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Jun Wu, Zijiang J. He, Teng Leng Ooi; Optic flow influences the visually perceived eye level. Journal of Vision 2002;2(7):512. doi: https://doi.org/10.1167/2.7.512.
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As one walks forward in the visual space, an optic flow field is generated with its focus of expansion (FOE) centered at one's physical eye level (Gibson, 1950; Warren, 1998). Yet, it is uncertain if optic flow helps define the visually perceived eye level. To investigate, we measured perceived eye levels when viewing an optic flow field consisting of a volume of 150 dots (30 × 6 × 6 m) moving at a constant speed of 15 m/s, that was generated in a virtual environment (V8-HMD/IS-600/SGI). When the FOE was set at 15 deg below the physical eye level, perceived eye level was 6.1±1.7 deg (n=6) below the baseline. We then adapted observers to this optic flow field (FOE-15 deg down) for 25 minutes. Subsequent measurements performed outside the HMD in a dark room revealed a downward shift in visually perceived eye level of 4.1±0.8 deg. Perceived object location in the dark (measured by asking observers to walk blindly to the target and then gesture the remembered height) was systematically affected — object locations (3.75 and 5 m, placed either on the floor or 0.5 m above) were judged as farther and higher by an average angular declination error of 2.0±0.3 deg. Together, these experiments further confirm that the eye level is used as a reference for judging target locations in the dark (Ooi et al, 2001), and is shifted downward as an aftereffect of adaptation to an optic flow field with lowered FOE. Equally significant, our findings indicate that optic flow information partly contributes to the synthesis of one's visually perceived eye level, which is used as a reference for the perception of direction as well as distance.
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