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W.H. Seiple, M.E. Schneck, K. Holopigian, C. Clemens; Can Ramp Stimuli be Used to Selectively Isolate Responses to Stimulus Onset or Offset? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3099.
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© ARVO (1962-2015); The Authors (2016-present)
To test the implicit assumption that ramp stimuli effectively isolate onset or offset responses of the ERG.
Ganzfeld ERGs were recorded in response to an array of white LEDs driven in either ascending (rapid off) or descending (rapid on) ramps. The maximum intensity of the stimulus was 150 cd/m2. The duration of the ramp and inter–stimulus interval (ISI) were varied independently. The pupil was dilated and monocular ERG responses were recorded; the contra–lateral eye was patched. The input was amplified 10,000x and band pass filtered between 0.1 and 100 Hz and fifty responses were averaged for each condition. Five normally sighted subjects participated in this experiment.
Large increment (from 10 to 150 cd/m2, 1.2 log change), isolated (ISI > 200 msec), long duration ramps (> 200 msec) produced responses at both onset and offset regardless of the direction of the ramp. Both responses were measurable in recordings in which ISIs longer than 100 msec separated the ramps. The responses to repeated ramps (duration = 200 msec and ISI = 0) were complex in waveform, and the waveforms varied with the ramp direction. These responses appeared to be a complex addition of both onset and offset responses. For smaller increment ramps (75 to 150 cd/m2, 0.3 log unit change), only a response to rapid offset was observed when presenting ascending ramps. In the case of descending ramps, only a response to rapid onset was observed. The responses to repeated ramps using smaller increments had monophasic waveforms that again varied depending upon ramp direction.
Onset and offset responses can be isolated by using small incremental ramps on an adapting background. However, large increment ramps produce responses at both ramp onset and offset. Recent evidence using chemical dissection of retinal responses in monkeys has also shown that ramp stimuli do not necessarily isolate onset and offset responses.*
*Khan et al., 2005
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