Linear acceleration perception: frequency dependence of the hilltop illusion.

Periodical stimulation of the otoliths during linear horizontal displacement leads not only to motion and displacement perception but also to the "Hilltop Illusion", which is the perception of being transported not on an earth-horizontal path, but over a small hill. This phenomenon is explained by the partial reinterpretation of the linear acceleration to a tilt of the gravitational vertical. The dynamics of the illusion and its role in the perception of spatial orientation were investigated for eight frequencies. The subjects were exposed to sinusoidal linear acceleration and had to dynamically adjust a luminous line display to their subjective vertical. This served as a quantitative indicator of the amount of perceived tilt with respect to gravity. The high frequency part (0.1-0.33 Hz) of the experiment was performed on a conventional linear acceleration device (sled), the low frequency part (0.0083-0.05 Hz) on the sled centrifuge (a rotating sled). The amplitude ratio of the settings of the subjective vertical was found to be similar to that of a lowpass with a 15 s time constant. However, the phase shifts never exceeded a 40 lag, which together with other experimental results led to the conclusion that a non-linear, possibly predictive type of information processing must be at work.