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

Figure 1A shows performance in a monkey when two visual targets were presented, one in the upper and the other in the lower visual field as indicated in the inset. Plotted are the percent of choices made to the lower target (Y axis) as a function of the contrast of the visual stimulus (X axis). The equal probability choice for the upper and lower target occurs when the two stimuli have the same contrast. In Figure 1B data are shown under conditions when instead of the lower target electrical stimulation is applied in concert with the upper visual target. The area stimulated contains neurons whose receptive fields were located in the same region as where the lower visual target had been presented in A. Data were collected in blocks using three different current levels of electrical stimulation. The data show that the visual stimulus which elicited an equivalent choice occurred when the visual target had a rather low contrast. The point at which there was equivalence between the visual target and the electrical stimulation shifted to higher contrasts with increasing current but only over a small range (between 5 and 10 percent contrast). Thus it appears electrical stimulation in the monkey under our conditions produces a rather weak, low contrast percept.