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John S. Pezaris, R. Clay Reid; Microstimulation in LGN produces focal visual percepts: Proof of concept for a visual prosthesis. Journal of Vision 2005;5(8):367. doi: https://doi.org/10.1167/5.8.367.
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© ARVO (1962-2015); The Authors (2016-present)
Existing efforts to develop a visual prosthesis have concentrated on stimulation of the retina or the primary visual cortex. We are pursuing a third approach which targets the dorsal lateral geniculate nucleus of the thalamus (LGN). We hypothesized that highly localized electrical stimulation of the LGN would generate focal percepts corresponding to the receptive fields of cells at the electrode tip. To test this we compared visually-guided saccades made to targets presented on a computer monitor against those made to targets presented through electrical stimulation in the LGN.
Daily tetrode penetrations were made in the LGN of one alert behaving monkey. Receptive field locations were mapped for each electrode position using 2D white noise stimuli. Eye positions were then recorded while a center-out saccade task was performed by the animal where, in interleaved fashion, saccade targets were presented optically as points flashed on a computer monitor or electrically through brief stimulation between two wires in a tetrode.
Our primary observation is that the animal could readily saccade to electrical targets consistent with electrical stimulation generating small, focal percepts. Correspondence between electrical saccade endpoints and receptive-field centers was 3.0±0.6° (n=18, with eccentricities of 3–26°), versus optical saccade accuracy of 1.7±0.5°. Electrical and optical saccades were comparable: endpoint scatter was 1.4±1.2° vs 0.8±0.2°, latency was 185 vs 160 ms. At times, however, rather than saccading to a presumed phosphene, in electrical trials the animal held it's gaze steady, as if continuing to foveate the extinguished fixation point.
Our observations demonstrate the possibility of creating artificial visual percepts through electrical stimulation in the LGN. Such percepts are visual in nature insofar as an animal immediately generalized to them in a visual task. Further, these percepts are focal as saccades to them are tightly clustered.
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