December 2022
Volume 22, Issue 14
Open Access
Vision Sciences Society Annual Meeting Abstract  |   December 2022
Examining increment thresholds as a function of pedestal contrast under hypothetical parvocellular- and magnocellular-biased conditions.
Author Affiliations
  • Jaeseon Song
    University of Georgia
  • Bruno G. Breitmeyer
    University of Houston
  • Jeffrey Nightingale
    University of Georgia
  • James M. Brown
    University of Georgia
Journal of Vision December 2022, Vol.22, 3225. doi:
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      Jaeseon Song, Bruno G. Breitmeyer, Jeffrey Nightingale, James M. Brown; Examining increment thresholds as a function of pedestal contrast under hypothetical parvocellular- and magnocellular-biased conditions.. Journal of Vision 2022;22(14):3225.

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      © ARVO (1962-2015); The Authors (2016-present)

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We used the pulsed- and steady-pedestal technique to examine how increment thresholds (ΔC) varied with pedestal contrast (C). Theoretically, the pulsed-pedestal method tracks ΔC for the parvocellular (P) system yielding a linear ΔC vs. C function; the steady-pedestal method tracks ΔC for the magnocellular (M) system yielding a nonlinear function. The M system also a) prefers low spatial frequencies (SFs) whereas the P-system prefers higher SFs and b) the M system is suppressed by red relative to green light. Our study used a 2 (background: physically equiluminant red or green) x 2 (pedestal SF: .75 c/d or 6.0 c/d Gabor) x 2 (pedestal type: pulsed or steady) x 6 (pedestal contrast: 0-0.64) design. Hypotheses: (1) For the low SF, 0.75 c/d pedestal, a) the pulsed-pedestal method yields a linear ΔC vs. C function, whereas b) the steady-pedestal method tracking the M system yields a nonlinear function. Moreover, in the latter case, since the M system’s contrast sensitivity should be lower for the red than the green background, ΔC values are higher for the red background. (2) The 6.0 c/d pedestal yields similar results, but since this higher SF is not optimal for the M system, the effects of pulsed vs steady pedestals and of red vs green backgrounds are attenuated, if not eliminated. Results unexpectedly showed the P models fit better than the M models across all experimental conditions, except for one. Linear functions fitted even better than the P models. The linearity of the ΔC vs. C functions across these experimental conditions indicates that the P-channels were predominant throughout. These findings question the general validity of the pulsed- vs steady-pedestal method as differentiating the P from M system and explain the lack of evidence of red-light suppression of the M system.


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