Figure 4. Shape.

(A) Shape of cortical activations generated in 2 animals by SE (blue) and wMEA (red) stimulation at high current intensity (top) and their corresponding 20° visual stimulus (white, bottom). (B) Aspect ratio (AR) of cortical activations (Wilcoxon rank sum test for paired data, ***p=1.06 10^–4, n = 44, N = 7). (C) Predictions of the elongation of electrical activations as a function of the contribution of axons en passant and the distance to the optic disk. Insets correspond to a model of retinal activation due to direct isotropic activation plus passive electrical diffusion and anisotropic activation due to axons en passant recruitment for 3 different electrode sizes. The brightness codes the strength of the response. Center of the white dashed target: position of the optic disk; black circle: position and size of the MEA active surface; gray lines: 'shadow cone' angle sustained by the MEA active surface respective to the optic disk location; colored contour: size and shape of the global retinal activation for an axons en passant contribution of 1 (alpha, see Materials and methods). (D) Elongation of electrical activations relative to their corresponding visual activations (AR electrical/AR visual) as a function of the 'shadow cone' angle. (E) Cortical radial organization of prosthetic activations. Solid segments: orientation of cortical activations; dashed segments: optimal radial orientation towards the black disk; segment crossing; geometrical center; red dot: center-of-mass of cortical activations; Dark disk: cortical position that optimized radial organization; gray disk: median position of the optic disk. The blue lines connect the center-of-mass to the geometrical center of activations. Scale bar: 0.5 mm. Inset: distribution of median angular deviation expected by chance compare to our observation: blue segment. (F) Top: centered and reoriented deviations of the center-of-mass (blue disks) to the geometrical center (center of the representation), horizontal dashed axis corresponds to the orientation of the radial organization. Bottom: averaged, centered and reoriented SE (with AR > and < than 1.6, left and middle respectively) and wMEA maps (right). White circle: center-of-mass; additional dashed contour corresponds to a Z-score of −4.5.

DOI: http://dx.doi.org/10.7554/eLife.12687.007

Figure 4—figure supplement 1. Model of retinal anisotropic activation.

Model of retinal anisotropic activation due to axons en passant for 3 different electrode sizes (rows in B&C). (A) Modeling steps used to compute the activation of axons en passant (see Materials and methods). The brightness codes the strength of the response, all representations are scaled between 0 and 1. (B) Direct activation with passive electrical diffusion (left column). Center of the white dashed lines: position of the optic disk; black circles: position and size of the MEA active surface. Anisotropic activation due to axons en passant recruitment (middle column). Size and shape of the global retinal activation (colored contours, right column) for an axons en passant contribution of 0.5 (alpha, see Materials and methods). (C) same as in A for an axons en passant contribution of 1.

DOI: http://dx.doi.org/10.7554/eLife.12687.008

Figure 4—figure supplement 2. Model of retino-cortical transformation.

To check whether retino-cortical transformation can introduce further bias to our predictions (Figure 4—figure supplement 1), we implemented a retino-cortical transformation based on the magnification measured in our data (A). This model simply generates a transformation using the formula RCM = 1 /(aR+b), where RCM is the retino-cortical magnification factor (mm/deg), R the retinal eccentricity (deg), a and b constants (along the horizontal meridian : a = 0.7; b = 30 & a = 0.4, b = 40 for the vertical dimension). In the Figure A, the resulting transformation is shown for a retinal pattern with circles of different diameter and eccentricity. (B) We show for 2 cone angles (blue and red) and ratio of axon-en-passant (same as Figure 4—figure supplement 1), what such transformation does. (C) From these maps, we calculated the cortical activation aspect ratio, normalized to the shape of activation without axon-en-passant (ratio = 0 not shown), plotted with the same convention as in Figure 4C.

DOI: http://dx.doi.org/10.7554/eLife.12687.009