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Vivian Holten, Stella F. Donker, Frans A.J. Verstraten, Maarten J. van der Smagt; Less ecologically valid optic flow causes more postural sway. Journal of Vision 2012;12(9):249. doi: https://doi.org/10.1167/12.9.249.
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© ARVO (1962-2015); The Authors (2016-present)
When we move through our environment we experience optic flow. Flow patterns typically contain a wide range of velocities. Radial optic flow patterns consisting of random dots, moving at a single speed, become less ecologically valid. Here we investigate to what extent ecological validity is a prerequisite for the experience of vection. As measure we use the amount of postural sway resulting from different types of expanding or contracting optic flow. We varied the validity of 3 radial optic flow types by manipulating their velocity profile (quadratic-speed gradient (Q), linear-speed gradient (L), single-speed (S)), assuming decreasing ecological validity. Four single-speed optic flow velocities were presented (6, 12, 24 or 48 deg/s), while in speed-gradient stimuli these velocities corresponded to half the integral of their speed-gradient curve. Stimuli (radius 43.5°, duration 4s) were presented in pseudo-random order, interleaved by dynamic visual noise patterns (duration randomly varied between 3.4-4.2s). Participants stood on a force-plate in a completely dark room. The effect of each optic flow stimulus was determined by calculating the postural sway on the anterior-posterior axis. In general we find that higher flow velocities lead to more postural sway. Interestingly, most sway was observed for the assumed less ecologically valid conditions. In addition, we find an anisotropy in that (QLS) contracting stimuli generate more sway than expanding stimuli. Specifically, for contracting optic flow the largest effect was found for the single-speed conditions, although all QLS stimuli resulted in significant sway. In contrast, for expanding optic flow, only the single-speed conditions resulted in significant sway. This study indicates that the experience of vection as measured by body sway is strongest for contracting, ecologically less valid optic flow. This might be explained by interactions between our visual and sensorimotor systems that are tailored towards compensating for ecologically valid stimuli.
Meeting abstract presented at VSS 2012
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