Although it is clear that the habituation was reduced during the intervening six months that the subjects were not exposed to virtual environments, it does not appear to have been entirely eliminated. Six participants took longer to report a symptom change during this final exposure than they had on the first exposure, whilst only one took less time to report a change.
At first sight, our finding of a reduction in habituation is puzzling in the light of the results of Regan ( 1995), who reported a consistent decrease over time in the number of participants reporting symptoms. However, the difference can be explained by the different experimental conditions employed. Regan's participants were tested on four occasions, and there were breaks of (i) four months between the first and second immersions, (ii) four months between the second and third immersions, (iii) one week between the third and fourth immersions. During the week's exposure of our previous experiment, our participants were immersed every day and habituation was manifest as a consistent daily decrease in reported nausea ( Hill and Howarth, 1999). However, the habituation that Regan's participants experienced is likely to have been weak because of the long time intervals between immersions in her experiment. As a consequence, only a very small proportion of participants would be expected to show any change during her study - which is the picture seen - and if there was little habituation in the first place, there would be little scope for dehabituation.
The pattern of habituation to the appearance of motion, and the subsequent loss of habituation, is consistent with the human response to real motion. Griffin ( 1990) reports that some experienced boat owners are sick on their initial voyages of every season, after only a few months ashore. Our finding of a reduction in habituation after extended non-exposure to the stimulus is entirely consistent with this report, and even though the physical stimulus in the two cases differs it appears that each can produce habituation which is not fully maintained over a period of a number of months.
Hill, K. J. & Howarth, P. A. ( 1999). "Habituation to side effects of immersion in a virtual environment" (in preparation)
Ramsey, A. ( 1996). "Incidence and adaptation to VR induced symptoms and effects due to changes in passive display speed and repeated immersion in a VE". VIRART Report 96/129; Nottingham University, England