Fig. 16.5  Illustration of pillbox and several Gaussian dot profiles. The left half of the figure shows the profiles in isolation, while the right half shows the effect of interactions with neighbors in a square grid. Four different s values have been chosen to illustrate the effect of neighbor interactions. Inter-dot distances and peak intensity values have been normalized. Note that the radius of the pillbox equals 0.5 in this example, and that a modest increase in s (0.33–0.5) causes dramatic blurring, due to long-range Gaussian overlap

multiple tasks has not been performed; in principle a similar intensity modulation could be used with Gaussian dots, by modulating s rather than peak intensity. To our knowledge this has not been attempted.

•Grey scale/size resolution: Natural vision is capable of resolving subtle differences in shading, even in the absence of color, but this is unlikely to be the case in prosthetic vision. For this reason a number of simulation studies have examined subjects’ visual performance under conditions of reduced grey scale resolution. Typical resolutions used range from 2 to 16.

•Homogeneity: Most simulation studies use identically shaped dots, but it is inevitable that phosphenes perceived by prosthesis wearers will vary in intensity, size, shape, and other aspects. Until more information from prosthesis wearers is obtained it may be premature to build such inhomogeneities into simulations, but it may be an important future extension.

16.2.2.3  Temporal Properties

The rewiring taking place in the degenerating retina (see Chap. 3) and possibly other stages of visual processing are likely to include the loss of rapid signal processing and the creation of feedback loops. The effect of such changes will be that temporal properties of prosthetic vision will be much slower than in natural vision, as crudely represented in Fig. 16.6 by the “ghosting” of the visual percept of a maze.