestern.edu.

We have pioneered what we believe is a novel method of stimulating cochlear neurons, using pulsed infrared radiation, based on the hypothesis that optical radiation can provide more spatially selective stimulation of the cochlea than electric current. Very little of the available optical parameter space has been used for optical stimulation of neurons. Here, we use a pulsed diode laser (1.94 µm) to stimulate auditory neurons of the gerbil. Radiant exposures measured at CAP threshold are similar for pulse durations of 5, 10, 30, and 100 µs, but greater for 300-µs-long pulses. There is evidence that water absorption of optical radiation is a significant factor in optical stimulation. Heat-transfer-based analysis of the data indicates that potential structures involved in optical stimulation of cochlear neurons have a dimension on the order of 10 µm. The implications of these data could direct further research and design of an optical cochlear implant.