Do you see what I see? Linking involuntary non-retinal (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 investigating how voluntary visual imagery is related to involuntary forms of phantom vision (specifically visual illusions), by investigating 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, watercolour effect, neon colour-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 consistent significant reduction in, was the neon colour illusion. In a large online follow-up study we used the method of adjustment to obtain a more precise measure of the neon colour-spreading illusion in individuals with aphantasia and those with visual imagery. We found that this measure of neon colour was lower in aphantasia than those with visual imagery, as were their subjective ratings of the illusion. Importantly, there were no differences between the groups for catch/mock neon colour ‘illusion’ trials or a separate colour adjustment task. Together these data provide evidence that individuals with aphantasia experience the neon colour illusion at a lower intensity, supporting the hypothesis that some forms of voluntary and involuntary phantom vision depend on overlapping mechanisms.