Adaptation of Visual Straight Ahead Requires an Unrestricted Field of View

Traditional accounts of adaptation to a rotation of the optic array (due to prisms) identify both visual and proprioceptive sites (Redding & Wallace, 1985). Last year (Brandwood, Rushton & Charron, VSS 2009), we reported the results of a walking experiment: Through manipulation of observer's walking behaviour, wedemonstrated that the magnitude and site of adaptation depends on the availability of optic flow. Specifically we found that optic flow plays an important role in the recalibration of perceived visual straight-ahead (Held and Freedman, 1963). This year we have taken a different approach to the same problem: In a repeated measures design we manipulated visual information. Participants wore glasses containing paired horizontally orientated wedge prisms and walked back and forth between targets for a short period of time. During locomotion vision was (i) unrestricted; (ii) restricted to 90°, or (iii) restricted to 90° with 400ms snapshots (through the use of optical shutters). Perceived visual straight-ahead and perceived proprioceptive straight-head was measured before and after exposure to the prisms. Adaptation was defined as the difference between the before and after measures. In the natural, unrestricted, condition we found adaptation was primarily visual. However, the site of adaptation switched towards proprioception as vision became more restricted. Thus, in line with our previous study we found that the site of adaptation varied with the availability of optic flow. Interestingly, the reduction of adaptation in visual straight-ahead with the restricted field of view in our study may explain the lack of shift in visual straight-ahead in Bruggeman, Zosh & Warren's (2007) study: They used a HMD with a restricted field of view. To conclude, taken together with our previous findings, the results of this study provide further support to the contention that optic flow drives a recalibration of visual straight ahead.