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D. Y. Tso, J. B. Schallek; Chromatic Bleaching Reveals a Rod-Driven Component in Retinal Intrinsic Optical Signals. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1068.
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To determine the extent of a rod-driven component to intrinsic signals of the retina.
The retinae of anesthetized and paralyzed cats were imaged with a modified fundus camera. A dim near-infrared light (NIR, 700-900 nm) illuminated the eye while a CCD camera captured the NIR reflectance of the ocular fundus. Patterned visual stimuli (540 nm) were projected into the subject’s eye in randomized block-trials. The changes in NIR reflectance in response to the presentation of visual stimuli represent the intrinsic signals. The adaptation-state of the retina was modulated from low-mesopic to photopic light levels by modulating the duration, intensity and chromatic spectrum of a 1-2 minute photobleach of the retina immediately before imaging. Bleaching intensities up to 5 log cd/m2 were used prior to imaging. Stimulus-evoked intrinsic signals were measured throughout 60-80 minutes after the bleach.
After a 60 second photobleach of the retina with 4 log cd/m2 white light, intrinsic signals decreased in magnitude by over 95% of pre-bleach levels. Intrinsic signals slowly recovered to pre-bleach levels over a time course of 40-60 minutes, consistent with a rod dark-adaptation time course. Conversely, when the eye was photobleached with 120 seconds 630+20 nm light (~130 cd/m2, photopic level), intrinsic signals remained at pre-bleach levels, showing little effect. In addition to the bleaching experiments, we found that intrinsic signals showed strong activations in rod dominated regions of the retina (>10 degrees from the area centralis) where rods are known to outnumber cones ≥60:1.
The slow recovery of intrinsic signal after a white light photobleach are consistent with the dark-adaptation time course of rods. Likewise, 630 nm light, which is largely outside the spectral-sensitivity of rods, had little effect on intrinsic signals along the same adaptation time course. Taken together with the observation that signals are strong in rod-dominated areas of the retina, the data suggest that intrinsic signals are sensitive to a rod-driven component.
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