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J Racine, S Joly, M Rufiange, E Simard, P Lachapelle; Scotopic Oscillatory Potentials: Of Mice and Men . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1823.
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Purpose: The scotopic electroretinogram (ERG) of mice is often presented subjectively as one with highly prominent oscillatory potentials (OP) compared to other species. The aim of this study was to document the above with objective measurements. Methods: Scotopic (Grass PS-22 photostimulator; intensity: -6.3 to 0.9 log cd.m-2.sec; duration: 20 µs) ERGs were obtained from adult albino and pigmented rats and mice as well as from normal human subjects. The ERGs were recorded after a period of dark adaptation of 12 hours (rodents) or 30 minutes (human subjects). In order to objectively evaluate the impact of the OPs on the ERG b-wave, the amplitude of the b-wave was measured as follows: from the trough of the a-wave to the peak of the b-wave (b-wave amplitude), and from the trough of the a-wave to the peak of the first OP, and then from trough to peak until the peak of the b-wave was reached (Sb-wave amplitude). An OP index was obtained by dividing the Sb-wave amplitude over the b-wave amplitude. Results: In all species, analysis of the luminance-response function showed that a gradual increase in the intensity of the flash caused a progressive increase of the OP index until it reached a maximum following which further increases in flash intensity reduced the OP index. In albino rodents the maximum OP index was of 1.73±0.18 (mice) and of 1.52±0.15 (rats) compared to 1.60±0.19 and 1.39±0.22 for pigmented mice and rats respectively. In mice, the maximum OP index was reached with an intensity of -1.2 log cd.sec.m-2 compared to 0.3 log cd.sec.m-2 in rats. In humans, the maximum OP index was of 1.22 ± 0.24 and was obtained with an intensity of -0.96 log cd.sec.m-2. In all subjects (rodents and humans) full dark adaptation yielded a maximum of 5 OPs on the ascending limb of the b-wave. This number was reduced to a maximum of 3 with a shorter dark adaptation period. Conclusion: Our results reveal that the OP index is highest in mice and the intensity needed to reach it is the lowest. This gives support to the subjective claims that the scotopic ERG of mice is OP-prone. Our results further reveal that the OP index is greater in albino rodents than pigmented ones, and also greater in rodents compared to human subjects. It remains to be determined if the above species differences are based on specific differences in retinal physiology and/or structure(s). Kowledge of the latter will not only be instrumental in explaining the above reported OP differences but also provide key information that will help identify the intra-retinal origin of the OPs, a topic which remains debated.
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