Selection of motor responses on the basis of unperceived stimuli

In a previous study, the sensory phenomenon of "backward masking" was used to demonstrate that subjects can preprogram a single stereotyped voluntary movement or movement-sequence and that such a movement can be triggered in response to a stimulus that is not perceived (that is, a stimulus of which the subject is unaware). In the present study, visual stimuli were presented at random in one of two different locations to normal human subjects in a choice reaction-time (RT) task. When the stimulus appeared in one of the locations, subjects made a motor response. When the stimulus appeared in the other location, subjects made 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 testing. Despite not perceiving the test stimulus in either of its randomly selected locations, subjects were able to select and execute the motor 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 appropriate programs for two separate movements can be simultaneously held ready for use, and that either one can be executed when triggered by specific stimuli without subjective awareness of such stimuli and so without further voluntary elaboration in response to such awareness.