This study investigated masking of letters by a bright flash of light or by a pattern. The results showed that (a) masking by flash is primarily a monoptic effect; masking by pattern occurs under monoptic and dichoptic conditions; (b) increasing the interstimulus interval decreases masking by pattern less than by monoptically presented flash; (c) repetition of trials decreases masking by pattern but not by flash. Different processes are involved when flashes and patterns are used as masking stimuli. (15 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)