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Experiment Author: Valentina Bruno
The experiment aims at investing an electroencephalography (EEG) measure, the readiness potential (RP). The RP is a negative potential observed from about 1–2 s prior to the onset of a voluntary action with EEG, and it arises before that a subject is conscious of performing an action. It has been firstly investigating by Libet et al. (1983), using a clock with a rapidly rotating dot: the subjects were asked to note the position of the moving dot when he/she was aware of the conscious decision to move a finger, while scalp EEG was used simultaneously to monitor brain activity.
In our E-prime experiment, we used a similar version of the Libet et al. experiment to investigate if the RP is shown not only in people with normal movements, but also in amputees with phantom limb movements when asked to perform actions with their phantom limb. In particular, after the amputation of a body part, amputees generally report the presence of their amputated limb (i.e. phantom limb syndrome) and sometimes they can voluntarily move their phantom at will.
The experiment consisted of a presentation of a clock with a rotating hand. Participants are requested to look at the clock and wait 5 seconds. After the 5 seconds of waiting, they are asked to perform a finger opposition movement either with the left or the right hand (i.e. a real movement in case of healthy people, a “phantom movement” in case of amputees) in a 10-second time window. Participants are requested to perform the movement whenever they want in the 10-second time window. After that, a slide asks participants to answer when they performed the movement, by indicating the number of the clock in which they perform the movement. In addition, the same slide asks the participant to answer with which finger they perform the movement. The hand with which participants have to perform the movement (right or left) is indicated before the clock compared.
The hand of the clock starts with one of four different positions after each trial, in order to avoid that participants move the finger every trial at the same time. An inline is devised to trigger the EEG to record the epoch in which the subject is asked to wait for the first 5 seconds (first trigger) and the 10-second window in which the movement has to occur (second trigger). In addition, in the 10-second window in which the subject is asked to move the finger, e-prime sends a trigger (by means of Gems packages) to the electromyographic (EMG) system that records the EMG activity of the hand. This is necessary in order to control that the answer of the subject (“when did you move?”) is reliable with the actual movement recorded with EMG
Libet B, Gleason CA, Wright EW, Pearl DK (1983) Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain 106: 623–642.