Characteristics of Percepts Evoked by Microstimulation in LGN J. S. Pezaris and R. C. Reid Department of Neurobiology Harvard Medical School Boston, MA 02115, U.S.A. Abstract Investigation into electrical stimulation in LGN with the eventual goal of creating a visual prosthesis has recently begun. To assess the characteristics of electrically-evoked percepts, we performed a parametric study of electrical stimulation of LGN through fine wire electrodes in awake behaving macaques, using a behavioral report to read out percept size and location. We used a simple center-out visually guided saccade paradigm where animals were required to foveate a central point and then saccade to briefly presented target stimuli. While most targets (and all fixation points) were presented on a computer screen, some targets were presented via electrical stimulation applied to fine wire electrodes placed in the LGN. Electrical stimulation was applied in differential mode (between electrodes separated by less than 10 microns) or in single-ended mode (between one electrode and a remote return). Stimuli were trains of 1 ms single-cycle sinusoidal pulses presented at 200 Hz with varying amplitude in either voltage or current mode. Data were gathered from two macaque monkeys (1 male, 1 female). Saccade endpoint cluster size was used to estimate phosphene size. The mean threshold for current mode was 40+-12 microamps (n=6). The mean threshold for voltage mode was 2.5+-0.6 (n=20) volts. The mean pre-stimulation electrode impedance was 540+-170 kOhms at 1 kHz. Saccades to single-ended stimulation targets had larger saccade cluster size (2.3+-1.6 degrees) than saccades to differential stimulation targets (1.1+-1.0 degrees). Current mode stimulation had larger mean cluster size (1.7+-1.1 degrees, n=11) than voltage mode stimulation (0.95+-1.0 degrees, n=35). We conclude that a mixture of differential (between neighboring electrodes) and single-ended (electrode with remote return) stimulation can be used to provide coarse control over phosphene size in an LGN-based visual prosthesis. Supported by: Kirsch Foundation, Dana/Mahoney Foundation, NIH R01 EY12815 and P30 EY12196. Fetch PDF of abstract (0.02 MB) Fetch PDF of slides (0.5 MB) John Pezaris, Harvard Medical School, 220 Longwood Ave, GB-203, Boston, MA 02115 pz [at] hms [dot] harvard [dot] edu, 10 November 2004.