On the basis of models of otolith functioning, one would expect that, during sinusoidal linear self-motion in darkness, percepts of body tilt are experienced. However, this is normally not the case, which suggests that the otoliths are not responsive to small deviations from the vertical of the gravito-inertial force vector acting on them. Here we show that this is incorrect. Subjects usually know on what kind of linear motion device they are (going to be) moved, having seen it prior to experimentation. This may result in a cognitive suppression of such otolith responses. In the present study, subjects were kept completely unaware of how they were moved and were asked to report on how they thought they moved. About 50% of the reports included tilt percepts almost immediately. It is concluded that this reveals the presence of otolith responsiveness to even small and short-lived deviations of the gravito-inertial force vector from verticality, a responsiveness which is suppressed when (prior) cognitions exist that the motion path is purely in the horizontal plane.

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A visual illusion is reported which comprises the following: when a monitor with a moving constant velocity grating is swayed in front of a subject, the grating may be perceived as freezing or decelerating on the screen. This percept appears to depend on the magnitude and direction of the retinal grating velocity, relative to that of the monitor. Various possible explanations are rejected. It is concluded that the illusion shows a resemblance to the phenomenon of "motion capture" but that it has certain new characteristics that require an explanation.

No abstract