Experiment AIE2 and Pilot Study AIP4 found at least a factor of 10 between-participant variation in the cross-over amplitude. What is the source of this variation? It was suggested in the previous chapter that this variation might reflect a sensitive measurement of field dependency. A second possibility is that it reflects individual variation in the degree to which the participant attended to the visual scene. Those who watched the scene more carefully would be expected to be more influenced by the vection stimulus, which would be reflected in a higher cross-over amplitude.
The relative influence of inherent individual differences and of variation in attention level could be assessed by providing a visual task. For instance, attention level could be controlled for by having participants push a button every time a red dot appeared on the screen.