Abstract
Abstract: :
Purpose:Infrared (IR) irradiation has been used to selectively stimulate photodiode/silicon–based retinal prostheses to prove their function in cats and other experimental animals. Data concerning the sensitivity of the cat retina to IR irradiation (Gekeler et al. ARVO 2002, Pardue et al. 2001) are contradictory. The aim of this study was to evaluate the sensitivity of the cat retina to IR–irradiation after prolonged periods of dark–adaptation. Methods:Electroretinograms (ERGs) were recorded from the corneae of three adult healthy cats after 60 minutes of dark–adaptation and in the light–adapted state. An IR–laser (λ=826 nm) and a visible laser (λ=670 nm) diffusely stimulated the eyes at 0.87 Hz with a pulse length of 4 ms. Laser power at corneal level was 23 mW for the IR–laser and 0.2 mW for the visible laser. Neutral density filters (D5.7 to D0) were used to attenuate irradiation power. For comparison with old studies also high–energy IR–emitting diodes (IREDs, λpeak=875 nm) were used. Results:IR–laser: Scotopic threshold responses (STRs) were elicited at intensities from 0.023 mW to 23 mW with an amplitude of –3 µV to –31 µV and implicit time of ca. 120 ms. In the light–adapted state no discernible ERG waves were observed. Visible laser: The amplitude of the STR increased from 0.0002 mW to 0.004 mW from –11 µV to –37 µV, but decreased with further increasing intensity; at 0.1 mW no STR was detectable. At 0.02 mW a b–wave developed on top of the STR which reached its maximum intensity at 0.2 mW with 77 µV at 87 ms. However, two peaks emerged with increasing strength of stimulation (one at 39 ms, the other at 87 ms) pointing to two different cell populations stimulated in the dark–adapted state. In the light–adapted state the typical cone–driven waveforms occurred at 0.1 mW. IREDs: STRs were elicited with amplitude of up to –25 µV and implicit time of ca. 95 ms. In the light–adapted state no discernible ERG waves occurred. Conclusions:We have determined retinal sensitivity of the cat to IR–laser and IRED irradiation after complete dark–adaptation of 60 minutes. The fact that the cat retina under certain conditions is indeed sensitive to IR–irradiation in the dark–adapted state requires close attention concerning the interpretation of results obtained after IR–stimulation attributed to a retinal prosthesis. Only in the light–adapted state IR–irradiation, either by diodes or by laser, seems adequate to discern responses elicited by retinal prostheses from those elicited by the photoreceptor system.
Keywords: retinal degenerations: hereditary • electrophysiology: non–clinical • retina