Abstract
Purpose: :
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.
Methods: :
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%.
Results: :
(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.
Conclusions: :
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.
Keywords: visual cortex • electrophysiology: non-clinical • ganglion cells