Visual Prosthesis
B. The visual percept elicited by electrical stimulation of area V1

To assess the nature of the visual percept created with electrical stimulation of a single site in area V1, we present stimuli under two basic conditions:

1. Two visual targets are presented simultaneously. The size, contrast, color or shape of one of the stimuli is systematically varied while the other is kept constant. One of the targets is presented in the receptive field of the neurons to be electrically stimulated.

2. One of the visual targets is replaced with electrical stimulation and is paired with the other visual target. This visual target is varied in either contrast, size, color or shape while the parameters of the electrical stimulation are kept constant in a block of trials.

The monkey has a free choice and is rewarded with a drop of apple juice after a saccadic eye movement is made to either of the two visual sites (condition 1) or to the site of the receptive field area of the electrically stimulated neurons (condition 2). When two identical visual targets are presented at two sites in the visual field that are equidistant from the fovea, monkeys tend to choose one or the other target with equal probability. However, when the stimuli are unequal in contrast or size, the animals have a strong disposition to pick the larger and/or higher contrast target.