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R. D. Hamer, F. A. Carvalho, D. F. Ventura; Contrast Dependence of Vernier (VRN) Displacement Thresholds Assessed by Steady-State Sweep VEP Supports the Hypothesis That Magnocellular Input to Cortex Underlies VRN Performance. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1498.
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Primate magnocellular (M) ganglion cells appear to have higher spatial precision (with higher SNR) than parvocellular (P) cells, and the dependence of M cell spatial precision on contrast (C), SF, TF and stimulus velocity is more similar to human psychophysical performance than comparable data from P cells (Ruttiger et al, 2002; Sun et al., 2003, 2004). These authors hypothesize that M ganglion cells provide the retinal signal to cortex adequate to support vernier performance. We measured the C-dependence of cortical vernier (VRN) thresholds (thd) using the Sweep VEP (sVEP) to help evaluate this hypothesis.
sVEP thds were measured in 12 young adults with normal vision. Vertical VRN breaks in colinearity were introduced to a horizontal squarewave grating (12.7x9.4o; 2 c/d; mean lum = 161 cd/m2), 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 zero mV. Cs tested: 4, 8, 16, 32, 64, 80%.
(1) Log VRN thd decreased linearly with log C with a slope of -0.5. (2) For C ≥ 16% , thds were hyperacuities (<1’). At high C, mean thd was 0.37’. (3) Thds for 2F had a different C-dependence, with little effect of C ≤ 16%. Thds for 2F were < 1F thds below 16% C, but were ≈ 1F thds beyond 16%. (4) The slopes of the sVEP extrapolation lines for 1F were 2-3 times > 2F slopes. (5) In a control protocol, symmetric, bidirectional displacements only generated 2F responses. (6) When the static and moving columns of bars had different Cs (16% moving; variable C static), a 1F response occurred when the static-bar C was as low as 1-2% C. No 1F occurred when the static bar had 0% C.
Results 3-5 imply that the 1F and 2F components derive from distinct neurons, and support the notion that 2F responses reflect symmetric cortical motion responses. The C-dependence of sVEP VRN (1F) thresholds is similar to prior psychophysics, and recapitulates M-cell C-dependence (Wehrhahn & Westheimer, 1990; Sun et al., 2003, 2004). Results 1 and 6 support the hypothesis that cortex extracts relative position information with hyperacuity precision preferentially from M cell signals.
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