Synesthetic colors act like real colors and interact with real colors

Abstract

People with color-graphemic synesthesia experience vivid colors when viewing achromatic alphanumeric characters. We tested two such individuals on tasks for which color is ordinarily important. Binocular Rivalry: Spatially distributed objects of the same color tend to dominate simultaneously during binocular rivalry. In synesthetic observers WO and LR, rivalry between pairs of achromatic letters was influenced by the relation between their associated synesthetic colors. In addition, a real color paired with a synesthetic color produced grouping comparable in strength to that produced by two real colors. Bistable motion: For non-synesthetic observers, color strongly biases perception of otherwise ambiguous apparent motion. For LR and WO, apparent motion between different pairs of achromatic letters was governed by the “color” of those letters. Similarly, motion between achromatic letters and real colored figures was determined by the similarity between the real and the synesthetic colors. McCollough Effect: Prolonged adaptation to color paired with orientation induces an orientation-contingent color aftereffect. LR and WO were alternately adapted to an achromatic “vertical grating” composed of letters synesthetically appearing green and to an achromatic “horizontal grating” composed of letters synesthetically appearing red. Upon viewing an achromatic test figure organized as horizontal and vertical stripes, LR saw horizontal as faint green and vertical as faint red. WO only saw faint red vertical, not green horizontal; intriguingly, WO also does not see “green” when tested using the standard McCollough effect procedure. Conclusions: These findings expand the range of phenomena influenced by synesthetic colors and reveal interactions between real and synesthetic colors.