The unique cytoarchitecture of the visual cortex, with cells of particular orientation-specificity arranged in vertical columns adjacent to other columns with somewhat different orientation-specificity causes subjective visual instabilities when viewing some repetitive grid patterns for normal subjects. Since caffeine increases cortical arousal by serving as an antagonist to the inhibitory neurotransmitter adenosine, caffeine intake might be expected to increase these subjective disturbances. Twenty subjects were administered a placebo, 100 or 200 mg of caffeine orally. After 40 min subjects rated the level of visual instability on nine subjective dimensions, while viewing grating patterns. Each subject was tested on all levels of caffeine intake, with at least 48 h between tests. With increasing caffeine dosage the level of reported visual disturbances increased. These data suggest that the increased cortical arousal associated with caffeine intake may interact with the structural properties of the visual cortex to increase the perceptual instability associated with viewing grid patterns.