Path integration requires keeping track of one's self-motion during navigation to update distance and orientation with respect to a starting point. Two main sources of information are available about self-motion: optic flow and body senses (a collective term for proprioceptive, vestibular and efferent information). People can perform path integration on the basis of optic flow or body senses (Loomis, et al, 1993; Peruch, et al, 1997; Riecke, et al 2000; Kearns, et al 2002), but when both are available the body senses dominate. However, the relative contributions of optic flow and body senses for path integration are not well understood. The current series of experiments used a triangle completion task and manipulated the gain of the optic flow during active walking in an immersive virtual environment (VE). Experiments 1 and 2 used a randomly textured VE that only provided optic flow information. In this case, behavior was dominated by the body senses, with a small, but significant, contribution of optic flow (∼15%). This was true with translational and rotational gains (distance and angular information about self-motion respectively). In Experiment 3, unstable landmarks were added to the VE, resulting in nearly equal contributions of vision and body senses to path integration. When landmarks remained in a fixed configuration (Experiment 4), vision dominated in some conditions. Experiments 5 and 6 used the same paradigm, but accuracy feedback was given at the end of every trial. If feedback agreed with visual movement, orientation accuracy improved. In a VE with local landmarks (Experiment 6), this same feedback also resulted in improved distance accuracy in visual space. Overall, these studies demonstrated that the relative contributions of vision and body senses for path integration are dependent on the visual environment.
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