LINEAR ACCELERATION PERCEPTION - FREQUENCY-DEPENDENCE OF THE HILLTOP ILLUSION

Periodical stimulation of the otoliths during linear horizontal displa cement leads not only to motion and displacement perception but also t o the ''Hilltop Illusion'', which is the perception of being transport ed not on an earth-horizontal path, but over a small hill. This phenom enon is explained by the partial reinterpretation of the linear accele ration to a tilt of the gravitational vertical. The dynamics of the il lusion and its role in the perception of spatial orientation were inve stigated for eight frequencies. The subjects were exposed to sinusoida l linear acceleration and had to dynamically adjust a luminous line di splay to their subjective vertical. This served as a quantitative indi cator of the amount of perceived tilt with respect to gravity. The hig h frequency part (0.1-0.33 Hz) of the experiment was performed on a co nventional linear acceleration device (sled), the low frequency part ( 0.0083-0.05 Hz) on the sled centrifuge (a rotating sled). The amplitud e ratio of the settings of the subjective vertical was found to be sim ilar to that of a lowpass with a 15 s time constant. However. the phas e shifts never exceeded a 40 degrees lag, which together with other ex perimental results led to the conclusion that a non-linear, possibly p redictive type of information processing must be at work.