Psychophysical detection thresholds in anterior horizontal translations of seated and standing blindfolded subjects

To help separate out the contributions of the somatosensory and vestibular systems to postural and sway control, short (1, 4 and 16mm) anterior translations of lengths less than the normal sway path length were made of a platform upon which blindfolded young adult test subjects (n=12) stood or sat. Acceleration detection thresholds from these short moves were compared in standing vs seated conditions using a 2-Alternative [Interval] Forced-Choice psychophysical test protocol. A negative power law trading relationship was found between peak acceleration threshold and move length and duration for standing subjects. For these same subjects while seated, acceleration thresholds for all lengths were nearly constant, and showed a weak positive power law trade between threshold and move length or duration. This latter observation is consistent with that of Benson et al ´86, who also observed a positive power law trade relationship between acceleration threshold and move duration for seated subjects. Thresholds were higher at 1mm for standing vs. seated tests; while at 16 mm, standing tests had lower thresholds compared to those obtained for the seated tests. These results suggest that the vestibular system provides the principal input for detecting these short translations while seated, but not while standing.