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N.W. Khan; Double Peaked Photopic Flicker Is Dependent on Stimulus Intensity . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4555.
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Purpose: The 30 – 32 Hz flicker elicited with xenon pulse stimuli has been shown to have a double peaked waveform in congenital stationary night blindness (CSNB)1,2,3. However, this type of notched waveform is also present in normal subjects and in some retinal dystrophies. The characteristics of these peaks were studied by manipulating stimulus intensity. Methods: Ganzfeld photopic flicker ERGs were elicited using 32 Hz xenon pulse stimuli across an intensity range of –1.5 to 0.7 log cd–s/m2 on a 32 cd/m2 background. Flicker ERGs were recorded after 10 minutes of light adaptation from 10 normal subjects, 4 congenital stationary night blindness (CSNB) with NYX mutations, 4 retinitis pigmentosa (RP) patients, and 5 X–linked retinoschisis (XLRS) genotyped patients. Results: Normal subjects show a single peak (P1) on the flicker response for stimuli less than –0.11 log cd–s/m2. Higher intensities elicit a second peak (P2) on the falling slope of the waveform. P2 implicit time shortens progressively as stimulus intensity increases, and by 0.22 – 0.46 log cd–s/m2, the waveform shows a notch in eight out of ten normal subjects. This double peaked (P1 + P2) waveform is prominent in some but not all normal subjects. With further increase in intensity, P2 forms the waveform peak, and P1 becomes smaller and faster. Two of four CSNB subjects showed a notch at 0.46 log cd–s/m2. RP patients had the smallest flicker amplitude (1.5 µV to 28 µV), yet the notch was present in three of four subjects at intensities similar or below those for normals. Two XLRS patients showed a double peak at –1.11 log cd–s/m2, however for intensity 0.46 log cd–s/m2 and above, P2 was dominant and P1 was on the rising slope of the waveform. Conclusions: The double peaked flicker waveform is seen in most normal humans and in some retinal dystrophies. The implicit time of P1 and P2 varies with stimulus intensity, and there are differences in intensity at which they form a notch. In some subjects, both normal and retinal dystrophy, P2 implicit time shortens more rapidly causing it to merge with P1. Therefore, for these subjects the notch may not be apparent at intensities used clinically. The notched waveform is not characteristic of any particular retinal dystrophy. From our normal data, the flicker amplitude and implicit time measured for a clinical flash represent predominantly P2. 1. Miyake et al. (1987) Invest Ophthalmol Vis Sci 28:1816–1823. 2. Kim et al (1996) Vision Res. 37: 2471–2475. 3. Allen et al. (2003) Br J Ophthalmol 87:1413–1420.
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