Figure 3.

Linearity of current tradeoff between electrodes, with all tested electrode pairs. A, Histogram of nonlinearity indices (see Materials and Methods), for all 15 cells tested (7 ON parasol, 4 OFF parasol, 3 ON midget, and 1 OFF midget) and 90 electrode pairs (6 per cell). Letters indicate nonlinearity indices of example electrode pairs shown in B–E, open triangles correspond to electrode pairs shown in Figure 2, and filled triangles correspond to pairs shown in Figure 5. The vertical dashed line indicates the minimum nonlinearity index for which fitting with a piecewise linear model was attempted (see Materials and Methods and Results, Piecewise linear model fitting). B–E, Measured thresholds and linear fits of four electrode pairs, illustrating the range of observed degrees of linearity. Shaded regions depict SDs of threshold estimates based on bootstrap analysis (see Materials and Methods). Insets indicate electrical image of targeted cell (see Materials and Methods) and electrodes used for stimulation; white is primary, black is secondary. Dashed line in C indicates the edge of the electrode array. Horizontal scale bars, 50 μm. Positive and negative current values signify cathodal and anodal pulses, respectively. B, ON midget cell. C, ON midget cell. D, OFF parasol cell. E, ON parasol cell.