Abstract
When an error is introduced into the mapping between visual direction and the movement of the feet, an observer will initially take a curved path when walking to a stationary visual target. After a period of walking, remapping, or adaptation, will occur resulting in a straighter walking trajectory. Held and Freedman (1963) were the first to report remapping and hypothesised that the discrepancy between the anticipated and actual patterns of optic flow that result from self-movement is used to drive an adaptation in perceived direction. Recently, Bruggerman, Zosh and Warren (2007) concluded that their data provided evidence against the suggestion that optic flow is involved in the recalibration of perceived visual direction, instead suggesting that flow is involved in the regulation of visuo-motor mappings. Here we revisit the issue. We introduce an error by standard means with prisms, and manipulate the availability of optic flow. We measure perceived visual straight-ahead and proprioceptive straight-ahead before and after a short period of walking. In a repeated measures design, optic flow was (i) continuously available during normal walking; (ii) intermittently available in a “stop & go” stepping condition, and (iii) removed in a condition in which observers have vision only during the stationary part of each step. The sum of visual and proprioceptive shift was approximately equal in each condition (over 50% of the induced error). However, proprioceptive and visual components varied. The ratios were approximately 85:15 (visual/proprioceptive shift), 45:55 and 30:70 for the continuous, intermittent and no flow conditions respectively. This pattern of data contrasts with the conclusion of Bruggeman et al. We find that the shift in perceived visual direction increases as the exposure to optic flow (broadly defined as change in the structure in the retinal array) increases. This raises questions about what factors influence the locus of adaptation.