Do you see what I see? Linking involuntary nonretinal (phantom) vision and mental imagery in aphantasia

Recent theories propose that, like endogenous and exogenous visual attention, voluntary and involuntary forms of phantom vision (e.g., mental imagery and dreams) are related and hence depend on overlapping mechanisms. However, the relationship between voluntary and involuntary phantom vision remains largely unknown. Here, we assess this relationship by examining how voluntary visual imagery relates to involuntary forms of phantom vision (specifically, visual illusions) in a unique population with no voluntary visual imagery (aphantasia). In our first study, we presented individuals with aphantasia with seven different visual illusions (Hermann grid, Ponzo illusion, Kanizsa triangles, Ebbinghaus illusion, watercolor effect, neon color-spreading, and rotating snakes). Compared to both a large group of undergraduates and an age-matched control sample, the only illusion in which individuals with aphantasia reported a significant reduction was the neon color illusion. In a large online follow-up study, we used the method of adjustment to obtain a more precise measure of the neon color-spreading illusion in individuals with aphantasia and those with visual imagery. We found that this measure of neon color was lower in those with aphantasia than in those with visual imagery, as were their subjective ratings of the illusion. Importantly, there were no differences between the groups for catch/mock neon color "illusion" trials or a separate color adjustment task. Together, these data provide evidence that individuals with aphantasia experience the neon color illusion at a lower intensity, supporting the hypothesis that some forms of voluntary and involuntary phantom vision depend on overlapping mechanisms.