Temporal dissociation of motor responses and subjective awareness. A study in normal subjects

The aim of the present study was to examine the timing of different responses given simultaneously to a single event, the sudden displacement of a visual object occurring at the onset of the grasping movement directed at that object. The subjects were requested to correct their movement in order to reach accurately for the object and to signal the time at which they became aware of its displacement by a simple vocal utterance (Tah!). The onset of the motor adjustment was measured using kinematic landmarks obtained from the hand trajectory. Movements executed during trials where the object was displaced had an earlier peak in acceleration (107 ms) than movements executed during control trials (120 ms). By contrast, the vocal signal occurred 420 ms following object displacement, that was more than 300 ms after the onset of the motor correction. Control experiments were performed in order to verify the influence of possible interferences between the two tasks. Motor corrections performed without vocal utterance had the same timing as when the vocal signal was produced. Vocal signals produced in response to object's displacements but in the absence of reaching movements had the same latency as when movements were performed. We conclude from these results that the two responses were generated independently of each other. Assuming that the vocal responses in this experiment did signal the subject's awareness, the observed delay between motor corrections and these responses suggests that neural activity must be processed during a significant and quantifiable amount of time before it can give rise to conscious experience. This dissociation between motor responses and awareness in normal subjects is discussed in the light of clinical cases where overt behaviour and conscious experience are dissociated by cerebral lesions.