Paolo Antonino Grasso¹, David Henry Peterzell, Federico Tommasi, Chiara Magnolfi, Linda Favillini, Rebecca Franconi, Elisabetta Baldanzi, Massimo Gurioli, Alessandro Farini; ¹University of Florence, Florence, Italy, ²National Research Council, National Institute of Optics, Florence, Italy, ³Fielding Graduate University, Santa Barbara, California, ⁴National University, Pleasant Hill, California

During simultaneous color contrast, observers perceive 'illusory' color in a stimulus influenced by its surrounding context. This study investigated factors driving individual differences in these perceptions to elucidate underlying visual processes. Seventy-four individuals participated. In an achromatic test stimulus presented with eight different colored surrounds, a color matching task measured perceived hue and chromatic induction strength ('saturation'), while a color classification task assessed whether the stimulus appeared as a color or gray. Individual chromatic sensitivity was assessed using the Color Assessment and Diagnosis Test (CAD), which provided thresholds for red-green (RG) and yellow-blue (YB) channels. Individuals varied considerably in their color contrast perceptions. (1) Most participants saw colors complementary to the surround, though 10-30% indicated a gray appearance. (2) The strength of chromatic contrast saturation was inversely related to chromatic sensitivity, especially in the YB channel, where higher CAD thresholds correlated with stronger chromatic induction. Strength of saturation was positively associated with age, with older participants experiencing stronger illusions. (3) Saturation strength across hue angles showed very high intercorrelations, consistent with a broadly tuned factor mediating chromatic induction. A two-factor factor analysis identified highly intercorrelated factors for inducing reds and blues (F1: 180° to 360°) or yellows and greens (F2: 45° to 180°), not supporting mechanisms tuned to cardinal color axes or narrowly tuned chromatic channels.(4) Multiple regression analyses revealed that age mediated YB-saturation correlations, while RG-saturation correlations remained independent of age. To conclude, our results revealed that individuals with the lowest sensitivity in YB and RG channels experienced the strongest saturation, potentially reflecting a chromatic induction process compensating for reduced chromatic sensitivity.

Acknowledgements: European Union - PON Research and Innovation grant 2014–2020