Jitter and size effects on vection are immune to experimental instructions and demands

Both coherent perspective jitter and explicit changing-size cues have been shown to improve the vection induced by radially expanding optic flow. The current study examined whether these stimulus-based vection advantages could be modified by altering cognitions/expectations about both the likelihood of self-motion perception and the purpose of the experiment. In the main experiment, participants were randomly assigned into two groups – one where the cognitive conditions biased participants towards self-motion perception and another where the cognitive conditions biased them towards object motion perception. Contrary to earlier findings by Lepecq et al (1995), we found that identical visual displays were less likely to induce vection in ‘object motion bias’ conditions than in ‘self-motion bias’ conditions. However, significant jitter and size advantages for vection were still found in both cognitive conditions (cognitive bias effects were greatest for non-jittering-same-size control displays). The current results suggest that if a sufficiently large vection advantage can be produced when participants are expecting to experience self-motion, it is likely to persist in object motion bias conditions.