The Rest Frame Construct

The RFC is a way of summarizing much of the literature on spatial perception. While the idea it captures is not fundamentally new, formulating it in a particular way is convenient for currently purposes. The RFC fairly naturally suggests lines of research involving visual-inertial nulling presence measures; techniques for manipulating presence; and techniques for reducing simulator side-effects.

The RFC derives from the observation that humans have a strong perception that certain things are stationary. For instance, we ordinarily perceive the Earth as stationary. We typically interpret relative motions between the Earth's surface and objects on or near it (such as bicycles or aircraft) as implying that the object in question is moving, while the Earth remains stationary (``at rest'').

From a mathematical point-of-view, our strong perception that certain things are stationary is quite strange. Given a relative motion between two entities, it makes as much sense to interpret either (or neither) as stationary. Our nervous system could, in principle, choose to agree with Copernicus that the Sun is stationary and provide us with a complex impression of our motion on the Earth with respect to the Sun. Or, in principle, if one moves one's hand back and forth in a room, one could perceive the hand as stationary and the room as moving in the opposite manner. Both of these possibilities are completely legitimate, mathematically, but are also completely foreign to our perception.

The suggestion made here is that the nervous system selects certain things as being stationary in order to minimize its calculations. For instance, if one is primarily concerned with navigating on the Earth, it is useful to assume the Earth's surface to be stationary and to use it as the basis for spatial comparisons. Similarly, it is much more efficient to compute the motion of one's hand with respect to a room which is assumed to be stationary than the converse. (For similar reasons, mathematicians and physicists are often concerned to choose a specific coordinate system which simplifies a particular problem.)

The RFC simply formalizes this discussion. Borrowing from physics, a coordinate system used to define positions, angular orientations and motions is called a ``reference frame''. The particular reference frame which a given observer takes to be stationary is called the ``rest frame'' for that observer.

The ``rest frame construct'' states that:

The nervous system has access to many rest frames. Under normal conditions, one of these is selected by the nervous system as the comparator for spatial judgments. We call this the ``selected rest frame.'' In some cases, the nervous system is not able to select a single rest frame.