Examining Increment thresholds as a function of pedestal contrast under hypothetical parvo- and magnocellular-biased conditions

Further Examination of the Pulsed- and Steady-Pedestal Paradigms under Hypothetical Parvocellular- and Magnocellular-Biased Conditions

“…Given previous studies that had reported evidence of such suppression, we were intrigued by why they found no evidence for the suppressive effect of red light. In our recent study [ 40 ], we applied Pokorny and Smith’s pedestal paradigms and used colored Gabor stimuli, presented on the same colored background, as pedestals to examine the effects of red vs. green backgrounds on the contrast-increment threshold (Δ C ) as a function of the Gabor-pedestal contrast ( C ). Like Hugrass et al, we observed that the background color (red vs. green) failed to influence increment thresholds.…”

“…Instead, they suggested that both pulsed- and steady-pedestal paradigms primarily track the P-system response. Hence, the paradigms’ inabilities to distinguish between the M and P responses may explain the absence of evidence for the suppressive effect of red light obtained by Hugrass et al and Song et al [ 40 ].…”

“…Only a subclass of M cells, the type-IV cells, are suppressed by uniform red light [ 21 ]. Hence, the red Gabor pedestals we used in our prior study [ 40 ] may not have been optimal in suppressing the type-IV M cells, since the red Gabor stimuli alternate between spatial increments of red light, leading to an increase in suppressive effects, and decrements of red light, leading to a decrease in suppressive effects. Pedestals consisting of spatially uniform increments of red light should therefore optimize the suppressive effects.…”

“…By setting C sat to 1.0, they derived the best-fitting estimates of the other two parameters: K , representing a scaling factor, and R max . In our study, we will likewise apply Equation (2) to obtain level-neutral estimates, not only for R max but also for C sat and δ as we did in our previous research [ 40 ]. The estimates that yield the best fit to our data will enable us to compute the percent contrast gain, expressed as ( R max / C sat )/100.…”

“…As noted above, our investigation centered around the variation of increment thresholds (Δ C ) as a function of pedestal contrast (C) when presenting stimuli in two distinct VFs (see Figure 1 ): the lower-left VF, preferred by the M system, and the upper-right VF, favored by the P system. Although we also aimed to assess the suppression of the M system under red light compared to green light, we based our following hypotheses on our previous findings [ 40 ] by assuming that only the P system is involved, regardless of stimulus color: The pulsed-pedestal paradigm would yield a steeper Δ C vs. C function in comparison to the steady-pedestal paradigm. This expectation arose from the premise that the pulsed-pedestal requires both detection and discrimination (i.e., a more difficult task), whereas the steady pedestal primarily focuses on discrimination (i.e., an easier task); Our expectations regarding visual-field influences on Δ C s varied for the pulsed-pedestal and steady-pedestal paradigms, in part due to these task-demand differences.…”