Normal subjects react more quickly to a pair of visual stimuli than to a stimulus alone. This phenomenon is known as the redundant signal effect (RSE) and represents an example of divided visual attention in which signal processing is carried out in parallel to the advantage of response speed. A most interesting aspect of this phenomenon is that it can occur when one stimulus in a pair cannot be consciously detected because of hemianopia or unilateral extinction resulting from brain damage. Here, we report that a similar dissociation between visual awareness and visually guided behavior is present in normal subjects who show an RSE even when the luminance of one of a pair of stimuli is below detection threshold. The observed RSE cannot be attributed to probability summation because it violates Miller's race inequality and is likely to be related to neural summation between supra- and subthreshold stimuli. Given that a similar implicit RSE is present in hemispherectomy patients, we hypothesize that the site of this summation might be the superior colliculus (SC).