This study examined the effect of perception without awareness on somatosensory simple reaction time using a backward masking paradigm in nine subjects ranging in age from 21 to 38 years.In the backward masking paradigm, two types of electrical stimuli differing in intensity(one type ranging from 1.2 to 1.5 times the sensory threshold of each subject, and the other ranging from 4 to 5 times the threshold)were successively delivered with a 40-ms interstimulus interval to the subjects' right median nerve at the wrist through disk electrodes.There were three stimulation conditions:weak stimulus alone, strong stimulus alone, and doubles stimulation(which was the backward-masking condition, with a weak stimulus always preceding a strong one).The subjects were asked to respond to either the weak or the strong stimulus as quickly as possible by clicking on the computer mouse button, without any knowledge that the double stimulation would be included in the experimental trials.The results showed that the mean reaction time for double stimulation was significantly shorter than that for the strong stimulus condition, ant that the reaction time for the weak stimulus was the longest among the three conditions.The latter result indicated that the weak stimulus could not be perceived quickly.All subjects reported that they did not perceive the weak, preceding stimulus used for double stimulation-suggesting that the weak stimulus was masked by the following strong stimulus.Therefore, the weak stimulus, without the subject being aware of it, may have facilitated the preparatory processes for performing response actions, resulting in the shorter reaction times observed with double stimulation.These findings are discussed in terms of several conceptual and empirical models of information processing for sensation, perception, and preparation of motor responses in simple reaction time under backward masking.