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.