several characteristics are common in the trigger stimuli of many people with pse. the patterns are usually high in luminance contrast (bright flashes of light alternating with darkness, or white bars against a black background). contrasts in colour alone (without changes in luminance) are rarely triggers for pse. some patients are more affected by patterns of certain colours than by patterns of other colours. the exact spacing of a pattern in time or space is important and varies from one individual to another: a patient may readily experience seizures when exposed to lights that flash seven times per second, but may be unaffected by lights that flash twice per second or twenty times per second. stimuli that fill the entire visual field are more likely to cause seizures than those that appear in only a portion of the visual field. stimuli perceived with both eyes are usually much more likely to cause seizures than stimuli seen with one eye only (which is why covering one eye may allow patients to avoid seizures when presented with visual challenges). some patients are more sensitive with their eyes closed; others are more sensitive with their eyes open.