Distribution of retinal responses evoked by transscleral electrical stimulation detected by intrinsic signal imaging in macaque monkeys.

PURPOSE: The distribution of the electrical current over the retina when electrical pulses are delivered transsclerally has not been clearly determined objectively and quantitatively in humans. The purpose of this study was to determine the pattern of electrically evoked neural activity in the monkey retina by using intrinsic signal imaging. METHODS: The intrinsic signals of monkey retinas were recorded as changes in the reflectance of infrared light from the retina after transscleral electrical stimulation by DTL electrodes. The effects of changing the stimulus parameters (e.g., intensity, duration, and frequency) of the electrical current, were investigated. RESULTS: Electrical stimulation evoked a uniform change in the reflectivity across the posterior pole of the retina; that is, the intrinsic signals changed uniformly. A peak of the intrinsic signal was not observed at the fovea. The threshold of the intrinsic signal was not significantly different for the macula, perimacula, and optic disc, and the threshold did not differ under dark- and light-adapted conditions. The strength of the signals increased with longer stimulus durations, and the maximum signals were obtained when the stimulus frequency was between 15 and 20 Hz. CONCLUSIONS: Intrinsic signals of the monkey retina evoked by transscleral electrical stimulation are elicited uniformly across the posterior pole of the fundus and most likely arise from activation of the inner or middle layers of the retina. These functional measurements could serve as a diagnostic tool for mapping the inner retinal activity, by which the site of a lesion can be noninvasively imaged.

Origins of retinal intrinsic signals: a series of experiments on retinas of macaque monkeys.

Diffuse flash stimuli applied to the ocular fundus evoke light reflectance decreases of the fundus illuminated with infrared observation light. This phenomenon, which is independent of the photopigment bleaching observed as an increase in the reflectance of visible light, is called intrinsic signals. Intrinsic signals, in general, are stimulus-evoked light reflectance changes of neural tissues due to metabolic changes, and they have been extensively investigated in the cerebral cortex. This noninvasive objective technique of functional imaging has good potential as a tool for the early detection of retinal dysfunction. Once the signal properties were studied in detail, however, it became apparent that the intrinsic signals observed in the retina have uniquely interesting properties of their own due to the characteristic layered structure of the retina. Experiments on anesthetized macaque monkeys are reviewed, and the possible origins of the intrinsic signals of the retina are discussed.

Transcorneal electrical stimulation of retina to treat longstanding retinal artery occlusion.

PURPOSE: To report the outcome of transcorneal electrical stimulation (TES) of the visual system on long-standing retinal artery occlusion (RAO). DESIGN: Open labeled, case series. PATIENTS AND METHODS: Two patients with central RAO (15 and 33 months respectively) and one with branch RAO (26 months) underwent TES therapy. Subjective and objective ophthalmological evaluations were performed before and after the TES. The ages of the patients were 38, 49, and 63 years. The TES (20 Hz biphasic pulses, 30 minutes, up to 1100 uA) was delivered by a bipolar contact lens electrode once a month for 3 months. Perimetric and/or electrophysiological examinations were performed as outcome measures. RESULTS: The visual acuity improved by more than 0.2 logMAR units in two cases, and the visual fields were improved in all three cases. The multifocal ERGs which had been reduced in the loci corresponding to the ischemic retinal area were improved after the treatment in two cases. Neither ocular nor systemic adverse effects were observed except for transient superficial keratitis. CONCLUSIONS: TES of the retina can improve retinal function in eyes with long-standing RAOs.