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K. F. Grieve, P. Tiruveedhula, E. A. Rossi, A. Roorda; Measuring Intrinsic Retinal Signals With the Adaptive Optics Scanning Laser Ophthalmoscope. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1954. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
In an effort to develop non-invasive means to relate structure to function in human eyes, we investigated the phenomena of intrinsic retinal signals at high resolution using an adaptive optics scanning laser ophthalmoscope (AOSLO).
AOSLO was used in dual-wavelength mode to stimulate the retina with 658 nm light and simultaneously image the retina with 840 nm infrared light. Modulation of each laser beam allowed for the delivery of complex stimuli into the scan, whose retinal locations could be recorded into the movie. 30 second movies were recorded with stimulation occurring at 5 seconds. Intensity changes in the infrared image in response to the visible stimulus were monitored over time. Two types of stimulus were used: i) a checkerboard or a ring pattern was used, and a difference image of the maximum response minus baseline computed; ii) a half-field stimulus was used, and after correction of eye movements, the ratio of the intensity of the stimulated region to that of the non-stimulated region was plotted. Stimulus parameters such as duration, power, frequency, form, light versus dark adaptation, and depth location were varied.
In two observers, difference images showed an increase in infrared light scattering in the stimulated region with respect to its surroundings. The intensity ratio of stimulated to non-stimulated regions showed an increase in scattered infrared light after stimulation, beginning at stimulus onset, reaching a peak around 5 seconds, and decreasing to baseline within another 2-5 seconds. Once past threshold, varying stimulus power or duration did not alter response magnitude, which remained at around 5% increase. 15 Hz flicker stimuli elicited more robust responses than 30 Hz stimuli. Response in a light adapted retina was reduced to about half that from a dark adapted retina on one observer. Preliminary attempts to localize the response in depth suggest that the photoreceptor layer gives the largest contribution to the measured signal. While two observers showed reliable and unambiguous responses, the findings were not repeatable in other eyes. The reasons for weak or absent intrinsic signals are not well understood, but may be due to artifacts from excessive eye movements, or low signal-to-noise in eyes with weakly scattering retinas.
We can measure intrinsic retinal signals at high resolution with the AOSLO. An increase in scattered infrared light in response to visible stimulation was observed in two subjects. The most robust response was elicited by a 2 second, 15 Hz flicker stimulus on a dark adapted retina.
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