conscious) decisions to engage, as in awakening, or to engage in a certain way . to rescue cognitive functions, much like the deliberate coping movements that . which describe the “intentional framework” and the problem of circuit Britten KH, Newsome WT, Shadlen MN, Celebrini S, Movshon JA () A relationship. Neuroscience of Consciousness, Volume , Issue 1, 1 January . both the subjective feeling of conscious intention and movements. would take place at the same time as the neural decision to move. Thus, it appears that a relationship may exist between this period of time and the control of action. Unexpectedly, the conscious awareness of the decision or “the urge to move” emerged only ms before movement, leaving therefore a time lag of () are central in understanding the relationship between the RP and the conscious . it can in the end only describe it it leaves everything as it is.
For example, the movements of a tennis player as she serves are voluntary, but their control involves many automatic subroutines in the cerebellum and elsewhere. Furthermore, to claim that conscious acts of will initiate voluntary movements is not to deny that the acts of will arise out of brain processes that are largely unconscious.
The use of such terms does not imply dualism.
The neural circuits involved in voluntary motor control are exceedingly complicated, and I here give only some simplified information that is necessary for understanding the Libet experiment.
Voluntary movements are controlled primarily by the motor cortex in the back part of the frontal lobe — Fig.
Motor commands are sent from the primary motor cortex and to some extent from other areas to motoneurons in the brainstem and spinal cord, which in turn control the muscles. The initiation and programming of movements depend on activity in many areas including the supplementary motor area Fig.
These areas feed directly or indirectly into the premotor cortex and motor cortex. Electrical stimulation of the motor areas produces movements, but not the will to move. The Libet Experiment, a Challenge to the Role of Conscious Will An important background to the Libet experiment was the discovery in the s that, before people make a voluntary movement, there is a slow build-up of electrical potential measured from the skull over the motor cortex, beginning as much as a second earlier for simple movements and even longer for complex series of movements.
Libet was interested in the relative timing of the RP compared with the movement and the conscious decision to move. He therefore asked his experimental subjects to perform simple movements, in most cases flexion of the fingers or wrist, and to estimate the time of conscious awareness of the urge or will or decision to move W by reporting the position of a spot moving in a circle on an oscilloscope screen.
Timing and awareness of movement decisions: does consciousness really come too late?
He also recorded the RP by electroencephalography, and the time of the movement itself was estimated from the electromyogram [measurement of the electrical impulses in muscles - ed]. Libet found that time W came only about msec before the movement, whereas the RP began much earlier, usually about msec before the movement Fig. The fact that the change in brain potential occurred before the conscious decision was interpreted by Libet and by many commentators to imply that our conscious decision to act is not the true cause of the movement.
They deduced that conscious will is too slow to make things happen, and that volitional acts must result from unconscious processes in the brain, not from conscious willing.
The Libet Experiment and its Implications for Conscious Will
This seemed to imply that our intuitive notion of conscious will must be an illusion. Schematised readiness potentials RPs preceding self-initiated voluntary acts, as in the Libet experiment. We follow Libet et al. He therefore argued that even though the initiation of the movement was not the result of conscious will, its vetoing was. This argument has not attracted great interest, but was supported by eminent free-will philosopher Robert Kane.
Single Neuron Recordings During the Libet Experiment The overall conclusion on timing has to be that the problems have not so far been resolved.
The Libet Experiment and its Implications for Conscious Will - 572233.info
It is rarely possible to record from single neurons in the brains of humans, but this can occasionally be done in epilepsy patients using electrodes that have been implanted to localise the zones that cause seizures. Thus, remarkably, Itzhak Fried and his collaborators managed to record from more than 1, neurons in the medial frontal cortex of epilepsy patients and especially in the supplementary motor area, which generates most of the early part of the RP as they performed the Libet experiment.
It was found that a few neurons changed their firing rate by an increase or a decrease almost 1. The authors conclude that their findings support the view that the experience of will emerges as the culmination of premotor activity starting several hundreds of msec before awareness.
Indeed, most specialists in the philosophy of free will who have addressed the Libet claim have rejected it. The published data of several groups do indeed support this claim, but critics have objected to the use of subjective recall after the event, because there is evidence that this can be very unreliable.
- Timing and awareness of movement decisions: does consciousness really come too late?
Furthermore, those such as Alfred Mele  who have tried the experiment for themselves have found that W is difficult to define. It would be useful to quantify the reliability of our judgements, but this is difficult for a purely subjective decision.
For this reason, several research groups have instead measured the reliability of timing judgements for perceptual events, which is easier to do. Results have been variable, but several groups found serious biases,  raising doubts about the interpretation of the Libet experiment. To try to solve these problems, Matsuhashi and Hallett devised an alternative methodology for estimating time W.
That is very different! The paper provided valuable information about brain activity leading ultimately to a decision, but did nothing to rescue the Libet experiment from the criticisms about timing. The overall conclusion on timing has to be that the problems have not so far been resolved.
Then the observer either added or subtracted the two numbers above the letter as the next two screens appeared. The next frame after that offered four numerical solutions as the corner numbers: The observer was asked to press one of four buttons corresponding to the solution of the arithmetical operation chosen.
Finally, in the last screen, a series of four letters were given corresponding to the four screens before the arithmetical operation, and the observer was asked to record by pressing a button which letter was on the screen when the observer decided to add or subtract. That corresponded to the time of the conscious decision.
Measuring the onset and content of spontaneous abstract intentions.
A trial began with a continuous series of stimulus frames refreshed every second, each consisting of a central fixation point, a letter below it, a single-digit number above it, and four single-digit response options, one in each corner. Immediately when participants felt the spontaneous urge to perform either adding or subtracting, they first noted the letter on the screen frame 0 relative to time of decision.
The chosen arithmetic task was then performed on the numbers presented above the central fixation in the next two stimulus frames frames 1 and 2. The response options for the numbers in frames 1 and 2 were randomly presented in the four corners of the subsequent stimulus frame frame 3: Participants selected the correct answer for the chosen task by pressing one of four corresponding buttons, thereby revealing the content of their abstract decision.
After the response was given, four letter options were presented from which participants selected the letter presented at frame 0, thereby revealing the time of conscious decision. The usage of the early component of the BP as neural marker of internal movement generation has been justified by the following findings Shibasaki and Hallett, The BP was initially thought to occur only before self-paced but not before externally-paced movements Libet et al. It is also usually absent before pathological and involuntary movements such as periodic leg movements Trenkwalder et al.
The onset of the BP seems to be earlier for preplanned than for spontaneous movements Libet et al. The early component of the BP has greater amplitude when a selection between 2 or more movements is made, compared to self-paced movements without selection Praamstra et al. Intracerebral depth recordings showed that the BP occurs mostly in regions belonging to the sensorimotor system, i.
There is a large body of evidence that these regions are implicated in movement preparation Fried et al. However, the following arguments suggest that the BP is in fact not an adequate marker for actual movement decisions, but that it represents a diffuse and non-specific preparation of the cortex for future tasks. The BP belongs to the family of direct current DC potentials, the origin and generators of which are incompletely understood and a matter of debate.
There is evidence for neuronal mechanisms generating DC potentials, including excitatory postsynaptic potentials at apical dendrites due to synaptic input from unspecific thalamic afferences and axonal collaterals Caspers et al. In addition, non-neural current sources such as glial cells, extracellular potassium concentrations, and potential differences across epithelia of the blood-brain barrier seem to contribute as least as much to DC potential shifts at the surface Voipio et al.
Negative shifts in DC potentials can be recorded under many different circumstances including during the transition from sleep to wakefulness, sensory stimulation, attention shifts, hypoxia, and epileptic seizures Caspers and Schulze, ; Caspers et al.
Given these mechanisms and circumstances, DC potentials likely represent unspecific regional workload, attention and vigilance rather than specific neural computation. Unlike initially thought, DC potentials similar to the BP can also occur before externally cued movements Thickbroom et al.
Furthermore, they can be observed before expected external stimuli without movements Brunia, or even before participants decide not to move Trevena and Miller, In these latter cases, the DC potentials are usually not named BP but contingent negative variation CNV or stimulus preceding negativity SPNdue to the different context of occurrence. Nevertheless, they share common mechanisms and configurations Brunia, A DC potential similar to the BP also precedes the onset of visual cues instructing the participants to move one of two buttons.
Hence, a BP-like potential started at least ms before participants could know when or which hand to move, thus suggesting that is related to task expectation rather than actual movement preparation Herrmann et al. Not all self-paced movements are preceded by a BP. Hence, the BP is not necessary for internal movement generation Pockett and Purdy, The amplitude and configuration of the BP can superimpose on other DC potentials related to concomitant tasks such as spatial attention or visual processing Lang et al.
In this regard, the concomitant clock-task used for determination of the onset of consciousness in Libet's experiment is of particular concern, as it may contaminate the onset of the BP. Several studies have found that this is indeed the case: Hence, the mere fact that the participants in Libet's experiment had to determine the onset of their conscious urge to move seems to have biased the marker used for assessing neural movement preparation.
Even random fluctuations in neural activity can produce a potential similar to the BP if the movement decision is based on a threshold crossing of these fluctuations Schurger et al.