This preliminary research has shown that the visual-inertial nulling measure is capable of finding a main effect in the predicted direction which agrees with a trend in reported presence scores, and that the test-retest reliabilities for the two measures are similar. It finds a weak trend for a stronger relationship between the visual-inertial nulling measure and field dependency (as measured by the Embedded Figures Test) than between reported presence and field dependency.
A nulling presence measure has considerable advantages over a presence measure based on self-report. Humans were evolved to make perceptual judgments. We rarely articulate spatial judgments; however our action reveals those ``judgments''. Spatial perception supports action, primarily by representing what actions the environment will permit. We were not evolved to rate presence on a numeric scale.
Because the nulling measure is more deeply rooted than reported presence, cross-over amplitudes found in different experiments may be less distorted by anchor effects than are reported presence ratings. This would allow knowledge to be built up systematically by pooling data across experiments, rather than being limited to within-experiment comparisons.
The prediction that the nulling measure is less prone to anchor effects than the reported presence measure could be tested by comparing a condition A to a condition B in one experiment, and by comparing condition A to a quite different condition C in a second experiment with different participants. The prediction would be that condition A would have similar values in the two experiments on the nulling measure, but significantly different values in the two experiments on the reported presence measure.
Finally, a measure which is much more convenient than the visual-inertial nulling measure described here, but which preserves its desirable properties, may be possible. This measure would be based on the induced-motion of a background grid, as reported in Appendix D and discussed in Chapter 8.