Our rheobase levels are on the order of magnitude of other studies of ocular surface stimulation. Delbeke et al.
8 found an average value for the rheobase in 10 healthy individuals of 0.28 mA using periorbital skin electrodes, which is an order of magnitude higher than our average rheobase of 0.038 mA (calculated from the Weiss model;
Table 1 ) and can be readily explained by the addition of a serial resistance by the closed eyelid. Dorfman et al.,
5 using a Burian-Allen contact lens electrode, found thresholds of between 2 and 3 mA at 1-ms pulse duration in four healthy individuals. This threshold is approximately an order of magnitude higher than our finding of 0.3 mA
(Fig. 2B) . Miyake et al.,
16 using a corneal contact electrode, found thresholds in healthy individuals at a 5-ms pulse duration of 0.29 mA, which is approximately three times higher than our value of 0.096 mA at 5 ms
(Fig. 2B) . It must be considered that Miyake et al.
16 and Dorfman et al.
5 used objective measurements of thresholds by registering cortical potentials that certainly yield higher thresholds than subjective estimation. Finally, Morimoto et al.,
10 using a Burian-Allen contact lens electrode, found a threshold in healthy individuals of 0.065 mA, which is still twice as high as our rheobase of 0.038 mA. We speculate that the psychophysical determination of thresholds yields lower values than objective tests, such as recording cortical potentials or pupillary responses, such as Morimoto et al. performed. In patients with RP, values of approximately 4 mA at 4 ms are have been recorded (with a Burian-Allen corneal contact electrode), which is 1.5 times higher than our 2.63 mA at 4 ms in these patients
(Fig. 6C) . Morimoto et al.
10 found a mean threshold of 0.0545 mA in patients with RP, which is somewhat lower than our rheobase of 687 mA. Both studies showed large interindividual variation (SE was 0.411 mA in Morimoto et al. and 0.105 mA in our study). Intraocular stimulation obviously requires lower thresholds, which are reported in the range of 0.1 mA at 16 ms, for epiretinal stimulation in patients with RP
37 and are six times lower than our threshold of approximately 0.6 mA with extraocular stimulation
(Fig. 6C) . For subretinal stimulation, no thresholds are thus far available in humans, but studies are under way (Zrenner E et al.
IOVS 2006;47;ARVO E-Abstract 1538). Ultimately, it must be the goal to establish a method that allows estimation of intraocular thresholds from external stimulation, to select valid patients for retinal prosthesis implantation.