When moving through space, we continuously update our egocentric mental spatial representation of our surroundings. We call this seemingly effortless, automatic, and obligatory (i.e., hard-to-suppress) process "spatial updating". Our goal here is twofold: 1) To quantify spatial updating; 2) Investigate the importance and interaction of visual and vestibular cues for spatial updating. In a learning phase (20 min) subjects learned the positions of twelve targets attached to the walls, 2.5m away. Subjects saw either the real environment or a photo-realistic copy presented via a head-mounted display (HMD). A motion platform was used for vestibular stimulation. In the test phase subjects were rotated to different orientations and asked to point "as quickly and accurately as possible" to four targets announced consecutively via headphones. In general, subjects had no problem mentally updating their orientation in space and were as good as for rotations where they were immediately returned to the original orientation. Performance, quantified as response time, absolute pointing error and pointing variability, was best in the real world condition. However, when the field of view was limited via cardboard blinders to match that of the HMD (40x30 deg), performance decreased and was comparable to the HMD condition. Presenting turning information only visually (through the HMD) hardly altered those results. In both the real world and HMD conditions, spatial updating was obligatory in the sense that it was significantly more difficult to IGNORE ego-turns (i.e., "point as if not having turned") than to Update them as usual. Speeded pointing tasks proved to be a viable method for quantifying "spatial updating". We conclude that, at least for the limited turning angles used (<60í), the Virtual Reality simulation of ego-rotation was as effective and convincing (i.e., hard to ignore) as its real world counterpart, even when only visual information was presented.
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