A series of studies was carried out in which it was shown that the amount of time that must intervene between 2 events in order for S to report correctly which of these 2 events preceded the other is approximately 20 msec. This holds over all modalities studied. "Whereas the time between successive stimuli that is necessary for the stimuli to be perceived as successive rather than simultaneous may depend upon the particular sense modality employed, the temporal separation that is required for the judgment of perceived temporal order is much longer and is independent of the sense modality employed." (PsycINFO Database Record (c) 2006 APA, all rights reserved)