Sex Hormones Influence the Helmholtz-Kohlrausch Effect

Saturated lights appear brighter than white lights of the same luminance. This is the Helmholtz-Kohlrausch (H-K) effect, and the phenomenon can be estimated by modeling achromatic luminance and saturation to total brightness. Current H-K effect models are different between women and men and are also more variable in women, which may be due to hormonal changes across the menstrual cycle (MC).

Total brightness (B) and achromatic luminance (L) were measured across blue, green, yellow-green, yellow, and red hues. These data were measured along with salivary hormone levels for nine cycling women and seven oral contraceptive (OC) users at points representing the menstrual, peri-ovulation, and luteal phases.

Simple brightness/luminance (B/L) ratio estimates of the H-K effect did not differ by OC use or MC phase, but B/L ratios were higher for the red stimulus in cycling women than OC users during the luteal phase. Estrogen, progesterone, and their interaction predicted 18% of the variation in brightness for cycling women. For OC users, only estrogen could be fit to brightness models where it accounted for 5% of brightness variance.

These findings first provide clear support for separating cycling women from OC users, particularly when examining long-wavelength mechanisms. Next, the interaction of OC use and MC phase on B/L ratios for the red stimulus adds to a rich history of long-wavelength mechanisms. Lastly, the current result amends previous brightness models with multiple hormone terms for cycling women but not OC users.