1 Introduction
One of the core experiences we share as human beings is the impact our intentions and decisions have on our life. We think that the decisions we make, from the major ones like getting married, to small ones, like taking a bottle of water from the fridge on a warm and sunny day, are what causes the actual physical overt actions. However, this shared conviction is problematic for a number of reasons, including questions like: do choices exist at all given the potentially deterministic nature of the universe; are our decisions causal in the action generation or rather the physical brain activations determine all our actions. Most of these questions are parts of a larger framework, the free will problem, which, rather than being a homogenous issue, is an umbrella term for a number of interconnected problems.
In recent decades, two most popular angles, discussed in connection to free will, although this division should not be treated as exhaustive, have emerged: (1) is free will possible given the deterministic nature of the universe, and (2) can the conclusions of neuroscientific experiments truly show that our intentions are not causal in the processes of action generation, and therefore we are not free. In this chapter I will argue that these discussions often dismiss one, important component, not only needed for the possibility of free will, but, moreover, implicitly assumed by most of the positions, namely the causal efficacy of the mental. I will argue that, if mental, as mental, does not have a causal impact on the physical, free will is in dire straits or, as Fodor famously put it:
[…]if it isn’t literally true that my wanting is causally responsible for my reaching, and my itching is causally responsible for my scratching, and my believing is causally responsible for my saying…, if none of that is literally true, then practically everything I believe about anything is false and it’s the end of the world.
fodor, 1990, p. 156
This chapter will have two central, interconnected, goals: (1) to argue that the causal efficacy of the mental is one of the necessary conditions for free will,
2 The Ubiquitous Assumption
Why is mental causation so important for the existence free will? In the discussions on voluntary or intentional action, it has become a standard to explain it though evoking a reason for action (e.g. Davidson, 1980), or an intention (e.g. Mele, 2009), both of which have two components: a belief and a desire. Both are often treated as goal directed (to further distinguish them from reflexes, which are not considered free), mental rather than purely physical, and not reducible to some specific brain activation. Therefore, the usual way of thinking about a voluntary action (without thinking about determinism or indeterminism) is as a result of a causal chain, where the cause of said action is either an intention, or a reason. Alternatively, a way in which an action is identified as free is through its mental cause. Depending on the time lapse between the intention and the action, we can identify distal (connected to actions far in the future), and proximal (liked to immediate action), but regardless of this, the presence of the intention as the main feature of the causal chain which results in a voluntary action seems to be necessary. All those components are
In the experimental domain, the interest in the causes of voluntary actions can be traced to the experiments by Kornhuber and Deecke (1965), but more frequently it is discussed in connection to the modified version of those conducted by Libet (Libet et al. 1982; 1985). For him an action is voluntary if it meets three conditions:
(a) it arises endogenously, not in direct response to an external stimulus or cue; (b) there are no externally imposed restrictions or compulsions that directly or immediately control the subject’s initiation and performance of the act; and (c) most important, subjects feel introspectively that they are performing the act on their own initiative and that they are free to start or not to start the act as they wish.
libet 1985, pp. 529–530
Here an action is voluntary (and therefore free) when it is not an automatic response to a stimulus coming from the agent’s immediate environment, it should be under agent’s control, and he/she should recognize themselves as the agent, thereby experiencing the feeling of acting. The feeling of acting out of own initiative is a key component and can be equated with making a decision to perform an action. Therefore, similarly to philosophical arguments, Liber thinks of a free action as one that is goal oriented, and is caused by a decision of an agent. Therefore if an action that an agent would consider as arising from his own decision, was in fact caused by other factors, it would not be defined as free. To further explore the origin of fully spontaneous actions (the only ones that could have the potential to be free) Libet conducted a series of experiments. He asked the participants to perform a simple overt action: move a finger or flex a wrist while looking at a dot of light projected on a clock-like surface. The participants were then asked to remember the position of the light point at the exact moment they decided to move, while the eeg machine was recording their neural activity. The results obtained by Libet showed a consistent spike of activity in the motor cortex (where the movement would be initiated) 500 ms before the onset of the movement. Yet, on average, the participants only reported their decision to move as taking place around 200 ms
Many problems with Libet’s experiments have been identified, from the doubious nature of the readiness potential, to the usage of the clock, which could possibly influende the readings. However the most basic assumptions of the experiment are rarely questioned. Libet clearly thinks that in order for an action to be free, it has to be caused by something mental (the decision) as opposed to something physical (the brain activation), and this assumption has remained the standard approach in neuroscience. The definition of the component might change- from decision to intention, but its nature, as a mental event, remains present. Therefore also here, as in philosophy, the next question that comes to mind is how.
While both philosophers (e.g. Mele) and neuroscientists (e.g. Haggard) seem to agree that a voluntary action requires a specific type of cause, namely a mental one, the relation between the mental and the physical and its importance to the argument on free will is often pushed to the side. At the same time intentions are sometimes defined as […] a mental state, which may be associated with particular brain states (Pacherie and Haggard, 2010, p. 70), which settles for a vague notion of association, without further explaining how such connection could impact the causal chain leading to the voluntary action. Sometimes dualism is assumed to be necessary for free will to have a chance at existence (e.g. Haggard, 2011, Desmurget and Sirigu, 2009), but those discussions are quickly dismissed (e.g. Mele 2009, 2014). Yet, Mele himself argues that Whenever human beings perform an overt intentional action, at least one of the following plays a causal role in its production: some intention of theirs; the acquisition or persistence of some intention of theirs; the physical correlate of one or more of the preceding items. (Mele, 2009, p. 11), thereby assuming that it is possible for a voluntary action to be caused by the physical item, only if it is correlated with the mental one.
The main advantage of Mele’s position is the space it leaves to incorporate the conclusions of neuroscientific experiments into a broader philosophical debates. It shows that the sole fact there is a physical activation linked to the mental is not in fact that surprising (after all we all have a brain), and that the physical can have a role in the causal chain that results in a free action, without automatically disproving free will. This can happen because of the relationship between the mental and the physical. The main disadvantage is the lack of
To summarize what we know so far:
- (1)the voluntary action is most often defined though its causes- if it is mental, the action could be voluntary (it could also not be, there is a number of other conditions, the mental causation does not solely guarantee free will), if it is physical the action surely is not voluntary;
- (2)this assumption is shared by both philosophers and neuroscientists alike, and seems to be the basic feature of what constitutes a voluntary action;
- (3)when it comes to the relationship between the mental and the physical, dualism is not necessary, and the fact that the physical is a part of the causal chain is not problematic. The physical could still play a role, but only in so far it is connected to the mental.
If we accept what has been said so far here, it is clear that there is a need for an account that shows exactly how the mental and the physical are connected, and how the mental plays the causal role (in its connection to the physical). If it is not dualism, it is either reduction or nonreductive physicalism. Since the first one leaves the mental (as mental) powerless, and therefore puts free will in peril, it seems that nonreductive physicalism is the better option, where the mental is linked to the physical through some form of dependence relationship, i.e. supervenience or realization, but cannot be reduced to it. Mental is defined not ontologically, but functionally, through the role it plays in the causal chain leading to the action-outcome. Moreover, nonreductive physicalism allows for multiple realization of the mental, one of the notorious problems of the reductive approaches, according to which the same mental event can be realized by a variety of physical realizations. It also allows for seamless accommodation of the scientific results to the philosophical debate, without entailing that free will is not possible. However, its ultimate success depends on finding a satisfactory solution to the Causal Exclusion Challenge, and without it positions such as the one held by Mele, are not sufficient to explain the possibility of free will.
3 The Causal Exclusion Challenge
While nonreductive physicalism, as we have seen before, is the most promising option for us, the consistency of its premises was challenged by the causal exclusion argument, most famously described by Jaegwon Kim. The four premises are (1) principle of mental causation, (2) principle of irreducibility,
- (1)Some physical effects have mental causes (the principle of mental causation)
- (2)Mental causes are distinct from physical causes (the principle of irreducibility)
- (3)If a physical event has a cause at t, it has a sufficient physical cause at t (the principle of causal completeness of the physical or the causal closure of the physical domain, Kim, 2005)
- (4)Events cannot have more than one sufficient cause occurring at a given time (the exclusion principle)
The first premise states that at least some mental events can have causal impact on the physical realm. The premise is very general and it allows for the mental to be either connected or disconnected from the physical realm, thereby being compatible with both nonreductive physicalism as well as dualism.
The second premise rejects the reduction of the mental to the physical realm. Also here the premise is very broad, and as such, is compatible with both dualism and nonreductive physicalism as the mental could also be independent from the physical.
The third premise necessitates that, if an event has a cause at all, it has a physical cause. The premise, by itself, does not limit the causes to one; therefore it allows for multiple causes.
The fourth premise of nonreductive physicalism limits the number of sufficient causes to only one, except for the cases of genuine overdetermination. This principle does not refer specifically to the realm of the mental, but rather states that each event has only one sufficient cause, while at the same time not claiming that such event has to be either mental or physical. However, when combined with the premise of the causal closure it entails that there can be only one physical cause.
Causal Exclusion has been one of the major philosophical puzzles, and we can identify two general groups of approaches: (1) to reject one of the premises of the causal exclusion, or (2) to redefine the notion of causation we are operating with. Often the two are combined, and one of the premises is rejected because the modified notion of causation allows for it (as in the case of interventionism).
List and Menzies (2017), who modified the first premise, to state that an agent’s action is free only if it is caused by the agent’s mental states, while other premises of the argument remained mostly unchanged, presented an approach particularly relevant to the free will problem. The authors explore which one of the premises could in fact be rejected, and conclude that the only
The approach of List and Menzies is similar to arguments by causal compatibilists, who also reject the no-overdetermination argument, and claim that in the case of mental causation we are dealing with two sufficient causes. There are two strains of causal compatibilism, where:
- (1)there are two, in some sense, independent causes, the mental and the physical, and the mental does not overdetermine the physical, but is the cause of the outcome, and
- (2)the mental and the physical, while connected, do cause something together but their causal contributions differ, and, in some cases or under some understanding of causation, the dynamics between the mental and the outcome is more telling than the one between the physical and the outcome.
List and Menzies’ account specifically resembles more the independent overdetermination solution. Then both the occurrence of the physical, as well as the occurrence of mental (in a world with free roaming souls), would bring about the same effect. Moore (2012) argues that the independent overdetermination solution will not work, and one of the reasons for it is the same as the one used in justifying the existence of the principle in the first place, namely the amount of overdetermination that would have to exist as a result. Mental causation is ubiquitous, and if mental overdetermines the physical, it would have to do so thousands of times a day, which seems to be against the odds. Secondly, accepting independent overdetermination, might lead to undermining the causal completeness of the physical. If the physical cause is sufficient, it seems no other cause is really necessary, however it seems permissible that, based just on this premise, two or more sufficient causes exist (depending on how we define sufficient). Yet, if the mental cause is sufficient, we could also conclude that the physical cause is not sufficient, which would violate the premise of causal closure directly. This, in turn, would result in the agent not being free once again.
Therefore we find ourselves in a predicament, we need causal efficacy of the mental for free will to be a possibility, but to get there we need to solve the
4 The Neuronal Basis of Mental Causation
Most of the experiments in neuroscience investigate the causes of our actions by focusing on the final stages of motor planning and the action initiation processes. There, deliberation processes and distal intentions are not taken into account, presenting a picture that could be considered limited. Additionally, they focus on overt actions, and some argue that only overt actions can be voluntary (e.g. Haggard, 2011). As such, the discussions often do not aim to explain the specific interactions between mind and brain, nor do they go beyond the immediate causes.
A different way to think about the free will issue was proposed by Tse (2013). He expands the actions under consideration to include internal ones, and sees free will as a part of the whole decision process, starting from the moment of acquisition of a new piece of information. In other words free will, rather than in the last moment of action execution, would be intertwined in the larger decision making process, where a variety of steps can play a significant role in the outcome, thereby making the last moment of decision less important. What becomes much more relevant, however, is the causal efficacy of the mental, which is dealt with by Tse through his criterial causation.
Criterial causation relies on a set of criteria that control the neuronal firings, conditions that have to be met in order to activate the neuron. If a new information fulfills those criteria, the neuron will react, which will lead to a cascade of neuronal activation that can ultimately lead to an action. If it does not, the neuron remains inactive, and nothing happens. The multiple realization, an important feature of nonreductive approaches (which it seems Tse favors, since free will is based on the mental being causal, as mental), is accounted for since the […] set of conditions on input […] can be met in multiple ways and to differing degrees. (Tse, 2013, p. 22). At its core Tse’s model of criterial causation is model of how an acquisition of a piece of new information can have a physical impact, and change the behavior of the whole system. This systemic change comes from the change of firing criteria of the neurons, based on a three stage model, where:
- (1)new physical/informational criteria are set in a neuronal circuit on the basis of preceding physical/mental processing at time t1, in part via a
mechanism of rapid synaptic resetting that effectively changes the inputs to postsynaptic neuron, - (2)at time t2, inherently variable inputs arrive at the postsynaptic neuron, and
- (3)at time t3 physical/informational criteria are met or not met, leading to postsynaptic neural firing or not. (Tse, 2013, p. 133).
Two things seem to be at play here- the information that changes the criteria according to which the neuron fires, and its physical realization, the vehicle of information. The physical realization of informational criteria happens on the neuronal level, where the information from the input triggers the neuron to fire if such criteria are met. Neurons transform, and communicate the information by changing individual action potential spikes into spikes that are being transmitted to other neurons, and place criteria for firing that can be met only by a specific subset of information, so only that subset can trigger the neuron to fire. Mental events can cause future mental and physical events by changing the criteria for firing used by neurons for the future inputs. The criteria that have to be met are somehow encoded in the incoming information, and as new piece of information has the power to change the current neuron-firing criteria. If those change, upon the subsequent arrival of a similar information piece, the neurons will react based on the new criteria. Tse illustrates this using the example of Orion, where prior to having any knowledge about the pattern that the stars of Orion create, no neurons will react to said pattern. However, once the criteria (i.e. the input looking like the constellation of Orion) is set, the next time we will see that pattern, a specific neuronal connection, or singular neuron will fire. Tse argues that the patterns of information (such as Orion) are multiply realizable, and the same content, while being realized in stars, beans or grains of sand, could trigger the neuronal reaction. At the same time, it is clear that both the physical realizer and the abstract informational content are taken to be causal on his account, thereby potentially necessitating the rejection of the no- overdetermination principle, which does not allow two sufficient causes (although it is not clear if, on Tse’s account both would be taken to be sufficient). As such, Tse’s proposed solution resembles views of causal compatibilists (e.g. Bennett, 2003; Moore, 2017 for an overview) and interventionists (e.g. Woodward, 2015).
However, a number of issues can be identified with Tse’s approach, most notably what he means by causation is unclear: an experimental approach could suggest interventionism, the description of neuronal firings could rather suggest a mechanistic approach. And that lack of clarity in terms of what is understood as causes is, in my opinion, detrimental to fully showing how this account has the potential to explain the causal efficacy of the mental realm.
5 Causality as Information Transfer
There are two main reasons to think that understanding causality as information transfer could be beneficial and applicable to the mind-brain debate: (1) the brain is an information processing machine, and (2) the way we talk about the mental to physical relationship seems similar to the relationship between the abstract content of information and its physical realization. According to Tse the transfer of information produces the change in physical world, at the same time not having to follow any of the energy conservation laws. In the pattern of Orion, it is not only the physical pattern of the stars, but the pattern combined with the abstract meaning that generates the change. Without the abstract content identified with Orion’s pattern, while we have seen the same pattern repeatedly, we can hypothesize it generated no change in the neuronal connections. If we see the mental and the abstract content of information as similar (which seems plausible), we could argue that through the transfer of information, the mental can make an effect on the physical world, through the abstract information level. Therefore, if there are parallels between the mental and the abstract content of information, and the physical and the ealization of that content, understanding causality as information transfer could provide us with the most promising platform to discuss causal efficacy of the mental. However, such approach has two challenges: (1) if the mental is similar to the meaning of a piece of information, how can that be shown, and (2) how would
Most of the accounts of information transfer come from mathematics (e.g. Shannon, 1948), but since they specifically get rid of the meaning of the piece of information as irrelevant, they would not serve our purpose. The most notable attempt in philosophy has been developed by John Collier’s (1999), and while his approach is specifically described as physical causation, and as such might not suffice to describe the relationship between mind and brain, an extension of his theory to include the semantic part of the information could be added (which Collier himself seems to allow). Such extension could be offered by Floridi’s (e.g. 2004, 2011) theory of semantic information, seen as orthogonal to the classic approaches to information. Recently other accounts (e.g. Illari, Russo, 2014) have emerged claiming that everything can be described informationally, therefore the position could be both highly applicable, and can be seen to avoid certain problems identified in connection to the standard production accounts (e.g. absences as I have come late to work because the bus did not arrive are problematic). While not yet accomplished, I want to argue that also in the case of mental to physical causation, causality understood as information transfer, could have potential to avoid certain issues.
Firstly, what is causality as information transfer. Collier himself defines it as […] the transfer of a particular […] quantity of information from one state of a system to another. Physical causation is a special case in which physical information instances are transferred from one state of physical system to another. (Collier, 1999, p. 215).
In his approach, Collier draws from Shannon’s theory and treats information quantitatively rather than qualitatively (which in the case of mental causation would be a major problem). Causality is understood here as a transfer of some piece of information in its physical form, which could be seen as compatible with the type of processing Tse has in mind when he describes the neuronal firings, which ultimately lead to a behavior. Therefore, a particular piece of information would be transmitted between neurons, leading to a change in the firing criteria. However, the picture is not complete without the abstract part of the piece of information, its meaning.
While the initial work of Collier bears strong resemblance to Shannon’s approach, in his later work Collier (2012) describes an information channel as a family of isomorphisms (also discussed by Dretske, 1999, and Floridi, 2010). Illari and Russo (2016) describe the covariance of isomorphisms as:
[…] two systems a and b are couples in such a way that a’s being (or type, or in state) F is correlated to b’s being (of type, or in state) G, then such correlation carries for the observer of a the information that b is G. For example, the dishwasher’s yellow light (a) flashing (F) is triggered by, and hence is informative about, the dishwasher (b) running out of salt (G) for an observed O, like Alice, informed about the correlation.
illari and russo, 2016, p. 260
Importantly, the ismorphisms definition described by Illari and Russo involves the agent, who is informed by the already existing correlation between the flashing light of the dishwasher and low salt levels. This assumes that, at some point, the new information (the flashing yellow light) conveyed a new meaning (the low salt levels) to the agent, and following Tse’s account, established a new firing criteria on the neurons. The addition of an agent, interpreting the information provided by the flashing light of the dishwasher, is new both to the information transfer accounts of Shannon and Collier, however, some form of coder and interpreter on both sides of the information channel could be assumed on both accounts. In both cases, Illari and Russo’s and Collier’s, isomorphism entails the existence of two systems, each consisting of a set of objects, and each of the objects, has a set of attributes. Isomorphism happens when attributes of one set of objects (like the flashing light) tell us something about attributes of another set of objects (like the low level of salt).
Up to this point the account can be used to explain how the agent gets informed, and acts based on the information. However, this by itself is insufficient to show that the mental is causal, to leave room for free will (even if we consider the whole, extended in time process that leads to action, from learning the new piece of information, to acting the next instance it comes into our attention again), or even to satisfactorily explain the agent’s actions. Or, as neuroscientists have put it
[…] even if a full specification of ordered synaptic potentials is the exclusive causal story, then, as functionalists of all sorts have long emphasized, reciting the specification would be a poor explanation of what happened, because there’s nothing systematically special about these particular synaptic sequences that ties them to bearing reports form one occasion to the next.
ross, spurrett, 2004, p. 615
Therefore the only physical causal chain is potentially insufficient, and the action of adding the salt to the dishwasher seems to need both: (1) the mental
6 Do We Even Need Another Notion of Causation?
What constitutes a causal link and what connection is there between causality and explanation? These are two of the questions we have to face when we think about causation. Throughout the ages, causality has been defined in a variety of ways, from law-based regularities (e.g. Hume), to modern production accounts (causality as transfer, e.g. Dowe, 1992), mechanistic approaches (Craver, 2007) and interventionism (Woodward, 2003).
Currently the most broadly applied approach, also to the mental causation problem, is interventionism, where causation is understood as making a difference (the cause should make a difference to the appearance, or the probability distribution of the effect). In its current form it was presented by Woodward (2003), and has been often argued to be the effective solution to the Causal Exclusion (e.g. Shapiro and Sober 2007; Woodward 2015). However, as pointed by Baumgartner (e.g. 2009, 2012, 2013) the mental to physical relation (supervenience) does not allow for independent manipulation of competing causes and, at the same time, holding all other variables in the set fixed. If the two are necessary for establishing a causal link, as interventionists suggest, such link cannot be identified. There has been a significant amount of back and forth regarding the problem, with Woodward (2015) arguing that the non-causal dependencies that hold between two variables should exclude those from the requirement of independent manipulation. This could be interpreted to entail that establishing that something is a cause, in case of two variables connected by non-causal dependencies, both of which could potentially be a cause, relies on the choice of which variable to manipulate, as this would automatically exclude the other variable from consideration. If we want to determine if our reaching for the fridge was caused by our intention to get a cold beverage or by
Another potential approach, applicable especially in neuroscience, is the mechanistic one, most fully described by Craver in his 2007 book. The approach extends beyond biology and neuroscience, including the classic examples of machinery. Yet, as Craver argues, equating mechanistic causality with a notion of a machine is too restrictive to account for the advanced neuroscientific processes. However, in spite of the account’s applicability in neuroscience, it is sensitive to the causal exclusion argument, mainly because of its multilevel explanations, which can refer to
[…] the behaviors of organisms, the processing functions of brain systems, the representational and computational properties of brain regions, the electrophysiological properties of nerve cells, and the structures and conformation changes of molecules.
craver, 2007, p. 9
Moreover, as was already pointed out in the quote from Ross and Spurret, and since the mental is multiply realizable, there is nothing special about the specific neuronal firings. Moreover, because of the causal exclusion, it is difficult to show how the mental could be causal via mental, which is difficult on the mechanistic approach.
With the variety of different types of causalities, some (e.g. Psillos, 2009) have argued that what is described there, are not different causalities but rather different symptoms of the same thing, like in the case of a common cold.
Given the problems encountered by the various theories of causality, especially when applied to the mental causation domain, and the potential need for a variety of evidence to even establish the causal chain, one might wonder if another notion of causality is what we need. So far in the article I have argued that yes, a new account is necessary, and seeing causality as information transfer can allow for a more natural way of thinking about causes and effects, especially when applied to mind- brain relationships. It seems that the notion of information transfer could very well fit both the neuroscientific jargon as well as the common intuitions about what happens in our brain. However to fully provide us with a picture of how this works, we would need another “level” of description- the meaning of information, and its causal powers in the causal chain. The account, while providing those two frameworks, does not require that two different types of causality are at work in generating one outcome, which would be unlikely. As such it is also compatible with the rejection of the no overdetermination principle, as, depending on how we define a cause, we are dealing with two, strictly linked, connected through supervenience, collaborating causes. But this might not be completely sufficient, since, as we have seen before, causal compatibilism (and such an account would potentially be an instance of it) falls short from providing a complete account of mental causation.
7 How to Move Forward
The discussions on mental causation, also those related to free will, seem to largely focus on two groups of issues: (1) what is causality, what does it mean to be a cause, and (2) how does the mental relate to the physical, and how it can be seen as a cause via being mental. However, it seems that the one thing missing is the question whether we are thinking about the causes themselves correctly. Also, here the information transfer account can help to clarify certain things. When we talk about a new piece of information being a cause, we never mean just the physical realization of the abstract meaning, nor do we ever talk about just the meaning. We tend to see information as one cause that consists of those two components: the physical vehicle and the meaning. Applying the same way of thinking to mental causation would leave us with one
The article presented here had two main aims: (1) to show that the causally efficacious mental is necessary for the possibility of free will, and (2) that none of the current approaches to causality provides a satisfactory solution to the problem, when discussed in connection to free will. I have argued that understanding causality as information transfer can avoid the shortcomings of the currently popular approaches such as interventionism. However, to show that mental can be causal qua mental, rather than just because of the physical properties, more is needed, firstly, an information causation account that includes the semantic level of causation, secondly, possibly the rethinking of the notion of a cause. I have argued that the account of causation as information transfer is a more natural way to talk about mental causation than the current positions, but the real work begins now, in finding a seamless way to bring these theories together.
References
Bishop, R.C. (2014). Review of the Neural Basis of Free Will, available at: http://ndpr.nd.edu/news/neural-basis-of-free-will-criterial-causation/.
Kornhuber, H.H., Deecke, L. (1965) Hirnpotentialanderungen bei Willkiirbewegungen und passivenBewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale. Pfliigers Archivfur Gesamte Physiologie, 284, 1–17. (in English: Changes in the brain potential in voluntary movements and passive movements in man: readiness potential and reafferent potential).