Inducing Combinational Percepts by Intracortical Electrode Stimulation in Macaque V1

Abstract

Abstract: : Purpose: To determine if a combination of percepts can be induced in the non–human primate by electrical stimulation of intracortical electrodes in area V1. In earlier experiments we demonstrated that 2–dimensional spatial information could be transferred to the primate cortex by training an animal to localize phosphenes. The goal of the present study is to demonstrate that orientation cues, using simultaneous stimulation on multiple electrodes can be transmitted to the cortex. Methods:The receptive field (RF) locations were mapped and the orientation tuning measured for each of 96 activated iridium–oxide intracortical electrodes chronically implanted into the caudal portion of the right operculum of a rhesus monkey. Fields were mapped with a 2° flicker stimulus shown for 1 sec in 100 different locations, 1° spacing (n ∼ 50). Because of the large stimulus size, field size was overestimated; however our interest was only in the center positions. Orientation tuning was measured with a field of randomly positioned line segments moving in directions {0, 22.5, <font face="symbol">¼</font>, 337.5°}.Owing to the natural cortical summation of many orientation cues to form an overall perception, an orientation task, rather than a phosphene task, was devised for testing the theory that stimulation over multiple electrodes can summate. We have designed a 2–alternative forced choice task based upon orientation. The animal will carry out the task using inter–stimulus angles of various sizes. From this we will be able to calculate d’, our basic estimate of sensitivity (performance). Once the animal learns the task and reaches a saturated level of performance, the first goal will be to study the dependence of performance on the number of channels stimulated. At present the animal is training in a visual version of the 2AFC task, and the electrical stimulation equivalent is in progress. Conclusions:Using a large number chronically–implanted intracortical electrodes it is feasible to obtain reliable spatial and orientation maps. Use of these maps in training the animal to master a multiple percept orientation task promises to be a significant step in determining how to use a multitude of intracortical electrodes in a cortical human visual prosthesis.

Keywords: receptive fields • visual cortex