In a previous study, the sensory phenomenon of ''backward masking'' wa
s used to demonstrate that subjects can preprogram a single stereotype
d voluntary movement or movement-sequence and that such a movement can
be triggered in response to a stimulus that is nor perceived (that is
, a stimulus of which the subject is unaware). In the present study, v
isual stimuli were presented at random in one of two different locatio
ns to normal human subjects in a choice reaction-time (RT) task. When
the stimulus appeared in one of the locations, subjects made a motor r
esponse. When the stimulus appeared in the other location, subjects ma
de a different motor response. Large and small stimuli were presented
in either location. In some trials, the small stimulus was followed 50
ms later by the large stimulus. The small stimulus was then ''masked'
' by the large stimulus and could not be perceived on forced-choice te
sting. Despite not perceiving the test stimulus in either of its rando
mly selected locations, subjects were able to select and execute the m
otor response appropriate for each location. The RTs for responses to
the masked stimulus and to the same stimulus presented without masking
(and so, easily perceived) were the same. This result implies that ap
propriate programs for two separate movements can be simultaneously he
ld ready for use, and that either one can be executed when triggered b
y specific stimuli without subjective awareness of such stimuli and so
without further voluntary elaboration in response to such awareness.