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X. Zhang, D. C. Hood; Isolation of a S-Cone Response With a Multifocal Visual Evoked Potential (mfvep) Technique. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6434.
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To test whether the LM cone contribution to the mfVEP can be removed from the response to a Blue/Yellow (SW) stimulus with the presence of high contrast achromatic (A) stimulus.
The mfVEP display was a 60-sector, dartboard array, 45° in diameter, with individual sectors presented as pattern onset stimuli. The contrasts for the A stimuli were 0 (A0: no pattern present), or 64% (A64). The RGB colors for the SW stimulus were [0.5 0.5 0] vs. [0.5 0.5 1] with the mean luminance of 62 cd/m2. There were two background conditions, 80 cd/m2 (similar to what is used in SW automatic perimetry SWAP test) yellow (590±12 nm LED) light background (Y80) and no background (Y0), and two patterns, A64/SW (A64 alternated with SW) and A0/SW for a total of 4 conditions. The LM contribution of the SW stimulus was approximately equal to an 8% achromatic stimulus, and the SW cone contrast is 86.5% based on the cone excitations . At any given moment, the display appeared a homogeneous gray equal to the mean luminance of the stimuli or a mix of gray sectors interspersed with sectors with the same A or SW pattern. All stimuli were modulated with m-sequences of 1024 steps with a screen refresh rate of 13.33ms/frame. The duration of the A64 stimuli was 26.6 ms, while the duration of the SW was 40 ms. The stimuli were spatially and temporally sparse . The minimum time between appearances of a pattern onset at spatially adjacent sectors was 53.2 ms between the same color stimuli for any sector was 213 ms, and between different color stimuli for a sector was 106 ms. Each run lasted for 3.6 min and was repeated twice for both OS and OD separately. A three-channel electrode placement was adopted . Six subjects with normal vision participated.
The Y80 background reduced the amplitude of both A64 and SW for both display patterns. While it prolonged the latency of the SW response in A0/SW pattern (an increment in delay of 4.3ms ±3.1ms, p less than 2%), it did not significantly prolong the latency of the SW response to the A64/SW stimulus (an increment in delay of 1.39 ms ±1.33ms, p>5%).
It is known that the S cone response has longer latency than the LM cone response and that the yellow background significantly reduced (>50%) the LM cone constrast in the SW stimulus. As expected based on the overlap between blue gun of CRT monitor and LM cone sensitivity, the yellow background remove some LM component in the SW response that is recorded alone. The fact that it does not significantly alter the latency of the SW response in the A64/SW conditions suggests that the LM component in the SW response is removed by the presence of the A64 stimulus.
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