Backward contralateral masking by light and by random pattern.

Letter-stimuli as targets were presented to the right or left visual fields and followed either by a flash of light or by a flash of light plus a patterned mask. The patterned mask always appeared in the opposite visual field of the letter targets. Analysis showed that masking occurred for both types of masks but that subjects produced more errors at each of five intervals between onset of the target and onset of the mask for the flash of light plus a patterned mask in the opposite visual field than for the flash of light alone. A pattern mask, when presented to the opposite visual field of a target stimulus, interferes with target processing at short target-mask intervals. These findings suggest that central backward masking may involve target-mask interactions beyond the visual cortex (Area 17).