The measures described in this section seek to evaluate spatial perception as directly as possible. They are similar in spirit to Brindley's ``Class A'' measures, but need not involve a sensory comparison. Rather, the effect of a stimulus is gauged in terms of its ability to alter perception in some way.
Many such measures take advantage of the close connection between visual and inertial perception. Perception of self-orientation and self-motion is served by a multisensory system that receives inputs from inertial receptors, including the vestibular apparatus and somatic receptors, as well as from the eyes [28,77,73,72]. Changes in inertial receptor response are usually interpreted as altered self-motion or self-orientation. Appropriate manipulation of stimuli to inertial receptors may result in self-motion perception, such as those produced by pitching the head forward while rotating about an earth-vertical axis (i.e., ``cross coupling'' - see Howard [44]) or by skin pressure cues from a ``g seat'' [65]. Similarly, visual field flow is usually associated with self-motion. To the extent that the visual surround is interpreted as indicating what is stationary, visual flow with respect to an inertially stationary observer may elicit perceived self-motion. The phenomenon of visually-induced perceived self-motion (vection) has been examined in numerous experiments and is the basis for many motion simulators (see Rolfe and Staples [88]). Multisensory integration of visual and inertial stimuli occurs in the vestibular nuclei. For example, Waespe and Henn [105] demonstrated that vestibular nucleus neurons may be excited by clockwise inertial rotation. Similar excitation from the same neuron may be evoked by counter-clockwise rotation of the visual surround (an optokinetic drum). These multisensory neurons, which may underlie self-motion perception, apparently do not distinguish between inertial and visual motion stimuli. Further, because these neurons are in the brain stem, self-motion perception associated with their response may provide a much more robust measure of spatial perception than self-report.
The multisensory nature of the self-motion perception system suggests procedures in which the visual salience of a virtual environment is measured in terms of its ability to overwhelm conflicting inertial perception cues.