The Influence of Conscious Intention on
Voluntary Movement
Version 2
Name: T. L. Pelsmaeker
Student ID: 10177590
Course: Bachelor thesis
Tutor: Dr. S. van Gaal
Date: 3-6-2014
Table of Contents
The influence of conscious intention on voluntary behavior 3
Philosophical aspects of consciousness 5
The classical experiment: conscious intention and the readiness potential 7
Brain processes of conscious intention and behavior 12
Conclusions and discussion 18
The Influence of Conscious Intention on Voluntary Movement
In a difficulty in the path of psychoanalysis Freud (1917) wrote the following: "Although thus humbled in his external relations, man feels himself to be supreme within his own mind. ... It is thus that psycho-analysis has sought to educate the ego. But these two discoveries - that the life of our sexual instincts cannot be wholly tamed, and that mental processes are in themselves unconscious and only reach the ego and come under its control through incomplete and untrustworthyperceptions - these two discoveries amount to a statement that the ego is not master in its own house."
As can be seen in the quote above, about a hundred years ago Freud opened the world to a view that there is more to the mind than just conscious experience. He called this the third great blow to human narcissism. First we had to deal with the fact that the earth isn't the center of the universe, then with the fact that humans are not different or better than other animals through Darwins theory of evolution, and now we couldn't even be sure that we are masters over our own mind. While most of Freuds theories about the mind are no longer considered to be of great
scientific value, they did have a significant impact on the way people view the mind. So in that sense Freud was right, his theory changed the human view of the mind forever. Nowadays the term unconsciousness is deeply rooted in folk psychology, as one can easily say that they did something because of their unconsciousness.
The view that behavior can be determined by more than consciousness alone did not only penetrate folk psychology, but also science. The field of modern neuroscience seems ruled by the idea that behavior can be determined by unconscious brain activity that predates conscious activity, as the experiment of Libet, Gleason, Wright and Pearl (1983) showed (Libet, 1985). Some scientists even believe that there is no functional role for consciousness in behavior. Even Freud did not go that far.
Since the impact of this Freudian thinking style is substantial, it seems paramount to get a clearer view of what is truly known about the workings of consciousness in the brain, and how it can or cannot influence behavior. But first the term consciousness itself needs to be defined and
specified, for its usage would otherwise lead to confusion, since consciousness has a broad meaning. Two subdivisions in the definition of consciousness can be made. The first is the distinction between state and content of consciousness. The state of consciousness is also called wakefulness or vigilance, and is thus the sense of being awake (Dehaene & Changeux, 2011). However, in the sentence "I was not conscious of the red light" conscious does not refer to state of consciousness but to the content of consciousness. The content of consciousness is the limited amount of information someone holds within awareness at a certain time (Dehaene & Changeux, 2011). In this paper consciousness will refer to the content of consciousness.
The content of consciousness can further be divided in long-term and short-term
consciousness (Haggard, 2008). Short-term consciousness consists of intentions for direct action, called conscious intention. Long-term consciousness consists of thoughts in conscious access that do not directly intent behavior, but might influence behavior in the long term. Planning and reasoning can for instance be seen as forms of long-term consciousness. Both types of consciousness are originally thought to cause behavior. Conscious intention might cause behavior by directly initiating it, and long-term consciousness might cause behavior since plans for future action are formed in long-term consciousness. However, the question if consciousness does really cause behavior in this sense is hotly disputed in modern brain science (Haggard, 2008). Most attention in research has been paid to conscious intention (Haggard, 2008). Therefore, and because the feeling of conscious intention seems most closely related to behavior, the focus of this thesis will lie on the influence of conscious intention on behavior.
Behavior that is thought to be caused by conscious intention is called voluntary movement. Voluntary in this sense does not mean that the action is caused by an absolute free will, but rather
that a feeling of volition towards the action exists, as opposed to reflexes. Therefore this thesis will focus on the relation between conscious intention and voluntary action.
Much has already been written about consciousness and conscious intention in the scientific literature, including issues about conscious intention and human volition (Haggard, 2008), about consciousness as a Global Neuronal Workspace (Dehaene and Changeux, 2011), about the influence of consciousness on behavior from a social psychology perspective (Baumeister, Masciampo and Vohs, 2008) and about the views of social psychology and neuroscience on consciousness combined (Brass, Lynn, Demanet and Rigatoni, 2013). What is missing however is an overview of the influence of conscious intention on voluntary movement, describing philosophically, neurologically and functionally how conscious intention might influence behavior. This thesis intents to fill that gap, by first addressing the question what philosophical framework is needed for a clear neuroscientific study of conscious intention. Second, the implications of the classical experiment of Libet et al. (1983) on the link between conscious intention and voluntary movement will be discussed. Lastly, it will be defined which brain processes are responsible for conscious intention and voluntary
movement. By addressing these questions, a clear picture of the influence of conscious intention on voluntary movement will be sketched.
Philosophical aspects of conscious intention
Before a scientific discussion about conscious intention is possible, it is necessary to provide a philosophical framework in which science can study the mind. When philosophers first started thinking about the mind two predominant theories emerged. The first theory is called dualism. Invented by Rene Descartes, this theory states that mind and matter are separate entities
(Dooremalen, Regt and Schouten, 2010). So according to dualism, next to matter there is a mind that is free of the rules that bind matter. However, in a dualistic fashion the mind can have a causal influence on the physical world, and vice versa, trough an unknown form of interaction. Materialism
contradicts this view. According to materialism everything is matter, and so is the mind (Dooremalen et al., 2010). In that fashion, the mind must reside somewhere in the brain and body. Holding a dualistic view is of little use for an experimental scientific understanding of the mind, because then the origins and working of the mind could never be fully understood by looking at the brain, which is something that neuroscience tries to accomplish. Thus when I speak of conscious intention causing voluntary movement, I mean thereby that some processes in the brain that are conscious intention caused the movement, and not some process that transcends matter.
Next to materialism a deterministic perspective is also needed in order to scientifically study consciousness. According to determinism everything that happened and will happen in the universe was predetermined from the beginning of time (Dooremalen et al., 2010). The logic behind this is that when the universe first came in existence so did the universal laws of nature. Matter is bound by these laws; they dictate the way matter behaves. Therefore, one with all knowledge of the universe could accurately predict how all the matter would behave in the future, and thus predict the future. So determinism states that every action is necessarily caused by previous actions by the laws of nature. According to this view conscious intention could never freely cause behavior without being caused by something else first, except with a dualistic viewpoint. Thus materialism and determinism together lead to the stance that there is no such thing as a truly free will. Conscious content and intentions are determined by previous events in the world. That does not, however, mean that consciousness cannot determine future events, because every event needs a cause, but every cause needs a cause as well. So every event in the universe can be seen as a link in this causal chain. It is now paramount to find out if conscious intention brain processes form a crucial link in the causal chain that lead towards behavior or are just byproducts of other processes with no causation
towards behavior. But it is clear that determinism and materialism do not exclude the possibility that conscious brain processes might be a cause of behavior. Therefore the lack of an absolute free will does not deem consciousness and conscious intentions useless in any way. Thus by taking a
on voluntary movement can proceed without having to deal with concepts that are scientifically inexplicable, like the absolute free will and a mind free of matter.
The Classical Experiment: Conscious Intention and the Readiness
Potential
Recently it has been scientifically disputed if the feeling of conscious intention does actually cause voluntary movement. This all started with the classical experiment of Libet, Gleason, Wright and Pearl (1983), that found the occurrence of unconscious movement preparatory brain activity before the onset of conscious intention, the so called readiness potential (RP). Following this
experiment, Libet (1985) theorized that conscious intention therefore cannot cause behavior directly, but might hold some kind of veto to stop planned movements just before execution. This is called the theory of free won't (Haggard, 2008). In this paragraph the classical experiment of Libet et al. (1983) and its theoretical implications will be discussed, in order to assess its influence on the link between conscious intention and voluntary movement.
Libet et al. (1983) showed that before the feeling of conscious intention arises, there is already action specific brain activity. Participants in the experiment were instructed to focus on the middle of a clock on a screen on which a spot of light rotated clockwise, completing a rotation each 2,56 seconds. The participants were instructed to flex the fingers on the right hand when the urge to do so arose, after at least one full rotation of the light spot. The participants were further instructed to report the position of the spot of light when the intention to move first emerged. During these trials brain activity was measured using electroencephalography (EEG), and the onset of the self initiated movement was determined using electromyography (EMG). EEG works by placement of electrodes on the skull of the participant. These electrodes measure differences in electrical potential on the skull compared to a reference electrode (Luck, 2005). These differences are believed to be
caused by synchronized firing of many neurons, and therefore are thought to represent general brain activity (Luck, 2005). This measured activity can subsequently be time locked to a certain event, in this case the onset of movement, and averaged across trials. This creates an event related potential (ERP) of fluctuations in brain activity potentially related to the event. An EMG works in a similar way but with electrodes placed elsewhere on the body, in this case on the right hand, to measure electrical activity caused by muscle contractions (Luck, 2005). A negative shift in potential was seen on the ERP about 500 ms before the reported intention, this is the RP. Intention to move occurred about 250 ms before the actual movement. This method and the RP are illustrated in figure 1.
Figure 1. An illustration of the method used by Libet et al. (1983) to measure the onset of the RP and conscious intention (above), and an schematic illustration of the RP waveform measured using this method (below). The time between the arrows is the time between the onset of the RP and the onset
of conscious intention. In this illustration the EMG is not shown. Reprinted from "Human volition: Towards a neuroscience of will." by P. Haggard, 2008, Nature, 9, p. 935. copyright 2008 by Macmillan
These findings led Libet et al. (1983) to conclude that conscious intention cannot be the cause of behavior because conscious intention is preceded by unconscious motor preparation. Libet et al. (1983) did however conclude that conscious intention could still influence behavior by making a last veto over an already initiated action (Haggard, 2008). So consciousness might be able to stop unconsciously initiated actions.
There are three grounds on which this experiment can be and has been criticized. First, the experimental method relies on the ability of participants to accurately register where the spot of light was when the urge to move was first felt. This is questionable because there is a certain latency time for the participant to read and memorize the position of the spot of light on the clock, and turbulence after the event can easily disturb subjective recall (Matsuhashi and Hallet, 2008). These disturbances can be harmful because the conclusions of this experiment are based on the time difference between the measured RP and the subjective feeling of intention, which was measured to be just 500 ms. Thus accuracy is needed for the conclusion to hold true, and because of the reasons mentioned above it is questionable that subjective reportage is accurate enough. Furthermore Lau, Rogers and Passingham (2006) showed that attending the onset of conscious intention might cause intention to be felt earlier than when no attention to intention is paid. Lau et al. (2006) measured activity in the pre supplementary motor area (pre-SMA) using fMRI, and found that attention to intention led to greater activity in the pre-SMA. This enhanced activity might have caused the participants to become aware of their intention earlier than normally would be the case. Therefore Lau et al. (2006) concluded that people might normally become aware of their intentions later than measured in the experiment of Libet et al. (1983), just 120ms before action execution instead of 250ms. This is too late for conscious intention to have veto over the action (Lau et al, 2006). The problems mentioned above show that Libet et al.'s (1983) method for measuring the onset of intention by subjective recall is too inaccurate to draw strong conclusions about the actual onset of conscious intention. However, later experiments found that when methods that don't require subjective recall are used, the RP is still found to occur before conscious intention, and conscious
intention far before the onset of movement (Matsuhashi and Hallet, 2008). Matsuhashi and Hallet (2008) instructed their participants to move the fingers on their right hand when the urge to do so arose, just like in Libet et al. (1983) experiment. However, a tone was played every 3 to 20 seconds, and the participants were instructed to cancel the finger movement if an urge to move was felt when the tone was heard. The researchers found that between 0 and 1,8 seconds before movement almost no tones were played, meaning that about 1,8 seconds before an action the conscious urge to move was felt. Using this method, the RP still occurred, and it occurred before the conscious
intention to move (2,17 seconds before the action). Thus it seems clear that some action related brain activity indeed occurs before the conscious intention to move arises.
Second, even when some brain activity occurs before the conscious urge, this does not mean that this RP is necessarily unconscious motor preparation for the specific action (Brass, 2013). The RP could possibly be equivalent to spontaneous fluctuations in neuronal activity that seem to gradually increase because the potential is time locked to the movement (Schurger, Sitt and Dehaene, 2012). Schurger et al. (2012) propose that sub threshold fluctuations in neuronal activity largely determine when the threshold of conscious intention is crossed. When convoluted to an ERP reading these random fluctuations might seem like a buildup of neuronal activity, like motor preparation, while they are not. This conclusion is strengthened by the fact that these sub threshold fluctuations are also found before fast unplanned reactions, when no motor preparation was possibly occurring (Schurger et al., 2012). This poses some troubles for Libet et al. (1983) original interpretation of the RP, because this shows that the RP is not an action specific phenomenon. It does not directly threaten the conclusion that conscious intention does not cause movement, because conscious intention is still proceeded by other unconscious brain activity.
The third critique targets the conclusion of Libet et al.'s (1983) experiment, that conscious intention cannot be a cause of behavior, for it is preceded by other unconscious brain activity. As stated before, from a materialistic deterministic perspective the cause of something is always
preceded by its own cause. Therefore the fact that Libet et al. (1983) found action specific brain activity before conscious intention emerged is no surprise, for conscious intention needs a cause just like anything else in the universe. Libet et al.'s (1983) conclusion that conscious intention does not cause voluntary action is therefore quite dualistic in nature, for it implies that conscious intention needs to be something more than brain activity to play a role in behavior. Also the proposed premise of conscious intention as a free won't instead of a free will doesn't escape the dualistic trap.
Conscious intention not to move, must just as well have a cause, for it is still not free from the laws of matter. This point is beautifully illustrated by the discovery that conscious intention not to move is preceded by positive brain activity, in contrast to the negative RP (Misirlisoy and Haggard, 2013). This also strengthens the point of Schurger et al. (2012) that the RP might be nothing more than just random fluctuations in brain activity. It seems like intention to act is preceded by fluctuations in the negative direction, and intention not to act by fluctuations in the positive direction. These
fluctuations are not some kind of unconscious motor preparation, but rather random noise that needs to cross a threshold to influence intention and behavior. This makes free won't just as unlikely as free will.
These three critiques change the way Libet et al.'s (1983) experiment should be interpreted. However, it can be concluded that conscious intention is indeed preceded by some brain activity in the form of a RP. But this does not mean that conscious intention can play no role in determining behavior. There is no free will and no free won’t, for conscious intention is not some transcendent phenomenon, but firmly grounded in the brain. Thus the question should no longer be if conscious intention is preceded by brain activity, but what this brain activity is, and how accurately these conscious and unconscious processes determine subsequent behavior. Then a clearer picture of the role of conscious intention in behavior can be sketched. These brain processes related to conscious intention and behavior will be discussed in the next paragraph.
Brain Processes of Conscious Intention and Voluntary Movement
It is now clear that conscious intention is preceded by brain activity, but the location of this activity in the brain and the location of conscious intention still remains unclear. The impossibility to determine the location of activity with an EEG, caused by the fact that EEG measures a summation of currents with infinite source construction possibilities (Luck, 2005), is the reason that the location of the RP could not be specified by experiments using the methodology of Libet et al. (1983). But for a clearer view of the way that brain processes determine behavior the where, next to the when, of brain activity must also be determined. By doing so it will be discussed if conscious intention is a necessary causal link in the path to behavior, or just the apparent cause for behavior that is actually mediated by unconscious brain processes (Wegner, 2003, in Haggard, 2008). Therefore the question which brain processes lead to conscious intention and voluntary movement will be discussed in this paragraph.Because of the low spatial resolution of EEG, researchers used other methods to determine the location of conscious and unconscious intention in the brain. Using functional magnetic
resonance imaging (fMRI), Soon, Brass, Heinze and Haynes (2008) showed that the choice to press a left or a right button could be predicted with 60 percent accuracy from activity in the frontopolar and parietal cortex about 10 seconds before the actual choice was made. It seems that a conscious choice might already be partly prepared for as long as 10 seconds before it enters consciousness. While an accuracy of 60% is well above chance level, it cannot be regarded as high. Measurement errors from various sources can cause this low predictability, for instance noise in the signal caused by random brain activity. It could also be that brain activity between the onset of this unconscious preparation plays a role in fine-tuning the actual behavioral response. This experiment reflects a simple choice, while choices in real life are a lot more complex that a choice between two buttons most of the time. More behavioral fine tuning is needed in those complex choices, thus it seems logical that a
tuned choice, possibly also conscious brain activity. Therefore, a better understanding of the brain activity occurring in the 10 seconds before an action is needed.
Somehow, the predictive brain activity in the frontopolar and parietal cortex leads to the RP and subsequent conscious intention activity. The RP seems to reside in the supplementary motor area (SMA) and pre-SMA (Fried, Mukamel and Kreiman, 2011). Fried et al. (2011) were able to predict the decision to move with 80% accuracy about 700 ms before conscious intention to move by using extracellular single-cell recordings of SMA and Pre-SMA neurons. A buildup of neural activity started about 1500 ms before movement was initiated. This buildup can be seen as an important contributor to the RP as measured by Libet et al. (1983), since this activity is correlated in time with the RP. It cannot be said with certainty that activity in the SMA and Pre-SMA are the sole
contributors to the RP. However, Lau, Rogers, Haggard and Passingham (2004) showed that greater Pre-SMA activity is correlated with earlier awareness of conscious intention, and that when pre-SMA activity crosses a certain threshold conscious intention arises. Since Libet et al. (1983) also found that RP activity needed to cross a certain threshold for conscious intention to arise, it seems logical that this pre-SMA activity might be a strong contributor to the RP. Therefore it can be concluded that the RP might originate from activity in the SMA and pre-SMA. Still, this SMA and pre-SMA activity only predicts subsequent movement with 80% accuracy. Next to measurement error this could mean that there is some final fine tuning brain activity preceding movement. This could be the brain activity of conscious intention, or some other unconscious brain process. For now it seems that activity preceding voluntary movement starts somewhere in the frontopolar and parietal cortices, which heightens the chance of an RP in the SMA and pre-SMA. This activity might lead to brain activity of conscious intention and other unconscious activity, finally leading to voluntary movement.
To close this causal link the final brain activity must be inspected. Lau et al. (2004) and Fried et al. (2011) already showed that buildup of SMA and pre-SMA crossing a threshold was correlated with conscious intention and subsequently voluntary movement, thus this activity might cause both.
Correlation does however not automatically means causation. But Fried et al. (1991) showed that directly stimulating the SMA creates the urge to move. Greater stimulation even led to actual onset of movement. This shows two things. First, it shows that the urge to move is a process that can be separated from movement itself. Therefore the urge to move cannot be caused by movement in retrospect, but is a brain process that occurs just before movement. It is probable that a first threshold of activity needs to be crossed in order to achieve the conscious urge to move, and a seconds higher threshold for actual movement. Second, it confirms the causal link between SMA activity and voluntary movement. SMA activity causes movement when strong enough, and this movement is always preceded by the urge needed for it to be voluntary (Fried et al., 1991). However, this urge is not necessarily equal to the entire feeling of conscious intention. Participants reported this urge to be very specific, like the urge to move the left leg, and to be very forceful, like the feeling of having to move or going to move. However, stimulation did not necessarily made the participants want to move. Wanting to move is an important part of the feeling of conscious intention. Therefore it seems that another brain process is needed to cause true intention. So it seems that at least two conscious processes in the brain are related with planning of voluntary movement: conscious urge is knowing and having to move, and conscious intention is wanting to move. While it is now clear that SMA activity is a cause of the conscious urge to move and movement itself, the cause of true conscious intention and its influence on behavior is still unknown.
The dissociation between urge and intention in the brain is further supported by the experiment of Desmurget, Reilly, Richard, Szathmari, Mottolese and Sirigu (2009), that showed that stimulating the posterior parietal cortex (PPC) led to the feeling of wanting to move and sometimes even the feeling of movement in absence of real movement (Desmurget et al., 2009). Furthermore, patients with a lesion in the PPC could not determine when conscious intention to move started (Sirigu, 2004). Patients became aware of the intention to move at the same time that movement was initiated, as opposed to healthy subjects that felt this intention about 250 ms before movement. This shows that the PPC is important for generating intentions prior to movement. However, stimulating
the pre-motor cortex led to movement without conscious awareness, intention and urge of the movement, an opposite effect (Desmurget et al., 2009). Thus it seems that the causal chain leading towards voluntary movement splits somewhere in the brain, leading to conscious intention and movement separately. The processes in the pre-motor cortex and PPC seem to be decoupled, since there is a double dissociation between movement and intention. Thus it seems that conscious intention might not determine behavior directly, but could be a sideway of the causal chain of brain activity that leads to behavior, predicting the behavior that is going to occur.
A couple questions are left unanswered by these findings. First, it still remains unclear how the SMA, conscious urge and the pre-motor cortex are related in causing voluntary movement. Since SMA activity leads to the urge to move and movement itself, and the pre-motor cortex to movement without urge, the SMA might be an earlier step in the chain of voluntary movement, that activates the pre-motor cortex to unconsciously prepare specific motor execution plans, that relate to the conscious urge felt due to SMA activity. It has been proposed that the pre-motor cortex plays an important role in comparing planned movement with actual movement, by comparing expected input to the somatosensory cortex with true input of the somatosensory cortex due to movement (Desmurget and Sirigu, 2009). This is supported by the discovery that the pre-motor cortex has projections to and from the somatosensory cortex (Christensen et al., 2007). In the experiment of Desmurget et al. (2009) movement by pre-motor cortex stimulation was not felt. This might be because the stimulation made the motor cortex to expect certain input to the somatosensory cortex, which was compared to the true input, and found to be correct (Desmurget and Sirigu, 2009). Because the comparison was true, no error signal was created in the pre-motor cortex, leaving the movement outside of awareness. Thus the pre-motor cortex might be responsible for monitoring ongoing behavior, and bringing it to awareness if something goes awry, like blockade of the movement by a wall (Desmurget and Sirigu, 2009).
Second, it remains unclear where activity of conscious intention splits of in the causal chain leading to voluntary movement. This split must occur before SMA activity, since SMA stimulation does not lead to intention (Fried et al., 1991) and PPC stimulation does not lead to movement (Desmurget et al., 2009). The dorsolateral prefrontal cortex (DLPFC) might be where this split occurs. It has been shown that the DLPFC projects to the both the SMA and the PPC, and receives projections from the frontopolar cortex (Desmurget and Sirigu, 2009). So the DLPFC might be where the path of activity of conscious intention, and conscious urge and movement splits, activated by the frontopolar cortex that has earlier been shown to be predictive for voluntary movement (Soon et al., 2008). The finding that DLPFC activity is far greater with self-initiated movement compared to externally initiated movement further indicates that the DLPFC is involved in voluntary movement (Jahanshahi et al., 1995). Interestingly, the DLPFC has been found to be activated only when conscious changes to a movement pattern are made, and not when this movement pattern is adapted unconsciously (Stephan et al., 2000). Therefore, the DLPFC might be important in conscious motor planning. This then might lead to conscious intention in the PPC and movement trough SMA an further motor cortices.
Last, it remains unclear what the actual function of conscious intention is if it does not directly cause voluntary behavior. It could be that conscious intention acts as a first messenger to other conscious and unconscious brain systems that a certain behavior is about to occur, enabling the brain to monitor and adapt the movement in real time without the need for sensory feedback. Movement outcome can subsequently be compared with movement intention in order to process if an error has occurred and to plan subsequent movement on the base of this prediction. Interestingly it has been shown that the PPC is important in adapting behavior in real time. Participants with a TMS inhibited PPC had difficulty with adapting the path of their ongoing movement due to a change of movement goals (Desmurget et al., 2001, Desmurget et al., 1999). The participants had to reach for a target with their right hand, which was hidden (Desmurget et al, 1999). Sometimes the target suddenly moved, forcing the participants to change the movement path. While control participants
had no difficulty in doing so, participants with TMS inhibited PPC's where unable to change the path of their hand. Thus the PPC is important for adapting ongoing movements to a changing
environment. I propose that conscious intention functions as a real time representation system of goal- directed movement, monitoring the goal and adapting ongoing behavior to reach the goal. Then there is no need for waiting for sensory input of ongoing movement, allowing movement corrections to be done much faster.
It can be concluded that at least two conscious brain processes are important in voluntary behavior, conscious urge and conscious intention. Both are integrated in a fine system of conscious and unconscious brain processes that is responsible for initiating, monitoring and adapting voluntary
Figure 2. The proposed neurofunctional model of conscious intention and voluntary movement. Red arrows represent activity preceding voluntary movement, black arrows represent activity after initiation of voluntary movement. The red arrows thus represent the causal system of brain processing leading to voluntary movement, and the black arrows monitoring of those movements.
movement. Based on the evidence in this paragraph, I propose a new neurofunctional model of conscious intention and voluntary movement. Planning for new voluntary movements starts in the frontopolar and parietal cortex about 10 seconds before movement is initiated (Soon et al, 2008). The frontopolar cortex projects to the DLPFC, where general movement plans for conscious movement are planned (Stephan et al., 2000). The DLPFC further projects to the SMA and PPC (Desmurget and Sirigu, 2009). If a certain threshold is crossed, the conscious urge to move will then arise in the SMA (Fried et al., 1991), and the conscious intention to move in the PPC (Desmurget et al., 2009). The SMA projects to the premotor cortex, where specific motor plans are formed unconsciously to guide the precise movement. While the premotor cortex will receive sensory feedback in order to monitor if the movement is executed correctly (Christiansen et al., 2007), the PPC will receive sensory feedback in order to monitor the goal of the behavior, and will compare this goal to the intention in order to adapt movement accordingly. (Desmurget et al., 1999). Together the PPC and premotor cortex will monitor movement and goals to ensure that the actual movement is equal to the intended movement. A summary of this model is presented in figure 2.
Conclusions and Discussion
It can be concluded that conscious intention is important for monitoring and adapting ongoing voluntary movement, but is not responsible for initiating voluntary movement. In the first paragraph it has been showed that whilst the deterministic and materialistic stance needed for a clear scientific study of conscious intention excludes the possibility of conscious intention acting on voluntary movement due to free will, an absolute free will is not necessary for conscious intention to influence voluntary movement. The second paragraph then made clear that conscious intention is preceded by unconscious brain activity in the form of an RP. This did however not exclude the possibility of conscious intention being a part of an causal chain of brain processes leading to voluntary movement. In the last paragraph this chain of brain processes had been identified, and a
new model for conscious intention in the brain has been proposed. This showed that brain processes of motor planning split of in two directions, one leading to conscious intention, and the other to a conscious urge and voluntary movement. Furthermore it showed that conscious intention adapts ongoing movement to a changing environment. Thus it seems that conscious and unconscious processes are delicately intertwined in determining voluntary movement and its outcome.
However, research on the topic of conscious intention and voluntary movement is far from complete, causing this thesis to leave some questions regarding these topics open for discussion.
First, not all steps in the proposed neurofunctional model of intention and voluntary movement are sufficiently supported by research, thus it must be seen as a first step towards a better understanding of conscious and unconscious brain processes causing voluntary movement. While the link between the (pre-)SMA, pre-motor cortex, conscious urge and voluntary movement, and the link between the PPC and conscious intention are substantially researched and corroborated by studies of imaging, laesies and stimulation, more research is still needed to corroborate and specify the links between the frontopolar and parietal cortex, the DLPFC, the SMA and the PPC. The DLPFC has only been shown to be connected to these areas (Desmurget and Sirigu, 2009), and to be important for planning of conscious, voluntary movement (Stephan et al., 2000), making it a
probable step in the neurofunctional model. The importance of the DLPFC for this model might however be made more clear by research of laesies of the DLPFC or research of stimulation of the DLPFC, because it can then be showed if the DLPFC is a necessary step for voluntary movement to occur, and if stimulation of the DLPFC does indeed lead to voluntary movement, accompanied by feelings of conscious intention and urge. Furthermore, since the DLPFC is a grand region of the brain, these kinds of research might help to determine the role of more specific areas in the DLPFC involved in causing voluntary movement. In order to strengthen and specify the neurofunctional model of intention and voluntary movement these types of research are necessary. The model does however create a good basis for future research on the topics of conscious intention and voluntary movement.
Second, conscious intention and urge are not the only processes of conscious content. As stated, conscious intention and urge are forms of short-term consciousness. Conscious content does however also contain more long-term processes of consciousness, for instance thinking about what to have for dinner later. These processes were beyond the scope of this thesis. However, better understanding of these long term processes might give a better understanding of the full influence of consciousness on voluntary movement, and the brain processes behind it. For instance it has been proposed that conscious content resides in an extensive frontoparietal network that acts like a global neuronal workspace (GNW) of the mind (Dehaene and Naccache, 2001). Combining the
frontoparietal network and GNW, and the neurofunctional model of intention and voluntary movement might give a broader understanding of the cause of voluntary movement. For instance, the frontopolar and parietal activity that is important for initiating voluntary movement might be caused by conscious activity in the frontoparietal network. Just as well, the DLPFC might be a part of the frontoparietal network of consciousness. Thus, in order to fully explain the origin of voluntary movement in the brain attempts must be made to unite models of conscious content and models of conscious intention.
While these shortcomings address sufficient topics for future research the conclusion of this thesis still stands. Processes of conscious intention and urge seem to be functional in voluntary movement, but both are caused and mediated by many unconscious brain processes. Thus in a sense Freud was right, and we must accept that we are no longer completely master in our own house
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The Influence of Dorsolateral Prefrontal Cortex
Activity on Conscious Movement Adaption: a Direct
Current Stimulation Study
Tom Pelsmaeker
Bachelor Psychology, University of Amsterdam. Under supervision of Dr. S van Gaal. Research
proposal, +- 2900 words, abstract 142 words. 21 July 2014.
1. Abstract
The dorsolateral prefrontal cortex plays a crucial role in conscious movement. In this study the influence of dorsolateral prefrontal (DLPFC) activity on conscious movement adaption is further investigated using transcranial direct current stimulation (tDCS). Thirty subjects will be given either anodal, cathodal or sham left and right DLPFC tDCS stimulation. The subjects will then have to finger tap to the rhythm of a tone. However, the rhythm of the tone will either be isochronous or fluctuated by 3, 7 or 20 percent. Cathodal stimulation might disturb the participants ability to adapt to these tones consciously, while anodal stimulation might increase the participants ability to adapt to these tones consciously, since cathodal stimulation decreases the excitability of the DLPFC and anodal stimulation increases excitability of the DLPFC. Such results would show the causal importance of the dorsolateral prefrontal cortex in conscious movement adaption. 2. Introduction
One of the great quests of neuroscience is unraveling the brain processes involved in consciousness and conscious intention. In my bachelor thesis I combined experimental data on brain processes of conscious intention in order to design a neurofunctional model of conscious intention (figure 1). However, the role of the dorsolateral prefrontal cortex (DLPFC) in conscious intention has not yet been adequately studied. Thus far, only correlational evidence on the role of the
DLPFC in conscious intention exists (Stephan et al., 2002, Jahanshahi et al., 1995). Therefore the topic of this research proposal will be the influence of the DLPFC on conscious movement.
It has already been shown that the DLPFC is connected to the frontopolar cortex, the posterior parietal cortex and the supplementary motor cortex, areas that have shown to be important in conscious intention and voluntary movement (Desmurget and Sirigu, 2009). Furthermore, Jahanshahi et al. (1995) have shown that DLPFC activity is greater in self-initiated movement than in externally caused movement. Last, Stephan et al. (2002) showed that the DLPFC is only active when movement is consciously adapted, and not when movement is adapted unconsciously. Thus it seems plausible that the DLPFC is somehow important in conscious planning of movement. However this evidence is only correlational, so no causal role between the DLPFC and conscious movement can yet be determined by these experiments. A stimulation study could aid to determine the causal role of the DLPFC in conscious movement.
Two forms of non-invasive brain stimulation are possible, transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) (Fregni et al., 2005). However, TMS might disrupt brain processes, while tDCS only enhances or decreases the excitability of a certain brain area (Fregni et al., 2005). Therefore tDCS is more suitable for the proposed experiment. Three types of tDCS stimulation are possible, anodal, cathodal and
Figure 1. The proposed
neurofunctional model of conscious intention and voluntary movement. Red arrows represent activity preceding voluntary movement, black arrows represent activity after initiation of voluntary movement. The red arrows thus represent the causal system of brain processing leading to voluntary movement, and the black arrows monitoring of those movement.
sham. Anodal stimulation has shown to increase the excitability of the DLPFC, while cathodal stimulation has shown to decrease the excitability of the DLPFC (Nitsche et al., 2008). Sham stimulation does not change activity in the DLPFC (Nitsche et al., 2008). Since it has been shown by Stephan et al. (2002) that DLPFC activity correlates with conscious awareness of fluctuation of a rhythmic tone and conscious adaption of finger tapping to that tone, it might be expected that increased excitability of the DLPFC due to anodal stimulation leads to more conscious awareness of rhythmic tone fluctuations and more conscious adaptation of finger tapping to the rhythmic tone fluctuations than sham or cathodal stimulation. Furthermore, it might be expected that decreased excitability of the DLPFC due to cathodal stimulation leads to less conscious awareness of rhythmic tone fluctuations and less conscious adaptation of finger tapping to the rhythmic tone fluctuations than sham or anodal stimulation. Last, since no changes of DLPFC excitability are expected due to sham stimulation, it might be expected that sham stimulation leads to the same amount of conscious awareness of rhythmic tone fluctuations and the same amount of conscious adaptation of finger tapping to the rhythmic tone fluctuations as found in the experiment of Stephan et al.
(2002). By combining the experimental paradigm of Stephan et al. (2002) with tDCS stimulation these hypotheses will be studied.
3. Method and Design Subjects
Thirty right handed subjects, either male or female, of adult age and good health will participate in this study.
Direct current stimulation
Similar to earlier studies using tDCS (Fregni et al., 2005), the direct current will be transferred using surface sponge electrodes (35cm2) and delivered by a constant current stimulator with a maximum current of 10 mA. The relatively large electrode size is suitable to stimulate a large brain area like the DLPFC (Nitsche et al., 2008). Since bilateral DLPFC activity was found in conscious movement adaption (Stephan et al., 2002), the entire DLPFC will have to be stimulated. However, since bilateral stimulation is impossible using tDCS (Nitsche et al., 2008), the left and right DLPFC will be stimulated separately. For anodal stimulation of the left DLPFC the anode will be placed over the F3 region according to the 10-20 international system for EEG placement (figure 2) (Rossi et al., 2001, Herwig
Figure 2. Schematic illustration of the electrode placement
for anodal stimulation of the left DLPFC according to the 10-20 internation system for EEG placement. The red square indicates the location of the anode, the blue square the location of the cathode. Reprinted from "Transcranial direct current stimulation: state of the art 2008". By Nitsche et al., 2008, Brain Stimulation, 1, p. 220. Copyright 2008 by Elsevier inc.
et al., 2003, in Fregni et al., 2005). The cathode will be placed over the contralateral supraorbital area. For cathodal left DLPFC stimulation the location of the electrodes will be swapped (Nitsche et al., 2008). For anodal right DLPFC stimulation the anode will be placed over the F4 region according to the 10-20 international system for EEG placement, and the cathode over the contralateral supraorbital area (Nitsche et al., 2008). For cathodal right DLPFC stimulation the location of the electrodes will be swapped. Sham stimulation will also be applied to the left and right DLPFC separately. For left sham stimulation the electrode placement of left anodal or cathodal stimulation will be used randomly across subjects, while for right sham stimulation the electrode placement of right anodal or cathodal stimulation will be used randomly across subjects. Anodal and cathodal stimulation will be delivered by applying a constant 1 mA current over a period of ten minutes, causing changes in excitability that last at least an hour (Nitsche et al., 2008). Sham stimulation will be delivered by initially applying 1 mA current but turning the stimulator of after 5 seconds. This will create the same itching stimulation at the beginning of sham stimulation as at the beginning of real stimulation, leaving participants unable to discriminate between different forms of stimulation (Nitsche et al., 2003, in Fregni et al., 2005).
Experimental conditions
Each participant would be subjected to all six stimulation conditions (sham, right-anodal, right-cathodal, left-sham, left-anodal and left- cathodal) over a period of five weeks. To protect against carryover effects there will be a period of one week between each stimulation condition, as advised by Nitsche et al. (2008). Furthermore the stimulation conditions will be counterbalanced across participants. After stimulation participants will perform the rhythmic finger tapping task as designed by Stephan et al. (2002). The participants will be instructed to tap with their right index finger on a sensor switch to four different rhythmic tone sequences: completely regular every 1250 ms (isochronous) and modulated 3, 7, and 20% of the stimulus interval (figure 3). Each of the four tapping conditions will be performed twice by each subject after each stimulation, counterbalanced within each subject and pseudo-randomized between subjects and stimulation conditions to control for possible carryover effects between tapping conditions. The eight finger tapping iterations will take about 45 minutes including pauses.
Control measures
In order to control if the sham stimulations are not noticed by participants the participants will be asked if they felt any difference between stimulation conditions (Nitsche et al., 2008) Participants who noticed the sham stimulation will be excluded of further analysis.
Figure 3. The four conditions of
the rhythmic finger tapping task. Reprinted from "Conscious and subconscious sensorimotor synchronization - prefrontal cortex and the influence of awareness". By Stephan et al. ,2002,
NeuroImage, 15, p. 348. Copyright
2002 by Elsevier Science.
Furthermore, it will be controlled if participants are able to follow the rhythms with their finger taps by checking that the average difference in time between finger taps is about the same as the average distance between the rhythmic tones on all conditions in statistical analysis. Participants that are unable to follow the rhythms will be excluded of further analysis
The participants will also be asked if the stimulus presentation was regular or contained any fluctuations with regard to rhythm, duration or pitch of tones (Stephan et al., 2002) in order to control when conscious awareness occurred and if it correlates with conscious adaption of finger tapping like in the experiment of Stephan et al. (2002).
4. Data analysis
The data analysis will match the analysis performed by Stephan et al. (2002). The finger tapping signals will be recorded on a PC via A/D card at a sampling rate of 1000Hz and stored for offline analysis. MATLAB mathematical software routines will be used to perform statistical analysis. For each subject, across subjects, across trials and across stimulation conditions the average synchronization error (SE) and average inter- response interval (IRI) will be calculated. The SE is the difference in time between the finger
tap and the tone, calculated by subtracting moment of the finger tap with the moment of the tone. The IRI is the interval between finger taps. The results on these measures in Stephan et al.'s (2002) experiment can be found in figure 4, as illustration of possible baseline results (sham stimulation) for this experiment.
First, the IRI will be compared to the interstimulation interval (ISI), the interval between the rhythmic tones, to determine if the participants are able to follow the rhythm. When significant differences between the ISI and IRI for a participant are found in a condition, the participant will be excluded of further analysis.
Then, the SE will be analyzed further. Using the SE it will be determined if the finger tapping task was executed in anticipatory or reactive mode. A negative SE indicates that the task is performed in anticipatory mode, meaning that the participants anticipate the tone and respond slightly before the actual occurrence of the tone. However, when the SE is positive the task is performed in reactive mode, meaning that the participants wait for the tone to actually occur before responding. The average SE's on all stimulation conditions for left and right, and all rhythmic fluctuation conditions will be compared using a 2x3x4 factorial repeated measures ANOVA, resulting in a within subject comparison of the SE outcomes over 24 conditions divided over
Figure 4. Average ISI, average IRI
with STD, and average SE with STD as found by Stephan et al. (2002) in the finger tapping experiment to illustrate the data analysis. Reprinted from "Conscious and subconscious sensorimotor synchronization - prefrontal cortex
and the influence of awareness". By Stephan et al. ,2002, NeuroImage,
15, p. 348. Copyright 2002 by
Elsevier Science.
side, stimulation and tone modulation as the three independent variables. A full factorial model will be used to test for all possible main effects and interactions. Furthermore, simple contrasts of all three independent variables will be calculated and modulation * stimulation * side plots will be made to specify the direction of possible main effects and interactions. Last a post hoc Bonferroni test will be used to compare main effects.
5. Interpretation of Possible Results
It was hypothesized that the sham stimulation will yield similar results as the study of Stephan et al. (2002), meaning that for the isochronous, 3% and 7% task after Sham stimulation an anticipatory mode is expected, while for the 20% task a reactive mode is expected. Anodal stimulation is expected to lead to more conscious awareness and adaption, and cathodal stimulation to less conscious awareness and adaption. No difference between left and right stimulation is expected. This can be translated to the two modes of responding. Stephan et al. (2002) found the anticipatory mode to be a form of subconscious adaption to the rhythm, while the reactive mode is a form of conscious adaption to the rhythm. Therefore anodal stimulation should produce more reactive mode responses of participant on the different rhythmic modulation conditions, and cathodal stimulation more anticipatory mode
responses, relative to other forms of stimulation.
Several ANOVA outcomes can thus be expected. The full factorial analysis should produce a significant main effect of rhythmic modulation, since it is expected that in different conditions different SE's will be found regardless of stimulation, in accordance with Stephan et al.'s (2002) experiment. A significant main effect of stimulation is also expected since increased or decreased excitability due to stimulation should influence the SE regardless of modulation, because it can be expected that stimulation changes the likelihood of anticipatory and reactive mode usage. No significant main effect of side is expected. A significant interaction effect between stimulation and rhythmic modulation is expected as well. between different stimulation conditions the difference in SE on different rhythmic modulation conditions is expected to differ. This can be expected because the continuous variable SE is determined by the two categorical modes of responding. It can be expected that a participant can be in anticipatory or reactive mode, but not somewhere in between. Looking at the results in figure 4 (Stephan et al., 2002), anticipatory mode is probably accompanied by a negative SE of about -35ms, while reactive mode is probably accompanied by a positive SE of 90ms. These values are expected to be the floor and ceiling for the SE. Since stimulation is
expected to change the probability of these modes it can be expected that different stimulation conditions will hit the floor and ceiling at different rhythmic modulations, thus creating the interaction effect. Of course values between the floor and ceiling can be expected when some but not all participants use the reactive mode. No interactions with side are expected
The contrasts will produce insight into the main effects and interaction effect. First, it is expected that all stimulation conditions differ from each other. Anodal stimulation is expected to differ significantly from sham stimulation, due to a higher average SE, and cathodal stimulation is expected to differ significantly from sham stimulation, due to a lower average SE than sham stimulation. Logically anodal stimulation is then expected to have a significantly higher SE than cathodal stimulation. However since simple contrasts only allow for one reference category this has to be proved using the post hoc Bonferroni test.
Second, some rhythmic modulation conditions are expected to differ. Since it is unlikely that participants become consciously aware of a 3% modulation, no significant difference in SE between the isochronous and 3% condition is expected. However, in the 7% condition awareness is more likely, thus a significant difference between the isochronous and 7% condition can be expected, due to a higher average SE in the 7% condition. A significant difference between the 20% and isochronous condition is highly expected, due to a higher average SE in the 20% condition. The post hoc Bonferonni test will provide further insight in differences between the conditions.
Last, interactions are expected between several conditions. In comparing the difference between sham and anodal stimulation on the isochronous and 3% condition no significant interaction is expected. In comparing the difference between sham and cathodal stimulation on the isochronous and 3% condition no significant interaction is expected as well, all
values are expected to be close to the floor. However, in comparing the difference between sham and anodal stimulation on the isochronous and 7% condition a significant interaction is expected. Anodal stimulation is expected to lead to more usage of the reactive mode on the 7% condition, thus creating a greater difference between SE in sham and anodal stimulation on the 7% condition than on the isochronous condition. A significant interaction between sham and cathodal stimulation on the isochronous and 7% condition is expected as well. Cathodal stimulation is expected to lead to more usage of the anticipatory mode on the 7% condition, thus creating a greater SE difference in sham and cathodal stimulation on the 7% than on the isochronous condition. However, a significant interaction between anodal and sham stimulation on the ischronous and 20% task is not expected, since both anodal and sham are expected to have floor values on the isochronous condition and ceiling values on the 20% condition. Last, a significant interaction between cathodal and sham compared on the isochronous and 20% conditions is expected, since more anticipatory mode usage on the 20% condition due to cathodal stimulation is expected. These expected interactions and main effects are summarized in the graph in figure 5 for the left DLPFC. Similar results are expected for the right DLPFC since no main effects or interactions with side are expected.
The results above would confirm the hypotheses, and show that there is a causal influence of DLPFC activity on conscious movement, since more activity leads to more conscious adaption and less activity to less conscious adaption. That would corroborate the position of the DLPFC in the neurofunctional model of conscious intention, since it shows the causal role of DLPFC activity in conscious, intentional, movement change. However, certain unexpected results might occur. the results of the sham conditions could deviate greatly from the results found by Stephan et al. (2002). several alternative conclusions could then be made. First, it could mean that the finger tapping task was not
Figure 5. Graph of possible
expected results of left side DLPFC stimulation. The floor and ceiling can be observed, as well as the expected interactions and main effects. It can be seen that on average anodal stimulation leads to a higher SE, and cathodal stimulation to a lower SE. Sham values are copied from Stephan et al. (2002). This graph was created using IBM SPSS statistics 20.
performed correctly in this experiment. However, this is unlikely since it is controlled for. Second, it could also mean that there is a placebo effect of stimulation, or an effect of PET scanning on task performance, which was used by Stephan et al. (2002). That would still allow for further conclusions to be drawn out of comparison between stimulation conditions. Last, it could mean that the reliability of the finger tapping task is low, producing different results each experiment. That could be caused by the low number of subjects participating Stephan et al.'s (2002) (N=9) experiment. Repeating that experiment using more participants could solve this problem.
But a significant main effect or interaction of side could also be found. That could indicate that only the left or right DLPFC is involved in conscious movement adaption, or it could indicate that the left or right DLPFC is only involved in movement of right body parts, and the other side of the DLPFC only in movement of left body parts. In both cases such results would help to further specify the neurofunctional model of conscious intention and the role of the DLPFC in conscious movement.
Last, it could be found that anodal stimulation does not cause more conscious movement adaption, or even decreases conscious movement adaption. In a similar fashion it could be found that cathodal stimulation does not cause later conscious movement adaption, or even increases conscious movement adaption. That could mean that either the experiment was not conducted correctly, or that the initial hypotheses are wrong. this could show that while the DLPFC is active during conscious movement adaptation, it is not the cause of this movement adaptation. Then the model would have to be adapted and the role of the DLPFC in conscious movement revised. Further research could then aim to unravel other brain areas that are important in causing voluntary movement, or to reproduce this experiment in order to attain alternative or corroborating results.
6. References
Desmurget, M., & Sirigu, A. (2009). A parietal-premotor network for movement intention and motor awareness. Trends in cognitive sciences, 13, 411-419.
Fregni, F., Boggio, P. S., Nitsche, M., Bermpohl, F., Antal, A., Feredoes, E., ... & Pascual-Leone, A. (2005). Anodal transcranial direct current stimulation of prefrontal cortex enhances working memory. Experimental Brain Research, 166, 23-30.
Jahanshahi, M., Jenkins, I. H., Brown, R. G., Marsden, C. D., Passingham, R. E., & Brooks, D. J. (1995). Self-initiated versus externally triggered
movements I. An investigation using measurement of regional cerebral blood flow with PET and movement-related potentials in normal and
Parkinson's disease subjects. Brain, 118, 913-933.
Nitsche, M. A., Cohen, L. G., Wassermann, E. M., Priori, A., Lang, N., Antal, A., ... & Pascual-Leone, A. (2008). Transcranial direct current stimulation: state of the art 2008. Brain stimulation, 1(3), 206-223.
Stephan, K. M., Thaut, M. H., Wunderlich, G., Schicks, W., Tian, B., Tellmann, L., ... & Hömberg, V. (2002). Conscious and
subconscious sensorimotor synchronization: prefrontal cortex and
the influence of awareness. Neuroimage, 15, 345-352.
