Does Vergence Affect Perceived Size?

Since Kepler (1604) and Descartes (1637), it’s been suggested that ‘vergence’ (the angular rotation of the eyes) plays a key role in size constancy. However, this has never been tested divorced from confounding cues such as changes in the retinal image. In our experiment participants viewed a target which grew or shrank over 5 seconds. At the same time the fixation distance specified by vergence was reduced from 50cm to 25cm. The question was whether the reduction in the viewing distance specified by vergence biased the participants’ judgements of whether the target grew or shrank? We found no evidence of any bias, and therefore no evidence that eye movements affect perceived size. If this is correct, then this finding has three implications: First, perceived size is much more reliant on cognitive influences than previously thought. This is consistent with the argument that visual scale is purely cognitive in nature (Linton, 2017; 2018). Second, it leads us to question whether the vergence modulation of V1 contributes to size constancy. Third, given the interaction between vergence, proprioception, and the retinal image in the Taylor illusion, it leads us to ask whether this cognitive approach could also be applied to multisensory integration.