Lemona Xinxuan Zhang¹, Abigail Flowers¹, Wumeng Wang¹, Lucy P Somers¹, Jenny M Bosten¹; ¹University of Sussex

Color experiences, typically elicited by colored light, can also arise in the absence of physically colored stimuli. Uniform-field (ganzfeld) intense flickering white light can evoke hallucinatory colors and forms. The mechanisms underlying this phenomenon, known as flicker-induced colors, are not well understood, but may provide insights into the neural basis of color perception. We aimed to investigate the correspondences between specific flicker frequencies and hallucinatory colors. In Experiment 1, participants completed a 2-session dichoptic color matching task. To one eye, we presented intense unstructured flicker using a custom LED stroboscope. To the other eye, we presented an adjustable colored disk on an LCD display. In session 1, participants explored the flicker frequency range (3-38 Hz), and adjusted the hue and saturation of the disk to match any (and all) hallucinatory colors they experienced at each frequency. In session 2, participants were presented with flicker at their previously matched frequencies, and were asked to match perceived colors again. A permutation-based analysis revealed that observed between-session color differences for same-frequency color matches were significantly smaller than permuted between-session color differences at randomly selected (unmatched) frequencies (p < .01). In Experiment 2, we investigated the frequency-specificity of flicker-induced colors by verbal report, avoiding dichoptic presentation. Over two sessions, participants reported hallucinatory colors while the flicker (3-38Hz) was presented binocularly. In concordance with the results of Experiment 1, we found a significant within-individual correspondence between flicker frequency and hallucinatory colors (p < .01). Our findings demonstrate reliable correspondences between the frequencies of flickering white light and induced illusory color experiences, compatible with a frequency-based component in the cortical coding of color.

Acknowledgements: This work was funded by the ERC grant 949242 COLOURCODE to JMB.