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S. Viswanathan, M.C. Simpson; Photopic Negative Response of the Human Electroretinogram: Effects of Test Flash and Background Intensity . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5701.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The PhNR of the cone ERG originates from inner–retinal activity and is reduced in optic neuropathies. We studied the effects of test flash and background intensities on the PhNR of the human ERG. Methods: ERGs were recorded with a LED–based ESPION system using DTL electrodes from 5 humans. Test flashes (640 nm, –3 to 0.2 log ph cd.s/m2) were delivered from darkness after 40 minutes of dark–adaptation and after 15 minutes of light adaptation on constant blue (440 nm) backgrounds (–2 to 1.9 log sc cd/m2) following initial dark–adaptation. Pupils were fully dilated and accommodation blocked. Results:Under dark–adapted condition, dim flashes evoked two negative potentials, the PhNR that was maximal at 70 ms after the flash and the Scotopic Threshold Response (STR) at 190 ms after the flash. With minimal increase in flash intensity, the cone– and rod b–waves emerged on the leading edges of the PhNR and STR at 46 and 125 ms respectively, the a–wave emerged on the leading edge of the cone b–wave. With further increase in flash intensity the cone and rod b–waves increased in size and the implicit time of the rod b–wave reduced, thereby causing the two b–waves to merge and give the semblance of a single positive potential. For the weakest background tested, the STR but not the PhNR was eliminated and the rod b–wave was considerably reduced in size. The intensity response curve (IRC) of the PhNR revealed that the amplitude increased with flash intensity and saturated around –0.4 log ph cd.s/m2 on the lowest background, whereas the b– and a–waves continued to increase in amplitude. The PhNR was desensitized at the higher backgrounds. With increase in background intensity, the IRC for the PhNR shifted to the right with the intensity at which the PhNR amplitude was half the saturated amplitude increasing by almost 1 log unit from the lowest to the highest background tested. The b– and a–wave amplitudes were less affected by the same adapting backgrounds. Implicit times of PhNR and cone b–wave did not change with test flash intensity for any given background level. However, the implicit time of the PhNR decreased from 95 to 68 ms from the lowest to the brightest background tested and for the cone b–wave the implicit time gradually reduced to about 32 ms with increase in light adaptation. The a–wave implicit time decreased from 23 to 14 ms with increase in test flash intensity under all adapation states. Conclusions: PhNR is a sensitive component of the cone ERG that can be recorded to dim flashes under dark–adaptation. The PhNR response saturates before b–wave saturation and is desensitized by adapting backgrounds to a greater extent than the cone b– and a–waves.
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