Resumo

Purpose: Primate magnocellular (M) ganglion cells appear to have higher spatial precision (with higher SNR) than parvocellular (P) ganglion cells, and the dependence of M cell spatial precision on contrast, SF, TF and stimulus velocity are more similar to human psychophysical performance than comparable data from P cells (e.g., J Vis. 232:2,2002; Vision Res. 19:44,2004). Thus, these authors hypothesize that M (but not P) ganglions cells provide the retinal signal to cortex adequate to support vernier performance. We sought to measure directly the contrast (C) dependence of vernier (VRN) thresholds (thd) at the level of human visual cortex using the Sweep VEP. Methods: VRN thds were measured in 12 young adults with normal vision using the steady-state Sweep VEP. Unidirectional periodic breaks in colinearity were introduced to a horizontal squarewave grating (12.7x9.4 deg; 2 c/d; Mn lum = 161 cd/ m2) so that each bar had 9 breaks yielding 5 moving and 5 static bar elements, for a total of &cong340 VRN breaks. During each of ten 10-s trials, the magnitude of the displacement (D) was swept logarithmically from ≤ 0.5′ to ≤ 7.5′ (¼ cycle). VRN thd was defined as D at the extrapolation to zero νV of the first harmonic (1F) component of the vector averaged response. Contrasts: 4, 8, 16, 32, 64, 80%. Results: (1) For C ≥ 16% , thds were hyperacuities (<1′). At high C, Mn thd was 0.37′. (2) Log VRN thd decreased linearly with log C with a slope of -0.5. (3) Thds for 2F had a different C-dependence, with almost no effect of C ≤ 16%. 2F thds were < 1F thds below 16% C, but were &cong 1F thds beyond 16%C. (4) Symmetric, bidirectional displacements did not generate 1F responses, only 2F. (5) The slope of the extrapolation lines for 1F was &cong3 times larger than for 2F. (6) F1 and F2 peak amplitudes increase linearly with logC up to 16% . (7) F1 pk amp grows &cong2.6 times faster 2F. (8) Pk amp satures for C ≥ 16%. (8) Results 3-7 imply that the 1F and 2F response components derive from distinct neural tissue, and support the notion that the 2F component derives from a symmetric cortical motion response. (9) When the static and moving bars had different Cs (16% moving; variable C static), a robust 1F response occurred when the static bar C was as low as 1-2% contrast. No 1F occurred when the static bar had 0% C. Conclusions: The C-dependence of Sweep VEP VRN thresholds similar to some prior psychophysical measures (Exp Brain Res 618:80,1990; Vision Res. 19:44,2004), and recapitulates M- but not P-cell C-dependence. The results support the hypothesis linking retinal M ganglion cell output to the cortical processing that extracts relative position information with hyperacuity precision.