 These results have been interpreted as evidence for greater noise in the visual system. Prolonged exposure to a single spatial frequency is thought to increase localized cortical excitation that spreads to adjacent cells in a different spatial frequency channel. This spreading of cortical activation is what causes the unpleasant symptoms. Studies by Wilkins and colleagues 1984 showed the most common spatial frequency at which unpleasant symptoms occurred was at 4 cycles deg-1 - the peak of the contrast sensitivity function. Conlon et al. 1999 presented a similar cortical model in which visual discomfort is caused by noise due to poor inhibitory connections between spatial frequency channels. 