12.4.1  Epiretinal Stimulation

Suzuki et al. [55] measured the thresholds of RGCs in wild-type and 16-week old rd1 mouse retinas to biphasic current pulses delivered through a 125-mm electrode positioned on the epiretinal surface. The thresholds were on average 20–50% higher in rd1 mouse retinas, depending on pulse duration. A description of the evoked responses was not provided so it is unclear whether the thresholds were from direct or indirect activation of the RGCs.

O’Hearn et al. [36] examined the thresholds for activation of RGCs in 8–12 week old wild-type and rd1 mouse retinas. Retinas were stimulated with a pair of electrodes (125-mm diameter) that were positioned either epiretinally or subretinally. With epiretinal stimulation, the thresholds for activation of RGCs in rd1 mouse retinas were 1.8-fold higher. The short-latency responses suggest that the RGCs were directly activated although this was not discussed by the authors. If so, then perhaps the properties of the sodium channel bands that presumably underlie RGC activation thresholds (described in Sect. 12.2.1.2) change during the degenerative process and the change results in a higher threshold.

12.4.2  Subretinal Stimulation

12.4.2.1  Response Properties of RGCs

Jensen and Rizzo [21] compared the electrically evoked responses of RGCs in wild-type and rd1 mouse retinas to stimulation of the neural network. Using biphasic current pulses, they found that RGCs in rd1 mouse retinas respond similarly to wild-type RGCs. In both wild-type and rd1 mouse retinas, three types of electrically evoked responses were observed (Fig. 12.14). Type I cells

Fig. 12.14  Dot plot of thresholds for activation of RGCs in wild-type and rd1 mouse retinas with biphasic current pulses. See Sect. 12.4.2.1 for a description of the three types of RGCs. Reprinted from [22], Fig. 4, with permission