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Lindsey Fraser, Bobbak Makooie, Laurence R. Harris; Contributions of the body and head to perceived vertical: Cross-modal differences. Journal of Vision 2014;14(10):1097. doi: 10.1167/14.10.1097.
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At whole-body tilts of 45°, a bias in the subjective visual vertical (SVV) towards the direction of tilt has been reported (the "A" or Aubert effect). This bias has been attributed to a tendency for the perceived direction of gravity to shift towards the longitudinal body axis (MacNeilage et al., 2007). However, it is unclear whether this bias exists in non-visual measures of gravity perception (e.g., Bortolami et al., 2006). Here we directly compared haptic (SHV) and visual (SVV) judgments of a rod's verticality relative to gravity. To assess the relative contributions of the head and body axes on verticality perception we varied body and head tilt independently. When the body was tilted 45° with the head upright, the SVV and SHV were both biased towards the direction of body tilt. When the body was upright with the head tilted 45°, the SVV bias was towards the head and increased in magnitude, but the SHV did not significantly differ from the gravity and body axes. Our findings agree with previous reports that SVV is biased primarily towards head position, but is also influenced to a lesser extent by body tilt. A novel finding is that biases of the SHV appear to be largely related to body orientation and not head orientation, potentially explaining some of the inconsistencies in the SHV literature. Clemens and colleagues (2011) have proposed two systems for estimating the direction of gravity, one using the head's position as a reference and the other using the body's position. Our results suggest the body system may play a stronger role in SHV than SVV, possibly because it is more computationally efficient to compare hand position to the body rather than the head.
Meeting abstract presented at VSS 2014
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