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Guillaume Giraudet, Clementine Faron, Jocelyn Faubert; Postural adaptation to large optic flow fields. Invest. Ophthalmol. Vis. Sci. 2013;54(15):572.
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The aim of the current study was to determine the effect of visual adaptation for contracting and expanding optic flow on postural stability.
Twelve subjects participated in the experiment. They stood in a fully immersive virtual reality environment. The stimulus was a 3D textured tunnel, either static or moving forward (expansion) or backward (contraction) at 3 different speeds: 0.275, 1.1 and 4.4 m/s. Each trial was composed of 3 stages. The first one was called “Baseline”, during which the tunnel was static. It lasted 2 seconds. Then the stimulus started to move forward or backward for 10, 20 or 40 seconds; “Adaptation” stage. For the third, “post-adaptation” stage the stimulus suddenly halted and remained static for 15s. Optical motion sensors, located on the stereoscopic goggles, were used to track and register body movements. Behavioral reactions to the various visual conditions were described by comparing postural stability between the 3 different stages described above.
Results showed that subjects get adapted to the expansion optic flows by increasing their postural stability. The longer the adaptation stage lasted, the more body movements were reduced. A similar stabilization effect was observed when movement speed increased. However, we did not measure any adaptation for the contraction optic flow. In this direction, postural behavior remained constant during the whole sequence, that is, during all 3 different stages of presentation and for all moving scene speeds.
The current study confirms that expansion and contraction optic flows are processed differently and have different impacts on visuomotor systems. Several neurophysiological and imaging studies reported an expanding motion bias represented as greater activation in motion sensitive cortical areas than for contraction optic flow (Ptito, Kupers, Faubert, Gjedde, 2001). Our results highlight that, in the context of visually guided actions, the control of body posture changes when the properties of the expanding optic flow are modified but it does not respond to properties of moving scenes in contraction.
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