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M. Rufiange, M. Garon, C. Casanova, M. Dumont, P. Lachapelle; Spectral Electroretinography: Typical S-Cone Response to Red and White Flashes . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1901.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To examine if the bi-phasic ERG waveform, suggested to result from the summation of the L-M- (early component) and S- (late component) cones, is unique to short-wavelength stimuli. Methods: Photopic ERGs were obtained from normal subjects with a LKC UTAS-E-3000 system. The recordings were performed against a white background light of 50 cd.m-2 with the use of 9 intensities of blue (max: 410nm, -2.40 to 0.30 log cd.sec.m-2), red (max: 640nm, -2.23 to 0.08 log cd.sec.m-2) and white (-2.23 to 0.64 log cd.sec.m-2) light. For each intensity, 300 responses were averaged with an inter-stimulus interval of 0.33sec. Results: With the amplitude matching approach used by others, equal b-waves of 5.5 ± 1.9µV were obtained in response to flashes of –1.6 log cd.sec.m-2 with the blue and –1.2 log cd.sec.m-2 with the red and white light. These b-waves, which peaked at 26.8 ± 1.4msec were followed by a second component, which peaked at 43.7 ± 4.3msec. In response to the blue flash, this second component peaked at an amplitude equal or above that reached by the initial b-wave, thus resulting in the typical morphology of the short-wavelength ERG, where the initial b-wave is claimed to originate from L-M-cone activation and the later one from S-cone activation. In contrast, in ERG responses evoked to the red and white flash this second component peaked at an amplitude significantly below that reached by the initial b-wave and was in fact often buried in the descending segment of the b-wave. This resulted in the typical long-wavelength ERG morphology given that only the initial "L-M cone" component is readily obvious. Interestingly however, a further 0.5-0.8 log unit reduction in the intensity of the red and white stimuli generated ERG responses that were identical in shape to that obtained with the blue flash, that is with obvious "L-M and S-cone" components. Conclusions: Our results suggest that the typical, bi-phasic, short-wavelength ERG claimed to result from the summation of the L-M and S-cones can also be generated with a long-wavelength stimulus as well as with white flashes provided that they are dim enough. Analysis of the white-flash luminance-response function reveals that what is identified as the S-cone contribution is most probably the i-wave, which is claimed to represent remnants of the OFF-response presumably originating at the level of the ganglion cells. Funded by CIHR, GRENE and Reseau-Vision.
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