SELECTION OF MOTOR-RESPONSES ON THE BASIS OF UNPERCEIVED STIMULI

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.