Abstract
The pulsed- and steady-pedestal paradigms are believed to track increment thresholds (ΔC) as a function of pedestal contrast (C) for the parvocellular (P) and magnocellular (M) systems, respectively. Plotted on linear ordinate and abscissa, the pulsed pedestal tracks ΔC for the P system, yielding a linear ΔC vs. C function; the steady pedestal tracks ΔC for the M system, yielding a nonlinear function. However, when we recently used Gabor stimuli biased towards the M or P systems based on their sensitivities to color and spatial frequency, we found that the P model generally fit better than the M model for both paradigms (Song, Breitmeyer & Brown, 2022). Here we used pedestals consisting of two-square pedestal arrays, with varied sizes (Experiment 1), colors (Experiment 1, 2), and visual fields (VF) (Experiment 2) to bias processing towards the M or P systems. To optimize activating what we assume are color asymmetric on-center type IV M cells (de Monasterio, 1978; Livingstone & Hubel. 1988, Wiesel & Hubel,1966), all stimuli were luminance increments on a uniform background. In Experiment 1, we used a 2 (color: equiluminant red or green) x 2 (square size: 0.2° or 1.6°) x 2 (pedestal type: pulsed or steady) x 6 (pedestal contrast: 0.0-0.64) design. The design of Experiment 2 was the same except instead of varying size, the two 1.6° squares were presented in either the lower-left or upper-right VF to bias processing towards the M vs. P system respectively. Data analyses from both experiments showed better fits to retinal, LGN, and V1P models than to M models for both paradigms, replicating our previous results. Our findings question the general validity of using the pulsed- and steady-pedestal paradigms to differentiate the M and P systems.