Lingyu Gan¹ (), George Sperling¹; ¹University of California, Irvine

Isoluminance describes a condition where lights of different spectral compositions exhibit equal luminance, allowing researchers to study visual processing independently of luminance cues. Typically, isoluminance is determined using conditions that predominantly stimulate the magnocellular pathway, such as flicker photometry, but it is often applied in conditions involving the parvocellular pathway. The magnocellular and parvocellular pathways serve distinct roles: the magnocellular pathway is sensitive to rapid changes in luminance, whereas the parvocellular pathway is tuned to high spatial resolution and color processing. This study specifically attempts to measure parvocellular isoluminance. The stimuli were the highest resolvable spatial-frequency gratings (28.6-35.8 cpd) presented within .15-.19 deg within the extreme central fovea, discernible only by the parvocellular system. Subjects briefly viewed high spatial-frequency yellow/red and yellow/green gratings and reported grating orientation. The yellow stripes were constant, and the intensity of the red or green stripes varied over the full available range to guarantee achieving isoluminance somewhere in the range. Results: (1) None of the 10 male and 10 female participants experienced a performance reduction to chance for any red or green stripe value (which included all possible isoluminant values). (2) Minimal visibility points in the central fovea generally did not coincide with isoluminant points determined by a motion-reversal paradigm. (3) Large between-subject differences in accuracy when judging the orientation of yellow/red versus yellow/green gratings formed three distinct response patterns: red-dominant, green-dominant, and red-green approximately equally dominant. For green-dominant and red-dominant subjects, luminance information was insignificant for grating acuity in gratings containing the dominant wavelength. Conclusions: (1) In the central fovea, the luminance system does not provide any advantage over the color system for grating acuity. (2) The concept of isoluminance for the multichromatic parvo system is fundamentally different from that of the monochromatic magno system, as typically determined by flicker photometry and similar methods.