The effect of probing modality on neural signatures and awareness of movement intent

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Since Libet et al. published their study on the timing of the conscious intention to act in 1983, this timing and neural signatures preceding voluntary movement have been the subject of much discussion. An innovative way to measure the onset of an intention was introduced in 2008, in which awareness of intention was estimated by participants’ responses to probe stimuli. In this work, the probing modality was manipulated to investigate the effect of 1) stimulus processing speed as well as 2) facilitating/interfering characteristics of the stimuli. The effect of these 2 stimulus characteristics was investigated for the timing of the awareness of intending to act and the neural signatures Readiness Potential (RP) and Event Related Desynchronization (ERD). Participants were asked to make self-paced voluntary movements while probing them at optimized intervals to target the times at which they are aware of their intention to act. When participants noted a probe while being aware of an intention to move, they were asked to stop (veto) their movement. This allowed us to estimate the time when participants were aware of an intention to act: the intention window. Four different probing modalities were used: auditory, visual, passive tactile and active tactile, chosen for their variance in processing speed and likely interaction with the participant’s awareness of intention. Probing modality was found to affect the size and onset of the intention window, where its onset can be explained by the different processing speeds of the probe stimuli and through intention facilitation. Trials in which a participant made a movement and trials in which they were not aware of an intention to act were found to differ significantly for both RP and ERD. This could suggest that the neural signatures of a veto lie in between that of a move and an ignored probe: possibly caused by there being an intention but not an actual move. In conclusion, probing modality influences the intention window, and the neural signatures seem to not only be related to the move itself but also to the intention. The expected difference in timing of neural signatures between different modalities was not found: the effect of modality on the neural signatures remains unclear.
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