Visual discomfort and cortical hyperexcitability

• Main Author: Haigh, Sarah
• Published: University of Essex 2012
• Subjects: 612.84
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.573696

Natural images contain statistical regularities that the human visual system is adapted to. Images that contain excess contrast energy at mid-range spatial frequencies are particularly aversive and can cause a variety of adverse effects including seizures, headaches and illusions. Some individuals experience more pattern-induced discomfort and illusions than others. However, the physiological mechanisms associated with the discomfort are unknown. Several well-known visual illusions appear to be caused by weak accommodation to the target. Individuals who experience pattern-induced discomfort and illusions show a weaker accommodative response suggesting that there may be a relationship between accommodation and discomfort. The aversion to chromatic and achromatic grating patterns that were either static or moving was investigated. Both sets of patterns varied in how epileptogenic they were. It was found that discomfort was not dependent on the epileptogenic properties of the pattern. Ocular accommodation to the patterns was then measured to see if the parameters of the patterns that induced discomfort produced a weaker accommodative response. There was no effect of the pattern on accommodation, suggesting that the discomfort is not caused by problems in accommodating to the pattern., Migraineurs and patients with photosensitive epilepsy show a heightened cortical response to uncomfortable grating patterns, suggesting that the discomfort might be cortical in origin. The haemodynamic and electrophysiological responses to the grating patterns were measured. The uncomfortable patterns produced a large change in the haemodynamic response, and a larger decrease in alpha power than the less uncomfortable patterns, both of which suggest that the discomfort is associated with an increase in cortical excitability I Page 1 Abstract Natural images contain statistical regularities that the human visual system is adapted to. Images that contain excess contrast energy at mid-range spatial frequencies are particularly aversive and can cause a variety of adverse effects including seizures, headaches and illusions. Some individuals experience more pattern-induced discomfort and illusions than others. However, the physiological mechanisms associated with the discomfort are unknown. Several well-known visual illusions appear to be caused by weak accommodation to the target. Individuals who experience pattern-induced discomfort and illusions show a weaker accommodative response suggesting that there may be a relationship between accommodation and discomfort. The aversion to chromatic and achromatic grating patterns that were either static or moving was investigated. Both sets of patterns varied in how epileptogenic they were. It was found that discomfort was not dependent on the epileptogenic properties of the pattern. Ocular accommodation to the patterns was then measured to see if the parameters of the patterns that induced discomfort produced a weaker accommodative response. There was no effect of the pattern on accommodation, suggesting that the discomfort is not caused by problems in accommodating to the pattern., Migraineurs and patients with photosensitive epilepsy show a heightened cortical response to uncomfortable grating patterns, suggesting that the discomfort might be cortical in origin. The haemodynamic and electrophysiological responses to the grating patterns were measured. The uncomfortable patterns produced a large change in the haemodynamic response, and a larger decrease in alpha power than the less uncomfortable patterns, both of which suggest that the discomfort is associated with an increase in cortical excitability. The discomfort may be a homeostatic warning that the cortex is showing a heightened response to the images. Increased neural firing comes at a metabolic cost, and so the discomfort may signal that prolonged exposure to the stimulus will produce a greater demand on the body's food and oxygen.