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X. Zhang, M. Wang, D. C. Hood; Simultaneous Recording of Multifocal Visual Evoked Potentials to Short Wavelength and Achromatic Stimuli. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6193.
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© ARVO (1962-2015); The Authors (2016-present)
To develop a paradigm that allows for simultaneous recording of multifocal visual evoked potentials (mfVEP) to both short wavelength (SW) and high contrast achromatic (A) stimuli.
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), 8 (A8) or 64% (A64). The RGB colors for the SW stimulus were [0.5 0.5 0] vs. [0.5 0.5 1]. There were four conditions, each with two patterns: A64/SW (A64 alternated with SW), A64/A8, A0/A8, A64/A0. The L & M contribution of the A8 stimulus approximately matched that of the SW stimulus, 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 and A8 was 40 ms. The stimuli were spatially and temporally sparse . The minimum time between appearances of a pattern onset at spatially adjacent sectors of any color was 53.2 ms, between the same color stimuli for a 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. A three-channel electrode placement was adopted . Six subjects with normal vision participated.
First, the new A64/SW paradigm yielded robust responses to both the A64 and SW stimuli. The average SNRs across the 60 local responses were 3.26±1.29 and 3.45±1.13 dB for A64 and SW, respectively. However, the A responses were larger and faster than those for SW at the center ring (1 deg in diameter) by 2.03±0.57 dB, consistent with the sensitivity of the S-cone system. Second, while there was a small response to the A8 stimulus when recorded alone (i.e. A8/A0), the A8 response was reduced to the noise level when recorded with A64 (A64/A8). Thus, L & M systems contribute little or nothing to the SW response in the A64/SW paradigm. Finally, a comparison of the A64/SW and A64/A0 conditions indicated that the presence of the SW stimuli in the A64SW paradigm reduced the SNR of the A64 response by 30%, as compared to the A64 response recorded alone (A64/A0).
A new paradigm can provide both SW and achromatic mfVEPs within one recording with a modest reduction in the achromatic mfVEP amplitude and a relatively pure S-cone mfVEP to the SW stimuli and , may provide better detection of glaucomatous defects.
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