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G. S. Souza, H. B. Shackelford, B. D. Gomes, M. E. C. Fitzgerald, L. L. Silveira; A Signal to Noise Comparison Between Pattern Reversal and Pattern-pulse mfVEPs. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5877.
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© ARVO (1962-2015); The Authors (2016-present)
To compare pattern-reversal and pattern-pulse mfVEPs at different stimulus contrast levels and eccentricities.
Two groups of four subjects (26.5 years-old ± 3.09) with normal or corrected to normal visual acuity were tested monocularly. Two subjects participated in both groups. The stimulus consisted of a standard dartboard pattern array, 40° of visual angle, composed of 120 sectors scaled in size across eccentricity to compensate for the variation of cortical magnification factor. Each pattern was made of a circular array of patches comprising 4 light checks and 4 dark checks whose spatial Michelson contrast was 100, 50, 25, and 12.5%. An m-sequence with 215-1 elements was used to drive stimulus presentation. The state 1 of the m-sequence consisted of pattern exhibition for both presentation modes, whilst the state 0 consisted of the same pattern with 180° spatial phase reversion for pattern-reversal or a homogenous area with same mean luminance for pattern-pulse modes; both states 1 and 0 lasted 26.6 ms. mfVEPs were extracted and analyzed to provide signal to noise ratio (SNR) of the first order kernel (1K) and first slice of the second order kernel (2.1K) for each presentation mode.
For pattern reversal, 1K was absent and 2.1K had 75% of reliable responses at 100% of contrast. When the contrast was changed 2.1K showed lower reliability than 1K. The SNR of 2.1K was better at 25%-50% contrast range. For pattern-pulse at 100% contrast, 1K was above of the noise level at all locations. The SNR ratio decayed linearly with stimulus contrast. 2.1K was above the noise level in 78% of all locations, showed the same behavior of 1K as a function of stimulus contrast, however with lower values. The SNR of 2.1K from pattern-pulse was 2.88 times higher than 2.1K from pattern reversal at 100% for the 48 central locations.
The pattern-pulse presentation mode is a better stimulation than pattern reversal mode to apply in clinical testing due to the efficiency in eliciting valid, above noise level responses and with higher SNR.
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