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Russell D. Hamer, Fabio A. Carvalho, Dora F. Ventura; The effect of Gap and Contrast on Sweep VEP Responses to Periodic Vernier Offsets. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6095.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the contrast (C) dependence of cortical vernier (VRN) thresholds (thd) using the Sweep VEP (sVEP) as a function of spatial separation between the VRN elements (gaps).
sVEP thds were measured in 10 young adults with normal vision. Vertical VRN breaks in colinearity were introduced to a horizontal squarewave grating (12.7x9.4 deg; 2 c/d; mean lum = 161 cd/m^2), generating 6 moving + 6 static interleaved vertical columns of bars, for a total of 418 VRN breaks. The VRN stimulus alternated between aligned (grating w/o breaks) and misaligned (w/breaks) states at 6 Hz. During each of ten, 10-s trials, displacement (D) was swept logarithmically from <=0.5’ to <=7.5’. VRN thd was defined as the D at which the rising slope of the vector averaged 1F response extrapolated to 0 microV. The extrapolated thd for the 2F component estimates a local relative Motion thd (e.g., Norcia et al., Vis Neurosci 16, 1123, 1999). Cs tested: 4, 16, 32, 64, 80%. Gaps tested: 0, 2’,5’.
(1) At 64% contrast, peak 1F amplitude decreases by ~2 - 3x for gaps of 2’ and 5’, and closely approximates the gap-dependence found by Norcia et al. (2) The gap effect also occurs at lower Cs, until 4% C is reached, at which point the 0’-gap condition only generates small amplitudes and is decreased little by gaps. (3) The 2F amplitudes show much less dependence on gap for all Cs tested (4) VRN (1F) thds increase by 1.5-2x with a gap of 2’, and by > 2x for 5’ gaps for all contrasts, including 4%. (5) The Motion (2F) thds are independent of gap at 64, 32, 16%, but increase by 2x between 2’ and 5’ at very low contrast (4%).
As is found psychophysically (e.g. Westheimer & McKee, Vis Res 17, 941, 1977) and electrophysiologically (e.g. Norcia et al.), we find VRN acuity is degraded by gaps in high-contrast stimuli. The effect of gap at different contrasts on the sVEP VRN responses has not, however, been examined before. The data show sVEP 1F (but not 2F) thresholds are highly sensitive to gap at intermediate and high contrasts. The different gap-dependence of 1F and 2F amplitude and thd results confirm the notion that they derive from different cortical generators, VRN and motion-sensitive neurons, respectively.
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