ON BENJAMIN LIBET: IS THE
MIND AHEAD
OF THE BRAIN? BEHIND IT?
by Ted Honderich -- Determinism and Freedom Website --
-------------------------------------------------------- 1.
Some Experimental Findings About Experiences Benjamin Libet and also
Libet and collaborators claim to advance a single hypothesis, with
important consequences, about the time of a conscious experience in
relation to the time when there occurs a certain physical condition in
the brain. This condition is spoken of as neural adequacy for
the experience, or, as we can as well say, neural adequacy.5
This finding has been taken to throw doubt on theories that take neural
and mental events to be in necessary or lawlike connection, and also
certain identity theories of mind and brain, as well as determinist
theories generally. The hypothesis derives in part from many previous
findings.6 Karl Popper and J. C. Eccles use it as
evidence for an ambitious view that asserts a kind of surveillance and
control of the brain by 'the self-conscious mind', despite some acting
of the brain on the self-conscious mind.7 What is maintained by the
experimenters derives from two sets of experimental findings pertaining
to certain neural activity and to the time-order of a subject’s pairs
of sensory experiences, and also from a finding having to do with some
electrical activity in the part of the brain that is the somatosensory
cortex. These findings are the set 1 to 3 below, and then the set 4 to
7, and then 8. Very roughly, findings 1
to 3 are to the effect that there is a certain delay after an
earlier event in the occurrence of something associated with a sensory
experience. But finding 4 apparently conflicts with this. Others of the
findings are used to give an explanation of the apparent conflict. (1) Experiments on
neurosurgery patients, with their agreement, and in conjunction with
surgical operations, are said to have established a fact. It is that
after the beginning of a small electrical stimulation applied directly
by inserted electrodes to part of the brain, the postcentral cortex,
there is a considerable delay, up to about half a second (500
milliseconds -- 500 msec) before something happens. That is electrical
activity in the cortex reaching 'neural adequacy' for eliciting a
certain conscious sensory experience on the part of the patient. What
is said to be delayed, to repeat, is precisely the condition of 'neural
adequacy', about which we are not told much, as distinct from the
conscious experience itself, whatever is to be said of its time
of occurrence. (2) There is said to be a
very similar delay in conscious experience, again up to about half a
second, with respect to direct stimulation of another part of the brain
-- direct stimulation trains to the subcortex. (3) There is also said to
be the same delay of about 500 msec in consciousness with a single
pulse of electricity to the skin of the hand -- peripheral
stimulation. As against these delays,
there are other findings, the first being crucial. (4) If the skin of the
hand of a patient is stimulated later than the beginning of a certain
stimulus direct to the brain, you would expect to hear from the subject
that the conscious skin-experience came after the other
conscious experience. More precisely, if a single-pulse stimulus to the
skin of the hand at just above threshold level is applied 200 or 300
msec after the beginning of a stimulus train direct to
somatosensory cortex, you would expect that subjects would
report that the conscious experience for the skin stimulus began after
conscious experience for the cortical stimulus. This would also be
expected on the basis of findings 1 and 3. Reports of tests, however,
very remarkably, were predominantly of experience for the skin stimulus
beginning before experience for the cortical stimulus. (5) There is no such
surprising order of conscious experiences reported with subcortical
stimulation. If a skin stimulus is applied later than the beginning of
a subcortical stimulus train, subjects report that the sensory
experience owed to the skin stimulus began after sensory experience
owed to the subcortical stimulation. When it should have, so to speak. (6) Similarly, if the
beginning of subcortical stimulation is simultaneous with a peripheral
stimulus to the skin of the hand, subjects report simultaneous sensory
experiences. (7) Similarly again, if a
skin stimulus is applied before the beginning of subcortical
stimulation, subjects report that experience of the skin stimulus came
before experience owed to the subcortical stimulation. (8) And finally, peripheral stimuli and subcortical
stimuli very quickly elicit some other electrical activity in the
brain. This is a relatively localized 'primary' evoked potential in
somatosensory cortex, owed importantly to a certain system in the brain
-- the specific (lemniscal) projection system. The onset of this
electrical activity, also referred to as the arrival of a fast
projection volley, is only about 15 msec after a stimulus to the hand.
Very quick. However, a stimulus train applied direct to somatosensory
cortex does not elicit a similar type of response. So the first set of
findings 1 to 3 are to the effect that neural adequacy for any sensory
experience is achieved only after a certain delay, about half a second.
But finding 4, which is indeed crucial, appears to conflict with this.
That is, the ordering by subjects of their experiences is this: conscious-experience-owed-to-later-skin-stimulus
came before
conscious-experience-owed-to-earlier-cortical-stimulus. This
suggests that the experience of the peripheral stimulus occurs
considerably sooner than about half a second later. This is
also the case with subcortical stimulation, but not with cortical
stimulation, as indicated by findings 5, 6 and 7. 2.
Statements of An Hypothesis The two sets of findings,
together with (8) the finding of some early electrical activity, are
said by Libet and collaborators to issue in what will concern us a lot
more, a single hypothesis with significant consequences. In itself, it
is about about the timing of conscious sensory experiences owed to
peripheral and subcortical stimulation. We are also given an
explanation of what is postulated in the hypothesis. The explanation of
what is postulated has to do in part with something of which you will
eventually hear the little bit more that is necessary -- the early
electrical activity in (8).
In fact some statements
made by the authors are of one hypothesis, or suggest it. Other
statements are of, or suggest, a different hypothesis. It will be
necessary, partly for fairness, to quote extensively. The following statements
S1 to S5 are somehow to this effect: a conscious experience occurs at a
certain time but is 'antedated' to the time of the early electrical
activity, the 'primary' evoked potential having to do with the specific
(lemniscal) projection system. (S1) '[The hypothesis]
postulates (a) the existence of a subjective referral of the timing for
a sensory experience, and (b) a role for the specific (lemniscal)
projection system in mediating such a subjective referral of timing.'8 (S2) '(1) Some neuronal
process associated with the early or primary evoked response, of SI
(somatosensory) cortex to a skin stimulus, is postulated to serve as a
‘time-marker’. (2) There is an automatic subjective referral of the
conscious experience backwards in time to this time-marker, after the
delayed neural adequacy at cerebral levels has been achieved (see Fig.
2).9 The sensory experience would be ‘antedated’ from the
actual delayed time at which the neuronal state became adequate to
elicit it; and the experience would appear subjectively to occur with
no significant delay from the arrival of the fast projection volley.'10 (S3) 'The results
obtained in these experiments provide specific support for our present
proposal, that is, for the existence of a subjective temporal referral
of a sensory experience by which the subjective timing is retroactively
antedated to the time of the primary cortical response (elicited by the
lemniscal input).'11 (S4) 'The specific
projection system is already regarded as the provider of localized
cerebral signals that function in fine spatial discrimination,
including the subjective referral of sensory experience in space. Our
present hypothesis expands the role for this system to include a
function in the temporal dimension. The same cortical responses to
specific fast projection inputs would also provide timing signals. They
would subserve subjective referral in such a way as to help ‘correct’
the subjective timing (relative to the sensory stimulus), in spite of
actual substantial delays in the time to achieve neural adequacy for
the 'production’ of the conscious sensory experience.'12 (S5) '…for a peripheral
sensory input, (a) the primary evoked response of sensory cortex to the
specific projection (lemniscal) input is associated with a process that
can serve as a ‘time-marker’; and (b) after delayed neural adequacy is
achieved, there is a subjective referral of the sensory experience
backwards in time so as to coincide with this initial ‘time-marker’.'13 These statements are
certainly not all clear -- I do not refer to their technicalities --
but their burden is that such a conscious experience as that of a
stimulus to the skin of the hand occurs only when neural adequacy is
achieved, but the experience is somehow 'antedated' or 'referred' to an
earlier time. That is, despite what is said of the experience’s
involving 'subjective referral backwards in time', the experience
itself occurs only about half a second after the beginning of
stimulation. The experience does not occur at the time to which it is
'referred'. This is also what is
conveyed by the experimenters, as will be worth noting, in one of their
diagrams and several other passages. Their
figure, reproduced
here as Figure 1, has to do in the main with
the early electrical
activity, the 'primary' evoked potential.14 (S6) It is stated of the
figure in part: 'Diagram representing the ‘average evoked response’
(AER) recordable on the surface of human primary somatosensory cortex
(SI) in relation to the...hypothesis on timing of the sensory
experience. Below the AER, the first line shows the approximate delay
in achieving the state of ‘neuronal adequacy’ that appears (on the
basis of other evidence) to be necessary for eliciting the sensory
experience. The second line shows the postulated retroactive referral
of the subjective timing of the experience, from the time of 'neuronal
adequacy’ backwards to some time associated with the primary
surface-positive component of the evoked potential.'15 Presumably 'neuronal
adequacy' is not taken as necessary for eliciting what has already
happened, earlier in time. The experience, at the later time, is merely
'referred' to the earlier. (S7) It
is stated that
the hypothesis in question 'deals with the problem of a substantial
neuronal time delay apparently required for the ‘encoding’ of a
conscious sensory experience, by introducing the concept of a
subjective referral of sensory experience in the temporal dimension.'16 Finally, there is what is
said of a quite different idea, owed to Donald MacKay. MacKay’s idea 'accepts
our proposal
that there is substantial delay in achieving neural adequacy with all
inputs, peripheral or central; but it would argue that, in those cases
where there is apparent antedating of the subjective timings of the
sensory experience, the subjective referral backwards in time may be
due to an illusory judgement made by the subject when he reports the
timings… For example, it could be argued that during the recall
process, cerebral mechanisms might ‘read back’ via some memory device
to the primary evoked response and now construe the timing of the
experience to have occurred earlier than it in fact did occur.'17 An
amendment of this idea from MacKay is contemplated, one that is said in
fact to turn it into the hypothesis of Libet et al: (S8) '…if any ‘read back’
to the primary timing signal does occur, it would seem simpler to
assume that this takes place at the time when neuronal adequacy for the
experience is first achieved, when the ‘memory’ of the timing signal
would be fresher; such a process would then produce the retroactive
subjective referral we have postulated.'18 So -- the burden of all
that has been reported here so far, in statements S1 to S8, is the delay-and-antedating
hypothesis. Such a
conscious
experience as that of a skin stimulus occurs only at the time when
'neural adequacy' has been achieved, about half a second after the
beginning of stimulation, despite the fact of 'antedating'. By
way of a headline, the mind is not ahead of the brain, despite the
mind's antedating capability.
On the other hand the
following statements S9 to S15 are different. Some have to do with the
authors’ figure reproduced here as Figure 2,
pertaining to the crucial
finding 4, about the subjects’ surprising ordering of experiences. It
could hardly be clearer in its quite different import.19 (S9) In the figure the
S-experience (experience of a skin stimulus) is specified as 'actually
before C-experience' (experience owed to cortical stimulation). It is
shown as occurring only a few msec after the stimulus-pulse (S-pulse). (S10) In the note to the
diagram it is stated: 'If S were followed by a... delay of 500 msec of
cortical activity before neural adequacy’ is achieved, initiation of
S-experience might...have been expected to be delayed until 700 msec of
C [stimulus train to somatosensory cortex] had elapsed. In fact,
S-experience was reported to appear subjectively before
C-experience....' The note, although perhaps less definite in its
intention, thus accords with the diagram. (S10) What the crucial
finding 4 established, it is said and implied, is that conscious
experience of certain stimulation did not occur at the time of neural
adequacy. 'If the subjective experience were to occur at the
same time as the achievement of neural adequacy in the case of either
stimulus, one would expect the subject to report that the conscious
sensory experience for the C stimulus began before that for the
threshold S pulse (Fig. 120)…. However, the pooled reports
were predominantly those of sensory experience for the C (cortical)
stimulus beginning after, not before, that for a delayed
threshold S pulse...'21
(S11) It is stated, of
the crucial finding 4, that 'the subjective experience of the skin
stimulus occurs relatively quickly after the delivery of the S pulse,
rather than after the expected delay of up to about 500 msec for
development of neural adequacy following the S input.'22
That is, the skin-stimulus experience itself occurs earlier, rather
than after the expected delay. (12) It is flatly stated
that 'subjective experience of a peripherally-induced sensation is
found to appear without the substantial delay found for the experience
of a cortically induced sensation.'23 (S13) Very importantly,
it is noted that findings 1, 2 and 3 above, about delay in achieving
neural adequacy, are not to be taken in a natural way, as
asserting or implying that the experiences in question are subject to
the given delay. That is left open. 'The two timings, for subjective
experience vs. neural adequacy, might not necessarily be identical.'24 (S14) It is stated that
the hypothesis in question introduces 'an asynchrony or discrepancy
between the timing of a subjective experience and the time when the
state of ‘neuronal adequacy’ associated with the experience is
achieved.'25 (S15) It is stated that
there is 'a dissociation between the timings of the corresponding
‘mental’ and ‘physical’ events.'26 The burden of all these
statements (S9 to S15), although some phrases might be taken as
ambiguous, is not the delay-and-antedating hypothesis but what we can
call the no-delay hypothesis. A
conscious experience
occurs earlier rather than later, i.e. before about half a second after
the beginning of stimulation rather than about half a second after the
beginning of stimulation. By
way of a headline, and one is certainly needed for the news, the
mind is ahead of the brain. As remarked, Popper and
Eccles use the hypothesis of Libet and collaborators, whatever it is,
to argue for the existence of 'the self-conscious mind'. Eccles states
the hypothesis a number of times. Compare (a) and (b) with (c) and (d). (a) 'The experiments of
Libet on the human brain...show that direct stimulation of the
somaesthetic cortex results in a conscious sensory experience after a
delay as long as 0.5 sec ... although there is this delay in
experiencing the peripheral stimulus, it is actually judged to be much
earlier, at about the time of cortical arrival of the afferent
input.... This antedating process does not seem to be explicable by any
neurophysiological process. Presumably it is a strategy that has been
learnt by the self-conscious mind.., the antedating of the sensory
experience is attributable to the ability of the self-conscious mind to
make slight temporal adjustments, i.e. to play tricks with time...'27 (b) '...Libet developed a
most interesting hypothesis, namely that, though a weak single (SS)
just threshold single skin stimulus requires up to 0.5 sec of cortical
activity before it can be experienced, in the experiencing process it
is antedated by being referred in time to the initial evoked response
of the cortex....'28 (c) 'The...experimental
design tested the supposition that a just-threshold single skin
stimulus (SS) was effective in producing a conscious sensation after
the same incubation period. . . as a just-threshold train of cortical
stimulation (CS), which is as long as 0.5 sec. If that were so, when
the SS was applied during the minimal CS train, the SS should
be experienced after the CS; but it was usually experienced before!'
29 (d) 'There can be a
temporal discrepancy between neural events and the experiences of the
self-conscious mind.'30 Passages a and b are to
the effect that the experience is later (i.e. about half a second after
the beginning of stimulation) and is somehow antedated. Passages c and
d say otherwise. The experience is earlier -- i.e. not nearly so
much as about half a second after the beginning of
stimulation. There is the same inconsistency suggested by two
parts of a diagram31 reproduced here as Figure
3. Part D has to do with
tests whose details are not all relevant to the present point. What is
relevant is that a just-threshold single skin stimulus (SS2)
is shown as giving rise to an experience whose 'actual time' is about
600 msec later. There is 'antedating', somehow involving 'ER [evoked
response] time', which is not further explained. Consider part B of the
diagram, however, which pertains to the crucial finding (4) above. CS
is cortical stimulation. The SS experience is shown in the diagram as actually
occurring earlier rather than later. The diagram specifies 'SS
experience before CS [experience]'. 3.
Inconsistency, Delay-and-Antedating We need to come to a
first conclusion. There is an inconsistency involving two
hypotheses in the work of Libet et al. and Popper and Eccles --
and, incidentally, in other reports of the work of Libet et al.32
The inconsistency is worth noting for itself. Not both the hypothesis
that there is delay in experience but antedating, the
delay-and-antedating hypothesis, and the hypothesis that there
is no such delay, the no-delay hypothesis, can be true. That conclusion carries
corollaries, of course. One is that there must be doubts about the
putative findings. Certainly if the delay-and-antedating hypothesis
is true, then if there are putative findings that entail the no-delay
hypothesis, those findings are false. The like corollary has to do with
taking the no-delay hypothesis to be true. Any putative findings that
entail the delay-and-antedating hypothesis are then false. It is not
within my competence, however, to examine these findings in detail,
notably the crucial finding 4. The inconsistency itself
and hence the fact that there are two hypotheses in question are
important, partly for further conclusions of mine. But it can
confidently be said that it is the delay-and-antedating hypothesis to
which the authors, at bottom, are committed. It is certainly the
hypothesis to be preferred. In my view, the authors’ failure to notice,
despite their commitment, that there are two inconsistent hypotheses in
question, has had a certain effect on their understanding of the
consequences of the delay-and-antedating hypothesis, consequences for
the mind-brain issue. It is my suggestion that failing to notice the
inconsistency, and hence that there are two hypotheses on hand, has
been significant in enabling Libet et al. and Popper and
Eccles,33 to come to certain conclusions about lawlike
connection between simultaneous mental and neural events, about
identity theories, and about the self-conscious mind and free
will. To consider another
matter first, however, how are we to understand the preferable
hypothesis, the one that does not put the mind ahead of the brain? It
is in part that an experience is somehow 'antedated' or 'retroactively
antedated' or 'referred back' to a 'time-marker'. What does that come
to? Very little account of the supposed phenomenon itself is given, as
distinct from what is supposed to explain it. The central idea appears
to be that a subject has a conscious sensory experience including or
involving or accompanied by the belief or impression that it is not then
happening. He may have a conscious sensory experience,
reportable as 'sensation in skin of hand', which conscious sensory
experience occurs about half a second after stimulation, and after
another sensory experience. His experience of the skin sensation
includes or involves or is accompanied by the belief or impression that
the sensation occurred significantly earlier than about half a second
after stimulation, before the other sensory experience. There seem to
be very great difficulties in this idea -- which, it must be
remembered, is crucially different from Mackay’s simpler idea noticed
earlier. It is true, surely, that
the actual having of any conscious sensory experience includes
or involves or is accompanied by the belief or impression that the
experience is present. That is, it is now. It is happening.
Further, it might be said that there is the belief or impression that
the experience is after another experience, immediately prior
in time. The having of any experience, that is, somehow brings in a
belief or impression as to a temporal property (presentness) and
perhaps also a temporal relation (after another experience).34 But then the supposed
phenomenon of a conscious sensory experience we have been considering,
the phenomenon of 'antedating' or 'referring back’, involves imputing
something very like certain self-contradictory beliefs to subjects. It
involves, more precisely, imputing something like simultaneously-held,
fully explicit self-contradictory beliefs. This is quite distinct from
the common sort of self-contradiction where the conflicting beliefs are
not brought together. The supposed phenomenon, by way of a kind of
summary description, is the phenomenon of a conscious experience which
a subject might describe in the words 'present-sensation past' or
'now-sensation then', or perhaps 'later-sensation earlier'. Libet et al. say
that the processes in referral or antedating are to be regarded as
'unconscious and "automatic" in nature and…not distinguishable by the
subject'.35 What processes are there in question is not
entirely clear. However, we cannot choose to regard a conscious
sensory experience as something of which the subject is unaware.
It seems, to repeat, that a conception of presentness and perhaps of a
temporal relation enters into the having of any sensation. Are there really
certain experiences such that the involved belief or impression as to
presentness is, so to speak, simultaneously denied? On the assumption
that a belief or impression of a temporal relation enters into the
having of any sensation, can it be that there are certain experiences
such that the belief or impression is simultaneously denied? As illustrated above in
statement S4, it is said that the supposed antedating phenomenon is
related to something else, another fact also owed to the specific
(lemniscal) projection system. '…the concept of subjective referral in
the spatial dimension, and the discrepancy between subjective and
neuronal spatial configurations, has long been recognized and accepted;
that is, the spatial form of a subject sensory experience need not be
identical with the spatial pattern of the activated cerebral neuronal
system that gives rise to this experience.'36 Also, 'The
newly proposed functional role for the specific projection system would
be additional to its known role in spatial referral and discrimination.'37 In fact, however, there
is no relevant analogy whatever between 'temporal referral', of the
kind with which we are concerned, and the given obvious 'discrepancy'
known since the beginning of neuroscience between (i) spatial
experience or things as experienced in space and (ii) neural spatial
configurations. The neural bit for the tree on the left is not itself
to the left of the neural bit for the tree on the right. The latter
'discrepancy' obviously involves no kind of self-contradictory
experience, the simultaneous occurrence of contradictory beliefs or
impressions. Libet remarks that
hypotheses in general are the weaker for involving ad hoc assumptions.38
It is also said that hypotheses in general are the weaker for involving
'added assumptions'.39 It is my own second and tentative
conclusion here that the delay-and-antedating hypothesis is open to
objection along these lines. The delay-and-antedating hypothesis,
being factitious and also obscure, is at least open to doubt.
However, it is not within my competence to judge the findings that are
put in question, or whose interpretation is put in question, if the
delay-and-antedating hypothesis is rejected. 4.
Mind and Brain, Free Will, A Conjecture, Contradiction To turn now to mind-brain
theories, it is said that 'on the face of it, an apparent lack of
synchrony between the ‘mental’ and the ‘physical’ would appear to
provide an experimentally-based argument against ‘identity theory’, as
the latter is formulated by Feigl, Popper, etc.'40 However, it is allowed
that a certain reply to the argument is possible. The reply is not
specified. It is then said, presumably about psychoneural lawlike
connection, that 'a temporal dissociation between the mental and the
physical events would further strain the concept of psychophysiological
parallelism or, if one prefers, of co-occurrence of corresponding
mental and neural states. It could thus have an impact on the
philosophical interpretation of such parallelism or co-occurrence when
formulating alternative theories of the mind-brain relationship.' In a later article it is
said, differently, that 'dissociation between timings of the
corresponding "mental" and "physical" events' raises serious but not
insurmountable difficulties for identity theories, but that the
dissociation does not contradict 'the theory of psychophysical
parallelism or correspondence'.41 The seeming change of mind
about these theories is not explained. In a still later article
it is said that 'the temporal discrepancy creates relative difficulties
for identity theory, but.. . these are not insurmountable'.42
It is said, further, that the data are 'compatible with the theory of
‘mental’ and ‘physical’ correspondence',43 and that the data
do 'introduce a novel experimentally-based feature into our views of
psychophysiological correspondence, with some interesting philosophical
implications that merit analysis'.44 Further, it is said,
without explanation: 'What we are discussing is not any denial of
correspondence between mental and physical events, but rather the way
in which the correspondence is actually manifested'.45 I am uncertain what to
make of what is said, so vaguely, of psychophysical 'correspondence'.
Can such 'correspondence' hold between events at different times? Does
such 'correspondence' require only such a loose connection between the
brain and the 'self-conscious mind' as posited by Popper and
Eccles? It is clear how the
findings as to the timing of conscious experiences may be taken to
threaten the hypothesis of psychoneural lawlike connection, of which
you heard earlier, or certain identity theories, or parallelist
theories. Evidently a conscious experience cannot be identical in a
real sense with a neural event if the mental and neural items have
different temporal locations or durations. Consider the claim that a
conscious experience was identical with the physical process that
constituted 'neural adequacy' for it. The claim must be false if the
experience and the processes occurred at different times. Again, the hypothesis of
psychoneural lawlike correlation naturally takes the correlated mental
and physical items to be simultaneous. That is part of the theory.
Evidently, if the studies of Libet et al. did establish of
certain mental and neural items that they were not simultaneous, this
would indeed raise a difficulty for the given theory of psychoneural
lawlike correlation. The same is true, evidently, of a parallelist
theory denying psychophysical lawlike correlation but involving
psychophysical simultaneity, the parallelism taken as simply
inexplicable or somehow owed to ongoing divine intervention. It is a
theory, perhaps, which has no contemporary defenders. Finally, if there were
mental events separate in time from their neural bases, that might be
taken as going some way towards supporting the theory of the
self-conscious mind. Given what seems to me the obscurity of that
theory, I shall not attempt to say more about why that might be true. A
remark of Popper and Eccles in a passage quoted above (p. 000) carries
the idea that something plays tricks with time, which thing is the
self-conscious mind. There, admittedly, they are to be taken as
speaking of the 'trick' of 'antedating'. If there were mental
events separate from their neural bases, by the way, that would not
only affect precisely the hypothesis of psychoneural lawlike
correlation in a theory of determinism and also the theory of the
self-conscious mind. It would affect thinking about freedom more
generally. Partly because of later work of his on determinism and
freedom, it seems to me likely that Libet is of this attitude in the
work we are considering. What this thought amounts to, put it one way,
is that if the mind is ahead of the brain, there is a strangeness or
mystery about the mind that does not sit well with determinism, or
determinism as we naturally conceive it. There might conceivably be a
determnism of or covering the mind in its very surprising independence,
but it would be odd indeed. It would be a determinism separated from a
determinism of the brain. It is clear, to sum up,
that the findings as to the timings of conscious experiences may be
taken to have these various consequences if the
findings are taken as issuing in the no-delay hypothesis. That is one
fact, about which I shall say a word more in a moment. Another fact, to
repeat, is that clearly it is the delay-and-antedating hypothesis that
is favoured by the authors, despite their inconsistency. It is my conjecture that
the authors, not having clearly distinguished the two quite different
hypotheses about timing, have supposed that the studies in question
have somehow established the no-delay hypothesis, to the effect that
certain mental and physical items are not simultaneous. That,
fundamentally, is why they draw their conclusions about the mind-brain
relation. However, they must choose one or the other of the two
hypotheses, and the one they favour, the delay-and-antedating
hypothesis, is not at all to the effect that certain mental and
physical items lack simultaneity. The idea that the mind is ahead of
the brain, if you could hold it, would indeed affect a good deal, but
it is not the idea you can think about, or think much about. The experience of the
skin stimulus, on the delay-and-antedating hypothesis, is simultaneous
with the neural process that is taken to constitute 'neural adequacy'
for it. The experience may indeed be of a strange self-contradictory
kind. This in itself, so long as psychoneural simultaneity is
preserved, is no problem whatever for the proposition of psychoneural
lawlike correlation or identity theories. Nor, evidently, is a problem
raised by the fact that the experience in one part somehow has
reference to an earlier time. That in itself does not make the
experience non-simultaneous with its physical process -- no more than
does the very different referring feature of an ordinary memory or
recall-experience make that experience non-simultaneous with its
'neuronally adequate' physical process, or time-distortion in an
hallucination make that experience non-simultaneous with the related
physical process. It may be supposed, I
have said, that both the hypothesis of psychoneural lawlike
correlations and identity theories would be affected by the no-delay
hypothesis, the idea to which the authors do not incline, but
which appears to have influenced their thinking. Still, as you will
indeed have anticipated, that is not all that is to be said about the
idea. It would not only put two things at different times, thereby
threatening the mentioned mind-brain theory and some others. The
no-delay idea, to remember, is as follows. Neural
adequacy for
a certain experience is achieved only about half a second after the
beginning of stimulation, but the experience occurs before then. What is this 'neural
adequacy'’? It appears to be neural condition that is a kind of
sufficient condition for the emergence of the experience. There are
certain quite general philosophical problems here,46 but it
must appear that the mind-ahead-of-brain hypothesis is in fact false
because self-contradictory: it asserts or very strongly implies both
that something cannot occur before a certain time -- before an
explanatory sufficient condition occurs -- and that it does. It is
true, as allowed above, that if the studies of Libet et al. did
actually establish of certain mental and neural items that they were
not simultaneous, this would raise a difficulty for, say, the theory of
psychophysical lawlike correlation. But it must be false that a mental
item occurs before the physical item on whose later existence it
depends and which explains it. 5.
The Later Research There have been, so to
speak, two periods, tranches or lines of research and theorizing by
Libet and his collaborators. In the first, as you have heard, to revert
to the philosophically relevant headline, what does the work is an
idea, not brought out of a confusion, that the mind is ahead of the
brain. In the second period, things are very different, and the
consequences for freedom and determinism are made more explicit. In
this case, what will be provided is an overview leaving out almost all
experimental details and the like.47 The research of the first
period had to do with the brain and with sensory experiences --
such experiences as a feeling in the skin of a hand. The experiments of
the second period, also carried out on consenting neurosurgery
patients, had to do with what are yet more directly relevant to
determinism and freedom -- what are named voluntary actions. These are
said to be actions wanted or initiated by the person and also not
prompted or cued in any way by anyone else, or anything external to the
person. The patients were asked
to perform a simple voluntary action, a flick of the wrist, whenever
they wished during a period when their brain activities were being
recorded. They might do so 40 times. What was confirmed by the
experimenters was that a certain electrical change in the brain, a
neural process named the readiness potential or RP, occurred about a
half-second before the wrist muscle was activated -- an average of
about 550 msec before. When did the conscious wish or intention or
willing to perform the action occur? Before or at the time of the brain
process? 'In the traditional view of conscious will and free will,'
according to Libet, 'one would expect conscious will to appear before,
or at the onset of the RP, and thus command the brain to perform the
intended act.'48 A good method of enabling
the subjects to report the clock-time of this event of willing was
developed, tested and used. The willing, intention or wishing -- the
subject's first awareness of it -- was in fact not before the brain
process but about 350 or 400 msec after it started. So the mind
is behind the brain. 'The initiation of the freely voluntary
act appears to begin in the brain unconsciously, well before the person
consciously knows he wants to act!' But a question is immediately
asked. 'Is there...any role for conscious will in the performance of a
voluntary act?'49 Libet's answer is that there can be, that
there is in some cases. The mentioned conscious
willing 350 or 400 msec after the brain process RP is still about 150
msec before the wrist muscle is activated. So there is time for some
more conscious willing. 'Potentially available to the conscious
function is the possibility of stopping or vetoing the final progress
of the volitional process, so that no actual muscle action ensues. Conscious
will could thus affect the outcome of the volitional process even
though the latter was initiated by unconscious cerebral processes.
Conscious-will might block or veto the process, so that no act occurs.'50
There is a bit of indirect evidence, we are told, to support this.
A question arises, of course. As Libet asks, does the
conscious veto, like an earlier conscious wish, intention or willing to
flick a wrist, itself have its origin in a preceding unconscious
process? The answer: 'I propose, instead, that the conscious veto may not
require or be the direct result of preceding unconscious processes. The
conscious veto is a control function, differently from simply
becoming aware of the wish to act. There is no logical imperative in
any mind-brain theory, even identity theory, that requires specific
neural activity to precede and determine the nature of a conscious
control function. And, there is no experimental evidence against the
possibility that the control process may appear without development by
prior unconscious processes.'51 That is
not quite all.
There is, we are told, a deeper question. Think again of the story of
the brain process RP in advance, the later conscious willing, the
conscious veto if that happens, and the flicking of the wrist. Is all
of that subject to determinism or not? Or rather, could it be that 'our
consciously willed acts are fully determined by natural laws that
govern the activities of nerve cells in the brain'?.52 In
response to the question, the indeterministic interpretation of Quantum
Theory is mentioned, but it is concluded we cannot really answer the
question. We can say indeterminism is at least as good an option,
scientifically speaking, as the determinism. And that there is almost
universal experience that we make free, independent choices, which is
'a kind of prima facie evidence that conscious mental processes can
causatively control some brain processes.'53 So there
you have it. In
the earlier period of research, what led our researchers and
commentators to their conclusions about the mind-body problem and
freedom was the proposition, existing in a kind of confusion, that the
mind is ahead of the brain. In the later period of research,
what do we have?54 Well, the mind is sometimes or usually behind
the brain -- as where the brain process RP is followed by the conscious
wish to flick the wrist and then the flicking. But sometimes, when
there is a veto, the mind really is ahead of the brain. This
theory is to be
compared with the determinism sketched earlier. In that determinist
theory, a conscious or mental event such as an active intention or
willing is in necessary or lawlike connection with a simultaneous
neural event. There are no active intentions or other mental events
that are without neural correlates. Each psychoneural pair, like any
resulting action, is the effect of a causal sequence that almost
certainly has in it other psychoneural pairs. That is, there are phases
of the sequence that are both neural and mental. Also, there can be and
are likely to be phases that are wholly neural and/or wholly bodily.
Furthermore, the sequence consists in causal circumstances, as explained earlier (p. 00). As in the case
of any causal sequence, therefore, there are many causal circumstances
for the effect -- each, whether earlier or later or with elements at
different times, necessitates the effect. As
against this, with
respect to the second period of Libet research in particular, we seem
to have vagueness, difficulties and questions. This has partly to do
with the fact that the research involves no good grip on causation and
explanation. The verbs in the passages quoted above indicate this
clearly. What is it for something to command, initiate, have a role in,
stop or block, affect, require, be the origin of have as a direct
result, control, determine, develop or causatively control something
else? And what does all that talk come to if it is taken as consistent
with indeterminism, the truth of an indeterminist
interpretation of Quantum Theory and so on? Further, it is notable that
we are left uncertain, despite the use of the term 'voluntary action',
whether the freedom in question at several points is voluntariness or
origination as we have understood them. We could also go back to the
first period of research, incidentally, and wonder about something's
being neurally adequate for something else, but let us not. Also, in
the theory we
are now considering, how are the conscious events related to
what is going on in the brain itself, generally speaking? Can it be
that what is assumed is an interactionism of mind and brain, something
that seemed to have receded into the past? The idea of a mental process going on with no neural correlate at all,
precisely a ghostly or free-floating thing, but then causing a neural
process, and also the other way on?55 Further, as you will
have noticed about the theory under consideration, there is also a
problem of whether determinism is consistently understood. It sometimes
seems to be the proposition that mental or conscious events and also
our actions are somehow effects of only neural sequences, which
proposition includes epiphenomenalism. Sometimes, on the other hand,
determinism seems to be the proposition that mental or conscious events
and actions are somehow effects of both neural and mental sequences. It is my
impression that
the theory under consideration might not have come into being if
matters of these several kinds had been considered. Still, could the
Libet theory be put into better shape by attention to these matters?
Might it be saved? Well, there are some particular questions and
difficulties. We first
hear that for
each voluntary action there is a neural event, the readiness potential,
about half a second before, not accompanied by anything conscious. This
is taken to put in doubt that the subsequent action of wrist-flicking
is significantly a matter of conscious will or free will. Implicit in
this, evidently, is the other proposition that there was no relevant
conscious event of choice, decision, inclination, impulse or the like before
the neural event -- before the readiness potential. No evidence is
provided for this proposition. So it could be, despite the RP, that the
wrist-flicking was actually originated by a conscious act of
will or something of the sort. The wrist-flicking could be voluntary
too, of course, in our own defined sense, in virtue of the earlier
choice or of course a choice, decision, inclination or the like in
favour of the wrist-flicking after the RP. What is most
important here, however, is that what has been provided so far is no
evidence whatever against any decent determinism, as is allowed. We now
hear, however, of
the possibility of the vetoing, that second piece of conscious willing.
Are we to understand, as it seems we are to understand, that such a
conscious event is not only not owed to a previous brain process -- its
own readiness potential -- but also is independent of what is
then going on neurally in the brain? That would, I take it, be an
extraordinary claim. It is surely unbelievable either that there was no
neural correlate of the vetoing, that it was free-floating, or that the
conscious vetoing by itself was a causal circumstance for no
wrist-flicking. Certainly no evidence whatever is offered for either of
these propositions -- either of these ways in which it could be said
the mind is ahead of the brain. But the
short story here
is that this speculation as to a subject's changing his mind about an
action that was contemplated or in some sense begun is a speculation
that is again no ground at all for doubt of determinism. The
speculation is given no significant support at all by the
neurophysiological findings, notably the spare 150 msec left over for a
change or mind or the like. There is no reason whaever given for
describing such an episode in terms of origination or free will. As for
the indeterminist
interpretation of Quantum Theory, and the proposition that we are all
aware of making free and independent choices, let me leave these items
for consideration in another context. It is hard to resist a word,
though, on a sample opinion you have heard about mind and brain, that
'conscious mental processes can causatively control brain processes'.
What does it mean? If it means that a conscious process unconnected
with any brain process is a causal circumstance or the like for a brain
process, is it not safe to say that the whole of the rest of
neuroscience is against the specuation? In the
determinism
sketched earlier, the mind and the brain go together, neither ahead or
behind. It is my view that the two bodies of research we have been
considering do nothing to disturb such a view. It would be remarkable
if they did.
1.
See my On Consciousness (Edinburgh University Press, 2004), pp.
5, 16, 19, 46, 150, 204, 206. 2. The propositions have
also been looked at, for example, by Daniel Dennett and Marcel
Kinsbourne, in 'Time and the
Observer',
in The
Nature of Consciousness: Philosophical Debates (MIT Press, 1997),
edited by Ned Block, Owen Flanagan and Guven Guzeldere. See also Alfred
R. Mele, 'Decisions, Intentions, Urges and Free Will: Why Libet Has Not
Shown What He Says He Has', in J. Campbell, M. O'Rourke and D. Shier,
eds., Explanation and Causation: Topics in Contemporary Philosophy
(MIT Press, forthcoming). And ??? 3.
See, for example, the contributions to The Volitional Brain:
Towards a Science of Free Will, edited by Libet, Anthony Freeman
and Keith Sutherland (Imprint Academic, 1999), which also has much
comment on Libet's later findings in it. As for the philosophy and
science of consciousness, it is certainly not the case that
consciousness should be thought about only by the first. Both are
needed, See my On Consciousness, p. 1 and thereafter. 4.
For a kind of overview, see Neurophysiology
of
Consciousness: Selected Papers and New Essays by Benjamin Libet (Birkhauser, 1993). 5.
Libet, 'Neuronal
vs. Subjective Timing for a Conscious Sensory Experience', in Cerebral
Correlates of Conscious Experience, ed. P Buser, P. and A.
Rougeul-Buser (Elsevier, 1978); Libet, 'The Experimental Evidence of
Subjective Referral Backwards in Time', Philosophy of Science,
1981; Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979.
Libet replied to the original version of the paper you are reading. See
'Subjective Antedating of Sensory Experience and Mind-Brain Theories', Journal
of Theoretical Biology, 1984. See also my 'Mind, Brain and Time:
Rejoinder to Libet', Journal of Theoretical Biology, 1986. 6. Libet,
'Cortical Activation in Conscious and Unconscious Experience', Perspectives
in Biology and Medicine, 1965; Libet, 'Brain Stimulation and
the Threshold of Conscious Experience', in Brain and Conscious
Experience, ed. J. C. Eccles, Springer, 1966; Libet,
'Electrical
Stimulation of Cortex in Human Subjects and Conscious Sensory Aspects',
Handbook of Sensory Physiology, Vol. 2, ed. A. Iggo
(Springer, 1973); Libet, W. W. Alberts, E.
W. Wright, L. D. Delattre, G. Levin and B. Feinstein, 'Production of
Threshold Levels of Conscious Sensation by Electrical Stimulation of
Human Somatosensory Cortex', Journal of Neurophysiology, 1964; Libet,
W. W. Alberts, F. W. Wright and B. Feinstein, Responses of Human
Somatosensory Cortex to Stimuli Below Threshold of Conscious
Intention', Science 1967; Libet, W. W. Alberts, E. W.
Wright, and B. Feinstein, 'Cortical and Thalamic Activation in
Conscious Sensory Experience', Neurophysiology Studied in Man,
ed. G. G. Somjen, Excerpta Medica, 1972; Libet, W. W.
Alberts, E. W. Wright, M. Lewis and B. Feinstein, 'Cortical
Representation of Evoked Potentials Relative to Conscious Sensory
Responses, and of Somatosensory Qualities -- in Man', in The
Somatosensory System, ed. H. H. Kornhuber (Georg Thieme, 1975). 7.
K. R. Popper, K. R.
& J. C. Eccles, The Self and Its Brain, (Springer,
1977). 8.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 193. 9.
This is Fig. 1 in the present paper by me. 10.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
pp. 20 1-2. Cf. Libet, 'Neuronal vs. Subjective Timing for a Conscious
Sensory Experience', in Cerebral Correlates of Conscious
Experience, ed. P Buser, P. and A. Rougeul-Buser (Elsevier, 1978)
p. 75. 11.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 217. 12.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
pp. 220-1. 13.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 222. 14.
From Figure 2 in B. Libet, E. W. Wright, B. Feinstein, and D. K. Pearl,
'Subjective Referral of the Timing for a Conscious Sensory Experience',
Brain, 1979, p. 201. 15.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 201. 16.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 221. 17.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
pp. 219-20. 18. Libet, E. W. Wright,
B. Feinstein, and D. K. Pearl, 'Subjective Referral of the Timing for a
Conscious Sensory Experience', Brain, 1979, p. 220. 19. From Figure I in B.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl,
'Subjective Referral of the Timing for a Conscious Sensory Experience',
Brain, 1979, p. 199. 20.
That is, my Figure 2. 21.
B. Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
pp. 199-200. 22.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 200. 23.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 222. 24.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 200. Cf. Libet, 'Neuronal vs. Subjective Timing for a Conscious
Sensory Experience', in Cerebral Correlates of Conscious
Experience, ed. P Buser, P. and A. Rougeul-Buser (Elsevier, 1978),
p. 75. 25.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 221. 26.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 222. 27.
Popper & Eccles, The Self and Its Brain, p. 364. 28.
Popper & Eccles, The Self and Its Brain, 1977, p. 259. 29. Popper &
Eccles, The Self and Its Brain, p. 257. 30.
Popper & Eccles, The Self and Its Brain, p. 362. 31.
From Popper and Eccles, The Self and Its Brain, Fig. E2-3, p.
258. 32.
C. W. Cotman and J. L. McGaugh, Behavioural Neuroscience
(Academic Press, 1980), pp. 806-7. 33.
Popper & Eccles, The Self and Its Brain, pp. 531, 565. 34.
Honderich, T. (1977. 'Temporal Relations and Temporal Properties: Time
and the Philosophies, ed. P. Ricoeur (UNESCO, 1977). [Add Gale?] 35.
'Subjective Referral of the Timing for a Conscious Sensory Experience',
Brain, 1979, p. 220. 36.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, , 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
Brain, 1979, p. 221. 37.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, , 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
Brain, 1979, p. 222. 38.
Libet, 'Neuronal vs. Subjective Timing for a Conscious Sensory
Experience', in Cerebral Correlates of Conscious Experience, ed.
P Buser, P. and A. Rougeul-Buser (Elsevier, 1978), p. 74). 39.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, , 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
p. 220). 40. Libet, 'Neuronal
vs. Subjective
Timing for a Conscious Sensory Experience', in Cerebral Correlates
of Conscious Experience, ed. P Buser, P. and A. Rougeul-Buser
(Elsevier, 1978), p. 80. 41.
Libet, E. W. Wright, B. Feinstein, and D. K. Pearl, , 'Subjective
Referral of the Timing for a Conscious Sensory Experience', Brain, 1979,
pp. 221-2. 42.
Libet, 'The Experimental Evidence of Subjective Referral Backwards in
Time', Philosophy of Science, 1981, p. 196. 43.
Libet, 'The Experimental Evidence of Subjective Referral Backwards in
Time', Philosophy of Science, 1981, p. 182. 44.
Libet, 'The Experimental Evidence of Subjective Referral Backwards in
Time', Philosophy of Science, 1981, p. 183 45.
'The Experimental Evidence of Subjective Referral Backwards in Time', Philosophy
of Science, 1981, p. 195. 46. A
Theory of
Determinism: The Mind, Neuroscience and Life-Hopes,
or Mind and Brain, in both cases pp. 40-49. 47.
Libet, 'Do
We Have Free Will?', Journal of Consciousness Studies, 1999,
reprinted in Libet, Anthony Freeman, and Keith Sutherland, eds., The
Volitional Brain: Towards a Neuroscience of Free Will (Imprint
Academic, 1999). What I have to say is based on this summary article,
which is also reprinted in The Oxford Handbook of Free Will
(Oxford University Press, 2002), edited by Robert Kane. The article
contains references to various research reports. 48.
'Do We Have Free Will?', p. 49. 49.
'Do We Have Free Will?', p. 51. 50.
'Do We Have Free Will?', pp. 51-2. 51.
'Do We Have Free Will?', p. 53. 52.
'Do We Have Free Will?', p. 55. 53.
'Do We Have Free Will?', p. 56. 54.
There is very effective criticism of several parts of the story by Al
Mele in 'Decisions, Intentions, Urges and Free Will: Why Libet Has Not
Shown What He Says He Has' mentioned above in footnote 2. 55.
For a discussion of interactionism, see my A Theory of Determinism:
The Mind, Neuroscience and Life-Hopes, or Brain and Mind,
in both cases pp. 151-4, 161-3. ----------------------------------- The
piece is one of seven in On
Determinism and Freedom, a collection of
Ted Honderich's papers to be published by Edinburgh University Press. ---------------------------------- HOME
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