Does the redundant signal effect occur at an early visual stage?

To try and cast light on the processing locus of the redundant signal effect (RSE), i.e. the speeding of reaction time (RT) with two rather than one stimulus, we manipulated three features of redundant visual stimuli, i.e. exposure duration, intensity and interstimulus interval (ISI). We found an inverse relationship between stimulus duration or intensity and the maximum length of ISI at which an RSE occurred. These effects are broadly similar to those found in the measurement of visible persistence, i.e. the phenomenon that the sensation produced by a brief visual stimulus can outlast the duration of the physical stimulation. Therefore, we suggest that the RSE occurs at a visual processing stage. This conclusion does not rule out other subsequent stages when employing different redundant stimuli and task paradigms.