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I Vanzetta, DA Nelson, T Bonhoeffer, R Ofri, A Grinvald; Novel Intrinsic Optical Signals in Feline and Human Retina Evoked by Photic Stimulation . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4363.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Retinal reflectance changes in response to stimulation carry information about the metabolic processes underlying light responses in the retina. The Retinal Functional Imager (RFI) (Optical Imaging, Inc.) was used to image photic evoked reflectance changes in cat and human retina. Methods: The RFI measures functional signals by recording and analyzing image series at chosen wavelengths. In cat and human, a low-intensity 1-2s long visual stimulus of 1 µJ/cm2 (510-585 nm) flashing at 100 Hz was used, while recording in the infrared. Intense stimulation in humans was at 1 mJ/cm2 at up to 50 Hz for up to 100 ms; responses were measured either continuously in the infrared or using short (< 2 ms), visible-wavelength "test pulses" at different times during the response. Results: At least three different signals were resolved, varying in polarity, time course, and optimal stimulus. In cat, a fast decrease in retinal reflectance was recorded, followed by a slower and ∼20 times larger increase. The spatial extent of the signals was consistent with stimulus size. The reflectance increase clearly outlasted the offset of the photic stimulus, peaking ∼5 seconds after stimulus onset. The signal decayed to 50% in approximately 10 sec. The time course of the onset of this response is consistent with the positive C-wave component of an ERG, suggesting involvement of the RPE in this optical signal. Using the same paradigm in humans, only a small reflectance decrease was observed, which, based on time course, amplitude, and polarity is consistent with the hemodynamic signal measured in vivo in other neuronal structures, such as the neocortex. The more intense stimulus, however, produced a striking retinal reflectance increase, matching the polarity and amplitude of the slow cat signal. Conclusion: The functional signals imaged by the RFI are due to active retinal processes. Low-intensity stimulus paradigm: negative-reflectance changes cannot be due to pigment bleaching, and the slow time course of the reflectance increase in the cat continues to rise after the stimulus offset. High intensity stimulus paradigm (where some bleaching occurs): the measured signal is strong in the photoreceptor-free optic disk, implying that it must derive from factors other than photoreceptors bleaching alone. These results suggest that the measured optical signals result from activity dependent metabolic processes, and may reflect several aspects of the functional integrity of the retina.
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