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R. Windisch, A. F. Cruess, F. Tremblay; Modelling of the On and Off Interactions of the Photopic ERG Oscillatory Potentials. Invest. Ophthalmol. Vis. Sci. 2007;48(13):536.
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To investigate the variations of oscillatory potentials (OPs) in function of flash and background intensity, to provide an explanation for the photopic hill.
A total of 20 normal adult subjects were investigated with three electroretinogram (ERGs) protocols. General procedures included pupil dilatation, corneal anesthesia and use of a Lovac contact lens referenced to the forehead (ground at the earlobe). The first protocol included adaptation to background luminance of 34 cd/m2 for 30 seconds, followed by constant flash stimulation of 10.1 cd•s/m2 under randomly selected backgrounds levels of 0, 1, 2,5,10,20,25 and 30 cd/m2. OPs were averaged 20 times after analog filtration with a 100 Hz high pass filter. The second protocol looked at the ERGs generated by series of flash intensities varying from 0.3 to 10.1 cd•s/m2 in steps of 0.25 log, presented against fix background intensities of 6.8, 34 and 102 cd/m2. Finally, we also looked at long duration flashes of 4, 10, 50, 100 ms in the same conditions as mentioned above, to isolate the ON and OFF components. All data were exported to a spreadsheet to be analyzed by ANOVA with repeated measures in terms of amplitude and implicit time.
Early OPs (2-3) are sensitive indicators of background intensity, showing mean implicit time of 16.7±0.5 ms in conditions of darkness, 15.7±0.4 in mesopic and 16.0±0.3 ms in photopic background luminance. The 4th OP did vary linearly to background intensities. In response to variation in flash intensities, OP2 & 3 decreased in implicit time as flashes were increased, while OP4 did the opposite. With brighter flashes, the delay induced in OP4 unveiled other small OPs. The use of long-duration flashes confirmed these small OPs share the same properties as the early ones.
We propose that the early OPs belong to the ON system while the late ones are associated with the OFF system, both systems having different temporal response characteristics. The combination of these two sets of OPs could help explain the variety of morphological features demonstrated by the photopic ERGs as well as the photopic hill of the intensity-response curve
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