The influence of the menstrual cycle on visually-induced discomfort
Since the introduction of immersive virtual reality (VR) equipment, a number of studies have found that users can experience unpleasant side effects. A major focus of VR research and development has been on head-mounted display (HMD) based systems. Incorporation of an HMD provides the user with a sensation of immersion and presence in the virtual environment (VE), and it has been these systems that have attracted the most attention from both the media and human factors researchers over the last ten to fifteen years. Health and safety concerns associated with the use of this technology include symptoms experienced by users which resemble those of motion sickness (for example, disorientation, pallor, stomach awareness and nausea) and reports of visual discomfort, particularly eyestrain. The motion sickness-like symptoms have been referred to as virtual simulation sickness (VSS), and VSS is considered to be a form of visually-induced motion sickness. Susceptibility to motion sickness varies immensely, however females are generally regarded as being more susceptible than males to all forms of motion sickness, and it has been suggested that this heightened susceptibility in females could be attributed to the functioning of the female endocrine system. Changes to many of the body's systems have been reported to occur over the menstrual cycle, and it is reasonable to suggest that changes in susceptibility to nauseogenic visual stimuli may also occur. The first piece of research conducted in this thesis examined the influence of the menstrual cycle on susceptibility to VSS. 16 naturally cycling females were immersed in a nauseogenic YE on specific days (5, 12, 19 and 26) of their menstrual cycle, these days were chosen because they fall in line with peaks and troughs of ovarian hormone levels. The menstrual cycle phase of participants was confirmed by the measurement of salivary oestradiol and progesterone levels. The responses from the naturally cycling females (the experimental group) were compared to those from two control groups, consisting of 16 pre-menopausal females taking a combined monophasic oral contraceptive, and 16 men. Susceptibility to VSS did vary over the menstrual cycle, with susceptibility increasing on day 12, this was seen as both an increase in symptom severity and a decrease in symptom onset time. No consistent variation was seen in the females taking a combined monophasic oral contraceptive. In addition to susceptibility to VSS changing over the menstrual cycle, variations in reports of visual discomfort were also apparent, with visual discomfort also increasing on day 12. As no previous research has investigated the influence of the menstrual cycle on susceptibility to visual discomfort, two studies were conducted to determine whether such a relationship exists in other situations. The first was a laboratory based experiment whereby visual discomfort was induced by prolonged accommodative effort, and the second was a field study whereby reports of visual discomfort were examined in individuals performing intensive VDU work. Unlike the changes in visual discomfort seen over the menstrual cycle in females viewing a nauseogenic VE through an HMD, no changes in visual discomfort were found over the menstruaJ cycle when visual discomfort was induced by prolonged accommodative effort, or by intensive VDU work. It was concluded from this research that the previously seen changes in susceptibility to visual discomfort over the cycle, in females viewing a nauseogenic VE, were not a consequence of susceptibility to visual discomfort changing over the cycle. In the first experiment, susceptibility to VSS was seen to increase on day 12, as manifest by an increase in the severity of nausea reported on this day, and increases in general bodily discomfort. It is suggested that the change in visual discomfort reported over the cycle is a consequence of these general changes rather than having a specific ocular origin. This idea was supported in a fourth experiment which examined symptom reports collected from 88 participants immersed in a nauseogenic virtual environment at weekly intervals. A relationship was found between the severity of symptoms of VSS reported and the amount of visual discomfort reported.