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N.V. Rangaswamy, B. Digby, R.S. Harwerth, L.J. Frishman; Optimizing the Spectral Characteristics of a Ganzfeld Stimulus Used for Eliciting the Photopic Negative Response (PhNR) . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4762.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine flash and background colors in a ganzfeld LED stimulator that will best isolate the PhNR and maximize its amplitude in the photopic ERG. Methods: Photopic full–field flash ERGs were recorded differentially between the two eyes of anesthetized macaque monkeys (ketamine/xylazine) before and after pharmacologic blockade of Na+–dependent spiking activity with TTX (tetrodotoxin, 1–2 µM; n=2), or laser–induced monocular experimental glaucoma (n=3), as well as in several other normal eyes. The monocular stimulus was produced by an LED–based stimulator (ESPION Colorburst; Diagnosys LLC) with red (max=650 nm), green (max=510 nm)and blue (max=465 nm) LEDs. Photopically matched flashes of brief or long duration (200 ms) were presented on scotopically matched blue or white (CIE 0.333, 0.333) backgrounds of 3.7 log sc Td. Photopic luminance of the white background (3.3 log Td) was higher than that of the blue (2.7 log Td). Results: A negative–going component (PhNR) that could be eliminated by TTX or severe experimental glaucoma was present in responses to brief and long duration stimuli of all colors. In response to a brief flash, the PhNR appeared as a slow negative wave following the b–wave. In normal eyes PhNR amplitude (measured from baseline for stimuli of 1.6 and 1.9 log Td.s) was significantly larger for red flashes on a blue background than for white flashes on white background; amplitudes for other colors were not significantly different from those for red or white flashes. In response to long duration flashes, the PhNRon appeared as a slowly recovering dip in a plateau that was of lower amplitude than the transient b–wave, and lasted for the duration of the stimulus. The TTX (or glaucoma)–sensitive PhNRon amplitude was similar for all color combinations except for the white flash on a white background where it was smallest, due at least in part to the photopic strength of the background. For red stimuli removing the PhNRon elevated the plateau to the peak level of the b–wave whereas for white on white stimuli (and other colors) a plateau of lower amplitude than the b–wave remained. Therefore the negative–going PhNRon was best isolated when elicited by red stimuli. Conclusions: The best color combination to isolate and maximize the amplitude of the PhNR is red flash on a blue background. The blue background saturates rods, while hardly affecting cone responses. For the long stimulus, the red flash best isolates the PhNRon because it minimizes the lower level plateau overlapping the PhNR that has previously been shown to originate from hyperpolarizing second order retinal neurons.
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