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Time of Conscious Intention to Act in Relation to Onset of Cerebral Activity (Readiness-Potential): The Unconscious Initiation of a Freely Voluntary Act

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The recordable cerebral activity (readiness-potential, RP) that precedes a freely voluntary, fully endogenous motor act was directly compared with the reportable time (W) for appearance of the subjective experience of ‘wanting’ or intending to act. The onset of cerebral activity clearly preceded by at least several hundred milliseconds the reported time of conscious intention to act. This relationship held even for those series (with ‘type II’ RPs) in which subjects reported that all of the 40 self-initiated movements in the series appeared ‘spontaneously’ and capriciously. Data were obtained in at least 6 different experimental sessions with each of 5 subjects. In series with type II RPs, onset of the main negative shift in each RP preceded the corresponding mean W value by an average of about 350 ms, and by a minimum of about 150 ms. In series with type I RPs, in which an experience of preplanning occurred in some of the 40 self-initiated acts, onset of RP preceded W by an average of about 800 ms (or by 500 ms, taking onset of RP at 90 per cent of its area). Reports of W time depended upon the subject’s recall of the spatial ‘clock-position’ of a revolving spot at the time of his initial awareness of wanting or intending to move. Two different modes of recall produced similar values. Subjects distinguished awareness of wanting to move (W) from awareness of actually moving (M). W times were consistently and substantially negative to, in advance of, mean times reported for M and also those for S, the sensation elicited by a task-related skin stimulus delivered at irregular times that were unknown to the subject. It is concluded that cerebral initiation of a spontaneous, freely voluntary act .can begin unconsciously, that is, before there is any (at least recallable) subjective awareness that a ‘decision’ to act has already been initiated cerebrally. This introduces certain constraints on the potentiality for conscious initiation and control of voluntary acts.
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Brain (1983), 106, 623-642
TIME OF CONSCIOUS INTENTION TO ACT
IN RELATION TO ONSET OF CEREBRAL
ACTIVITY (READINESS-POTENTIAL)
THE UNCONSCIOUS INITIATION
OF A
FREELY
VOLUNTARY
ACT
by
BENJAMIN LIBET, CURTIS A. GLEASON, ELWOOD W. WRIGHT and
DENNIS
K. PEARL1
{From
the
Neurological
Institute, Department
of
Neuroscience,
Mount Zion Hospital and
Medical
Center,
the
Department
of
Physiology,
School of
Medicine,
University
of
California,
San
Francisco,
CA
94143 and the Department
of
Statistics,
University
of
California,
Berkeley,
CA)
SUMMARY
The recordable cerebral activity (readiness-potential, RP) that precedes
a
freely voluntary, fully
endogenous motor act was directly compared with the reportable time (W)
for
appearance
of
the
subjective experience of'wanting' or intending to
act.
The onset of cerebral activity clearly preceded by
at least several hundred milliseconds the reported time of conscious intention to act. This relationship
held even for those series (with 'type II' RPs) in which subjects reported that all of
the
40 self-initiated
movements in the series appeared 'spontaneously' and capriciously.
Data
were
obtained in at least
6
different experimental sessions with each of
5
subjects. In series with
type II RPs, onset of
the
main negative shift in each RP preceded the corresponding mean W value by
an average of about
350
ms,
and by a minimum of about
150
ms.
In series with type I RPs, in which an
experience of preplanning occurred in some of the 40 self-initiated acts, onset of
RP
preceded
W
by an
average of about 800 ms (or by 500 ms, taking onset of RP at 90 per cent of
its
area).
Reports of
W
time depended upon the subject's recall of
the
spatial 'clock-position' of
a
revolving
spot at the time of his initial awareness of wanting or intending to
move.
Two different modes of recall
produced similar
values.
Subjects distinguished awareness of wanting to move (W) from awareness of
actually moving (M). W times were consistently and substantially negative to,
in
advance
of,
mean
times reported
for M and
also those
for S, the
sensation elicited
by a
task-related skin stimulus
delivered at irregular times that were unknown to the subject.
It
is
concluded that cerebral initiation
of a
spontaneous, freely voluntary
act can
begin
unconsciously, that
is,
before there is any (at least recallable) subjective awareness that a 'decision'
to
act has already been initiated cerebrally. This introduces certain constraints on the potentiality
for
conscious initiation and control of voluntary acts.
1 Present address: Department
of
Statistics, Ohio State University, Columbus, Ohio.
Reprint requests
to Dr B.
Libet, Department
of
Physiology, University
of
California,
San
Francisco,
CA 94143, USA.
624 BENJAMIN LIBET AND OTHERS
INTRODUCTION
The 'readiness-potential' (RP), a scalp-recorded slow negative potential shift that
begins up to a second or more before a self-paced act (Kornhuber and Deecke,
1965;
Gilden et
al.,
1966),
can also precede self-initiated 'freely' voluntary acts which
are not only fully endogenous but even spontaneously capricious in origin (Libet et
al., 1982). The appearance of preparatory cerebral processes at such surprisingly
long times before a freely voluntary act raises the question of whether conscious
awareness of
the
voluntary urge or intention to act also appears with such similar
advance
timings.
The present study attempts
to
answer
this
question experimentally.
In the present study, the experience of the time of the
first
awareness of the urge to
move was related by the subject to his observed 'clock-position' of a spot of light
revolving in a circle; the subject subsequently recalled and reported this position of
the spot. Thus the experience of timing of the awareness was converted to a
reportable, visually related spatial image, analogous to reading and recalling the
clock-time for any experience. (The reliability and validity of this operational
criterion are further considered below.) This indicator of the time of the conscious
experience could then be related (1) to the actual time of the voluntary motor act, as
indicated by the electromyogram (EMG) recorded from the appropriate muscle,
and (2) to the time of appearance of the simultaneously recorded RP that is
generated by the brain in advance of each
act.
The voluntary motor acts under study
were those produced with minimal or no restrictions on the subject's independent
choice of when to act, and under instructions that encouraged spontaneity of each
volitional urge to act (Libet et
al.,
1982).
The present findings thus provide experimental evidence on the timing of the
conscious intention to act relative to the onset of cerebral activity preparatory to the
act, and on the roles of conscious processes in the initiation of a freely voluntary
motor act.
METHODS AND PROCEDURES
Subjects
Six right-handed college students were studied as two separate groups of three each. Group 1
comprised
3
females
(S.S.,
CM. and M.B.), but the quality of the EEGs and the minimal amplitude of
the RPs of
one
precluded using much of her
data.
Group
2
consisted of
2 males
and
1
female
(S.B.,
B.D.
and G.L.). Study of
this
group began a few months after completing the study of Group 1.
Recording
The d.c. recording and averaging of the EEG has been described (Libet et
al.,
1982). For present
purposes, analysis of
RPs
is made for those recorded at the vertex, where they were all maximal. (For
the first 4 experimental sessions with Group 1, only the contralateral precentral recording site is
available.) Linked mastoid electrodes served as the reference lead, with a ground electrode on the left
ear lobe. Controls excluded the electro-oculogram as a source of the slow potentials. In each
experimental series, 40 trials were performed and averaged by a computer of
average
transients (CAT
400B).
The
2 s
period of EEG stored by the CAT with each trial included a 1.4
s
period already on the
CEREBRAL
AND
CONSCIOUS TIMES
OF
VOLITION
625
recording tape before 'O-time'.
The
latter was signified
by the
EMG, recorded with bipolar electrodes
on
the
skin over
the
activated muscle
of
the right forearm.
Procedure
The subject
sat in a
partially reclining position
on a
lounge chair with
an
observer present
in the
room. Each trial
was
started only when
the
subject considered himself comfortably ready.
The
trial
began with
a
brief'get-ready' tone. This signalled that during the next
1
-3
s
the subject should relax his
muscles, especially those
of
the head, neck
and
forearm, blink
his
eyelids
if
he wished,
and fix his
gaze
on
the
centre of the
5
inch circular screen of a cathode
ray
oscilloscope (CRO) that was positioned
at
about
1.95 m
away
in his
direct line
of
vision.
At the end of
these irregular get-ready periods
the
operator activated
the
PDP-12 computer
to
initiate circular revolution
of
the beam
of
the CRO.
The
CRO spot
of
light revolved
in a
clockwise circle near
the
circumference
of the
screen starting from
the'
12-o'clock' position; this motion simulated
a
sweep secondhand of a clock
but
each revolution was
completed
in
2.56 rather than 60 s.
A
circular scale, with numbers
at
each
'5 s'
position, was mounted
at
the external edge
of the CRO
screen,
and a
plastic grille
on the
peripheral portion
of the
screen
displayed illuminated radial lines spaced
at
'2.5s.' intervals (each equal
to 107 ms of
actual time).
Subjects were asked
to
maintain their gaze fixed
on
the centre of the CRO screen
and not to
follow
the
CRO spot around, even though they were
to
report information relating the 'clock-position' of the spot
to
the
events (see below).
The
visual angle subtended between
the
centre
and the
peripheral position
of
the moving spot
was
small enough
(1.8 deg) to
present
no
difficulty from loss
of
visual acuity.
The
'clock-time' of the CRO spot
at
each event, namely EMG with motor act
or
stimulator synch pulse with
stimulus
to
skin,
was
recorded
by the
PDP-12 computer. Subjects were trained
to
make their
self-
initiated movement sufficiently brisk so that within
no
more than 10-20 ms from
the
start of any
EMG
potentials they achieved
the
amplitude pre-set
to
trigger
the
computer.
The subject was asked
not to
blink from the time the CRO spot started revolving until after the event.
To minimize
the
possibility that
the
need
to
blink might become
a
controlling 'external' factor that
compels
or
impels
him to act, the
subject
was
told that
he may
blink during
the
trial
if
the need arose;
but that,
if
he
did
blink
(or
made some other extranaeous motion),
he
should then simply wait
for the
CRO spot to make
at
least another full revolution before performing the quick voluntary movement,
as
at
the
start
of
the trial.
Two different kinds
of
series were studied.
(1) Self-initiated voluntary acts.
The
subject
was
asked
to
wait
for one
complete revolution
of the
CRO spot
and
then,
at any
time thereafter when
he
felt like doing
so, to
perform
the
quick, abrupt
flexion of the fingers and/or the wrist of his right hand (see Libet etai, 1982).
An
additional instruction
to encourage 'spontaneity'
of
the
act was
given routinely
to
subjects
in
Group
2 and
only
in the
latter
half to two-thirds
of
sessions with Group
1. For
this,
the
subject
was
instructed
'to let the
urge
to act
appear
on its own at any
time without
any
preplanning
or
concentration
on
when
to
act', that is,
to try
to
be
'spontaneous'
in
deciding when
to
perform each
act;
this instruction
was
designed
to
elicit
voluntary acts that were freely capricious
in
origin.
(2) Skin-stimuli
'at
unknown times'.
For
such
a
series the subject expected
to
receive
a
near-threshold
stimulus pulse
on
the back of the right hand. Delivery of the pulse was made by the operator
at
irregular
times that were unknown
to the
subject,
but
only after
the CRO
spot completed
its
first revolution.
They were actually delivered randomly during
the
second
or
third revolution
of the
spot (that
is,
between about
2.6 and 7.6 s
after
the
spot began
to
revolve); this range overlapped with that
for the
times
of the
self-initiated movements. These conditions closely paralleled
the
attentive
and
other
requirements associated with performing
and
recalling
the CRO
clock time
for
'spontaneous'
self-
initiated voluntary acts (see also Libet
et al.,
1982).
Subjects' reports of the time of an event. The 'clock position' of the revolving CRO spot
at the
time
of
the subject's awareness
of
an event
was
observed
by the
subject
for
later recall. Within
a few
seconds
after
the
event,
the
subject
was
asked
for his
report
of
that timing,
as in
recalling
a
spatial image
of
ordinary clock time
in
conjunction with another event.
It
was emphasized that only
an
after-the-event
626 BENJAMIN LIBET AND OTHERS
recall of the experience was required, and that the subject should not worry about the task in advance
of each event. Subjects became rapidly accustomed to this task during the training runs and did not
find it to be taxing or stressful; nor did this task have any detectable effect on RPs (Libet et al, 1982).
Modes of
recall.
Although each report depended on the subject having continously monitored the
revolving CRO spot and visually noting, to
himself,
the position of the spot at the actual time of his
awareness (of the event under study—see
below),
two different modes were employed for his after-the-
event recall of that spot position. With the (A) or 'absolute' mode, the subject was asked to look back
on the circular time scale mounted on the CRO and report the 'clock-time' of the spot position in
'seconds'. (Each 'second' on this scale corresponded to an actual time of 2560/60 or about 42.7 ms.)
With the (O) or 'order' mode, the subject was asked to report the order of the final stopped position of
the CRO spot, at the end of the trial, relative to his recalled position of the moving spot at the time of his
awareness. For this, the subjects simply reported 'CRO spot (stop-position) first', at an earlier clock-
position than the event-awareness; or 'awareness first', or 'together' (same position for both, insofar as
the subject could discriminate). The (O) mode of recall was found by most subjects to be somewhat less
demanding than the (A) mode.
The final stop position for the CRO spot following each event was arrived at in a complex manner
that differed for (A) or (O) mode of recall. When either of the modes of recall was to be used, the computer
continued the clockwise motion of the spot for a period beyond the time triggered by the event; this was
called the 'contination interval'. The continuation interval could have one of
20
different values, all in
the range between +500 and +800 ms (approximately 12 to 19 'seconds' of clock-dial). One of these
continuation values was selected by the computer, from a randomized series of the 20, for use in a given
trial. No violations of independence of answer were found in relation to the randomized continuation
intervals.
For the (O)-order-mode only, however, the CRO spot did not stop after reaching its continuation
interval; instead the spot jumped discontinuously, to stop at clock positions that were both before and
after the subject's recalled positions for his awareness. (1) 'Stopping range'. The clock-times within
which all of these final stop positions were included (the 'stopping range') ordinarily spanned 600 ms of
real time. The positive and negative end points of the stopping range, relative to zero trigger time for
each event (EMG or S-synch pulse), were chosen so as to span the entire range of times (relative to O) in
the reports for a given awareness (W, M or S, defined below). We usually succeeded in setting the
positive end of the stopping range well beyond the stop-times of the CRO spot for which all reports
were 'W-first' (or M- or S-first), that is, earlier than the stopped spot position; and the negative end of
the stopping range well beyond the stop-times for which all reports were 'spot first' (as in fig. 1B).
Actual distributions varied with the subject. The beginning and end points of the 'stopping range' were
therefore set individually before each series of trials, depending on previous results with the kind of
awareness to be reported and with that subject (see examples in fig. 1); the training series of 10 trials
that routinely preceded each regular series of 40 trials was useful for this purpose. (2) 'Stop-times'.
Within each selected stopping range that spanned 600 ms, one of 40 different actual stopping times for
the CRO spot, at intervals of
15
ms, was randomly selected by the computer for use after each of the 40
events. A different sequence of these randomized stop times could be preselected for the successive 40
trials in given series, so that a given sequence of stop-times was not repeated in a given session. The
length of the 'continuation-interval', that precedes the final jump of the spot to its stop-time (see above),
was randomly varied in a fashion that was independent of the randomized sequences selected for the
final stop-times. The objective of all this was of course to avoid providing any clues that might relate the
stop-time position of the CRO spot to the clock-time of the event
itself.
The O procedure would not seem to be subject to the kind of artefactual difficulty described by
Garner (1954). In the latter's case, subjects were asked to judge 'half-loudness' referred to a standard
acoustic stimulus; each subject gave reliably consistent judgements, but these turned out to be
appropriate only with reference to each different range of stimulus intensities presented rather than to
the standard stimulus. In our case, the stopping range of reference times was determined for each
subject from his own range of reported W times, as indicated by initial trials, rather than vice versa.
CEREBRAL AND CONSCIOUS TIMES OF VOLITION 627
There were some series for which the adopted stopping range did not appropriately span the full range
of potential reports by the subject; that
is,
the numbers of'W first' and 'spot first' reports were far from
equal, with few or no reports for one of these possible responses. This indicates that the subject was not
shaping his reports to correspond to the adopted range of stop-times for the CRO spot. In such
instances, the series had to be repeated, in the same or a later session, with a more suitable stopping
range that could result in a statistically usable analysis for the mean W time in the series. Additionally,
the subject had no prior knowledge of or consistent experience with the actual stopping ranges that
were used. In a usual given experimental session, each separate series for W, M and S awareness times
required a different stopping range. This meant that the subject did not have any consistent stopping
range with which he was repeatedly presented in successive series.
Type of 'awareness' to be timed by the subject. Three different subjective experiences were to be timed.
Each series of
40
trials involved reports of only one of these for all 40 events.
(W).
In W series, the subject was asked to note and later report the time of appearance of his
conscious awareness of'wanting' to perform a given self-initiated movement. The experience was also
described as an 'urge' or 'intention' or 'decision' to move, though subjects usually settled for the words
'wanting' or 'urge'. Subjects were told there were no right or wrong or inaccurate answers possible in
the W and M series; that we simply wanted their best recollection and report of their own experience.
Subjects were also asked, after completing a series of 40 self-initiated acts, whether any of the acts
occurred without their experiencing any prior awareness of an urge or intention to act, that
is,
whether
they were 'surprised' to realize a voluntary act had occurred without a conscious prior intention. In
three series, subjects (3 of the 5 studied) did report that some, usually a small number, of the 40 acts
'surprised' them (Libet et al., 1982), although the mean W time for such series of 40 was negative to
EMG zero time, as usual. Asking subjects about 'surprise' acts should have indicated to them that it
was acceptable even to have and to report the absence of a conscious urge or intention to act prior to a
self-initiated act. The fact that instances of 'surprises' were reported increases confidence that the
reports of timing prior to the act represented endogenous experiences not defined or induced by the
instructions.
It was emphasized that the reported time should refer to the earliest appearance of the W awareness
that might precede an actual movement; the subjects easily distinguished this specific urge to act from
any awareness of a more general preplanning nature that might precede W, as in association with type I
RPs (see Libet et al., 1982), and also from an occasional urge to move that was not followed by an
actual movement.
(M).
In M series, the time of subject's awareness that he/she 'actually
moved',
in the self-initiated act,
was to be noted and reported after each trial. This was intended to provide an instruction for timing an
experience related to but different from that of W, and thus to act as a partial test of the validity of the
W timings. Some subjects stated, on their own, that their mental set differed somewhat in W M M
series.
In a W series there was a feeling of active attention to or 'watching for' the awareness of wanting
to move, so as to be able to note the time of its appearance, although the urge to move arose
spontaneously with no preplanning; but M series proceeded without such advance 'watching' for the
event to occur. It should be emphasized that any such difference in mental set did not appear to affect
the associated RPs, which had similar forms and onset times for W and M series (see Results, below).
(S).
In S series, time of awareness of the sensation elicited by the near-threshold stimulus pulse to the
back of the hand, delivered at randomly irregular times unknown to the subject, was to be noted and
reported after each trial. The attentive and other conditions of this task closely paralleled those for the
W and M series, except that the event was an externally-induced sensory instead of a self-initiated
motor one. After each S series, whether for training or experimental purposes, the subject was given a
rough indication of how close he was to the actual times for the stimuli, as an aid in improving accuracy
in all the experiments. If the awareness times reported in an S series were to differ significantly from the
actual times of the stimulus pulse in each respective trial, the mean difference between the reported and
actual stimulus times may be regarded as a measure of that subject's 'bias', when observing and
reporting awareness times under the experimental conditions employed (see Libet et al., 1979). This
628
BENJAMIN LIBET AND OTHERS
would include errors
in
making simultaneous judgements
of
the times
for a
mental event (sensation
here)
and a
visual event (position
of the CRO
spot)
(see
Discussion). There were
in
fact mean
differences
or
'shifts'
for
S that were characteristic
for
each subject.
Progression of the experiments. The first (and,
in
some cases, the second) half-day session was purely
for training purposes. Subsequently, each subject
was
studied
in 6 to 8
regular half-day sessions,
usually
1
per week. Each of the first
4
regular sessions began with
a
training series of 25 trials with skin
stimuli intended
as a
retraining
of
reporting
the
recalled times
of
a subjective experience. This series
differed from the experimental S series (of 40 trials)
in
that, after each 5 trials of this series, the subject
was told what
the
actual 'clock-times' were
for the
skin stimuli, whose randomly irregular delivery
times were
not
known before
the
trials. Following this there were two 40-trial series
of
self-initiated
acts,
one W and one M, each preceded by
a
briefer 10-trial series
for
retraining purposes, and then
one
40-trial series
of
skin stimuli delivered
at
unknown times (S).
The
order
of
W
and M
series within
a
session was alternated for each successive session.
A
given mode for subjects to recall clock-positions of
the revolving spot,
(A)
vs
(O), was used
for
all series
in a
given session,
but
the modes were alternated
for successive sessions.
In
sessions after
the
first
4, few M or S
series were studied.
2
I
+128(ms)
'+2.5
s'
(W) time reported
W-lst
T
Spot-1st
IT
I
-400-250-10050200 (ms)
Stop position
of
CRO spot
FIG.
1. Examples of plots of a subject's reports of (W) time after each trial in series of 40 self-initiated, voluntary
movements.
A,
series in
session
3
for subject
S.B.
mode of recall
is
'absolute'
(A);
i.e.
subject reported 'clock-time' of
CRO spot at time
of
first
awareness of'wanting' to
move.
From this
value,
the 'clock-time' of the EMG-trigger
was
subtracted; the resultant
net (W)
time,
relative
to
EMG
zero
time,
was
plotted. Abscissa indicates both
the
net 'clock-
time'
in
seconds on the dial (arrows) and the corresponding net real times for
(W).
B,
series in session
2
for
subject
S.B. mode
of
recall
is by 'order' (O). The subject reported one of
three
alternatives
for
his recalled position of the
revolving spot at the initial awareness of W. As indicated on the ordinate these were 'W-first' (W time earlier than
the
final
stop-position of the CRO
spot);
or
'Spot-first'; or 'together', T
(W
time indistinguishable from or same
as
for the
final
stop-position of the spot). The 'stopping range', within which lay all the
40
different stop-positions of
the
spot,
randomly sequenced
in
the
series
of
40
trials,
was set
between
200 ms
positive
to
(i.e.,
later than) and
400 ms
negative
to the
EMG zero time
in
each trial. (Since both ends
of
the 600 ms stopping range were included,
the
computer actually designated
41
stop-positions at
15 ms
intervals,
but the subject did not report
W
time for the 41 st
trial of the
series.
This
accounts for
an
absence of
a
report at
the
stop-time
of
115 ms in
the
figure,
which happened
to be the randomly sequenced stop-time of the CRO-spot in the 41st trial
of
this series.)
CEREBRAL AND CONSCIOUS TIMES OF VOLITION 629
Statistical handling of the response times. This is described in relation to the data in two actual series,
one in (A) and one in (O) mode of recall (plotted in fig. 1). For each event in a series of 40 with (A)
mode of recall, the 'clock-time' in 'seconds' for EMG-trigger time is subtracted from the 'clock-time' of
the CRO spot at the time of awareness, as reported by the subject; this gives a net reported clock-time
(relative to 'EMG zero time'). Each net reported clock-time is then converted to real time, and these W
times (reported real time of each awareness of wanting to move, relative to EMG zero time) are plotted
for that series. (Each 60 s of 'clock-time' = 2560 ms actual time.) For example, in fig.
1A
we see that
subject S.B. reported W times of
43
ms twice, O ms 4 times, —43 ms 8 times, etc. (as converted from
net reported clock-times of 1, 0,1 'seconds', respectively). Averaging these values for the whole
series gives a mean shift for W, relative to EMG-O time, of —2.1 'seconds' of clock-time or —90 ms
of real time.
For the (O) mode of recall we have extended to trinomial data the idea presented by Church and
Cobb (1971). With this technique, the mean W shift was calculated as:
(upper, positive end of'stopping range')
(time interval between 'stop times') x (number of points
1 /2)
'Stopping range' and 'stop-times' are defined above. 'Number of points' is calculated by giving
1
point
for each time the subject says 'W first' (i.e. spot position at time of awareness has an earlier 'clock-time'
than the final 'stop-time' of the CRO spot), and 1/2 point for each response of 'together' (i.e. W time
and stop-time of spot appear the same to the subject). (In some series a trial was 'aborted' or a subject's
report was not available, for some technical reason. In such a case that trial was considered to
contribute a number of points equal to our estimate of the probability that the subject would say 'W
first' for that particular 'stop-time' of the CRO spot.) In the example shown in fig.
1
B,
the responses for
each of the different 'stop-times' are plotted. In that series, the upper end of the 'stopping range' was
200 ms after EMG-trigger time (lower end was —400 ms), with the usual minimum 15 ms time interval
between stop-times within the 'stopping range' of 600 ms. There were 19 'W first' responses and 8
'Together' responses; the remaining responses were of course 'Spot first'. There was one trial with a
missing report in this series, at the CRO stop-time of
115 ms; we estimate the probability of saying
'W first' as 1/2 in this case. Putting this together, our estimate of the mean shift for W is
200-15(19 + 8/2 +
1/2-1/2)=
-145 ms
The mean shifts for the awareness in an M or S series were computed in an analogous manner for A or
O mode of
recall,
respectively, using the stimulator-synch trigger for zero time in the S series.
RESULTS
/. Subjective Timings
The mean values of the 40 reported times of awareness (whether for W, M or S),
for each series in a given study session, are presented in Table 1,
A.
Each value is for
net time relative to 'zero-time' for each event, that is, W or M relative to EMG zero
time for activation of muscle in a self-initiated movement, or S relative to stimulus-
pulse time in the case of skin stimuli delivered at irregular times. There were no
obvious or consistent differences between sessions in which mode of recall of time
was 'absolute' (A) or by order (O) relative to final 'clock-position' of the spot (see
Methods).
The mean value of W in each series, that is of the recalled times for being aware of
'wanting' to move, was invariably in advance of or negative to the EMG zero time.
The average of all such mean Ws was about —200 ms (Table
2D).
Except for the
nature of the event, the basic procedures for attentive monitoring of the revolving
630 BENJAMIN LIBET AND OTHERS
spot
and of
noting visually
and
later recalling
the
clock-position
of
the spot,
in
connection with appearance
of an
awareness, were
the
same
for M
('actually
moved')
or for
S (skin sensation produced
by
irregularly timed, stimulus pulse)
as
they were for
W.
Reported times
for
S
might be expected on the average to be close
TABLE
1
Column
A.
Awareness times (W) and, column
B,
RP-onset times,
for
each
'W
series' of 40 self-initiated movements.
Awareness times also given
for
(M)
and
(S) series in same session. RP onsets are given
for
both
the
'main negative
shift' (MN) and for time
at
which 90 per cent of total area under the RP begins (see text for definitions of W, M and
S).
Column
c.
Differences (ms) between
RP
onset-times
and W
times ('uncorrected',
and
'corrected'
for
S), taking
RP onset either for the main negative (MN) component
or
for 90 per cent of the RP area, in each W series of 40
self-
initiated acts. (Instances in each series of 40 trials when the W time preceded [was negative to] onset
of
RP are given
after
RPMN
onset
for
the respective series.)
Sub-
ject
S.B.
G.L.
B.D.
S.S.
CM.
Ses-
sion
1
2
3
4
5a
b
6a
b
1
2
3
4
5a
b
6a
b
1
2
3
4
5a
b
6a
1
2
3
4
7a
b
c
1
Mode
call
A
O
A
O
A
A
A
A
O
A
O
A
0
0
0
O
A
O
A
0
O
0
0
0
0
0
A
A
A
A
O
(30
trials)
2
3
4
6
7a
b
8
0
0
A
A
A
A
A
X
-
54
-145
-
90
-188
-123
-119
-118
-161
-208
-422
-377
-258
-213
-283
-221
-271
-225
-145
-152
-142
-145
-108
-146
-235
-253
-255
-283
-248
-236
-209
-287
-223
-245
-132
-211
-260
-251
-204
A.
0
SE1
11
19
11
34
16
10
13
15
28
24
27
21
42
34
34
40
19
24
14
18
29
46
30
31
28
26
19
20
17
16
25
25
17
13
17
17
16
Awareness
times (ms)
(M)
It
-
21
-
48
-
72
-
95
-213
-172
-220
-201
+
92
-
3
+
76
+
40
-168
-
33
-153
-113
-138
-123
-
83
-
69
SE
15
20
12
20
28
22
26
25
I
1
10
25
12
21
25
19
24
9
21
20
10
(S)
It
-
12
-
53
-
42
-
53
+157
-147
-184
-217
-120
—164
+
135
+
45
+
61
+
90
-130
-
83
-
75
-157
-
63
-
23
-
23
+
29
SE
11
15
12
18
28
30
30
25
20
13
26
9
20
27
17
24
13
20
27
10
B.
Onset of RP (Wseries)
Type
Rf
II
II
I
I
II
I
I
II
II
I
I
II
I
I
II
I
II
III
II
II
III
III
II
c
11°
nc
mc
v
i°
"c
•MN-
-
550
-
900
-1100
-1150
-
800
-
950
-
900
-
600
-
500
-1200
-
900
-
800
-
900
-1200
-
600
-1400
-
400
-
225
-
500
-
425
-
250
-
325
-
650
-
250
-
400
-
400
-
300
-
900
-
400
-1100
-
500
-
400
90
% area
-1076
-
729
-
863
-
757
-
876
-
694
-
685
-
484
-
380
-
755
-
635
-
593
-
599
-
866
-
563
-
765
-
295
-
157
-
401
-
469
-
716
-
210
-
468
-
806
-
282
-
281
-
695
-
915
-
604
-
805
-
408
-
489
(no RP available)
II
Ic
"c
"c
-
600
-
400
-
400
-1000
-1050
-
450
-
475
-
520
-
781
-
227
-
703
-
694
-
368
-
479
c. (Onset RP)
minus
(W),
i.e.
(
MN
-
496
-
755
-1010
-
977
-
677
-
831
-
782
-
439
-
292
-
778
-
523
-
542
-
687
-
917
-
379
-1129
-
175
-
80
-
348
-
283
-
105
-
217
-
504
-
15
-
147
-
145
-
17
-
652
-
164
-
891
-
213
-
177
-
468
-
181
-
181
-
740
-
790
-
199
-
271
B)-(A),
toMN
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
1
0
0
0
|
0
3
0
2
17
0
4
0
0
0
1
1
0
0
2
1
using
Kr90%
-1022
-
584
-
773
-
584
-
753
-
575
-
567
-
323
-
172
-
333
-
258
-
335
-
386
-
603
-
342
-
494
-
70
-
12
-
249
-
327
-
571
-
102
-
322
-
571
-
31
-
26
-
412
-
667
-
368
-
596
-
121
-
266
-
388
-
570
-
16
-
443
-
434
-
117
-
275
(Onset
Rl
(W-S),
RPMN
-
508
-
808
-1052
-1030
-
625
-
282
-
439
-
962
-
740
-
662
-
543
-1293
-
40
-
35
-
287
-
195
-
145
-
230
-
220
-
174
-
274
-
200
-
439
f
Fninus
using
RP90%
-1034
-
637
-
815
-
637
-
410
-
166
-
319
-
517
-
475
-
455
-
506
-
658
+
65
+
33
-
188
-
239
-
691
-
112
-
102
-
569
-
184
-
289
-
359
* All values for
subjects
S.B., G.L. and B.D. are for RPs
recorded
at the
vertex.
For
subjects
S.S. and CM. the relevant
RPs were
recorded
only at the
contralateral
prccentral
area for
the
hand,
as
designated
by
subscript
c, except for
some
vertex
recordings
noted
by
subscript
v.
Simultaneous
values
for v
and
c
are
given
for
sessions
6 and 7a
of
subject
CM.
f
SE
=
standard
error for our estimate
of
the
mean value.
CEREBRAL AND CONSCIOUS TIMES OF VOLITION
631
to zero (actual stimulus time) or possibly delayed slightly. But the actual mean values
for S were usually negative rather than positive or delayed, except for subject B.D.,
and they differed for each subject and with each session. The value obtained for
S
in
a given session could be regarded as at least a partial measure of the way the subject
is handling those reporting factors that S and W series do have in common. As an
approximation, one may 'correct' W for the subject's 'bias' in reporting awareness
time by our methods, by subtracting S from W for that given session. The average of
all W values (about
200 ms) would be changed to about
150 ms by subtracting
the average of about —50 ms for all S values (see Table
2D).
TABLE
2.
GRAND AVERAGES (MS) FOR ALL SERIES
IN
EACH COLUMN
OF
TABLE
1,
ACCORDING TO TYPE
OF
ASSOCIATED
RP
AND TO ORDER
OF
W AND
M
SERIES
IN
A SESSION
Type of RP,
for W
series
n
I
12
II
20
III
5
For all series
A. Awareness
times
W
-233
-192
-183
In sessions when
W series done before M series
In sessions when
M series donebefore W series
B.
Onset of RP
(in W series)
RPMN RP9OX
-1025
-784
-535
-527
-270
-517
W
n
X
37
-204
10
-191
10
-240
c. B-A; i.e.,
(Onset RP)
minus
(
W),
using
-825
-343
-87
np
90%
-522
-333
-334
D.
Awareness times
M
n
20
10
10
X
-86
-92
-80
n
6
14
3
(Onset RP)
minus ( W-S),
using
RPMN RP90%
-950
-585
-366
-323
-118
-409
S1
n
X
22
-47
10
-41
10
-53
Mean values
for M
series were also mostly negative (except
for
subject B.D.),
averaging about
85
ms for all mean Ms (Table
2D).
M was also slightly negative to
S in almost every individual study session (see Table
1,
column A); so that even if the
average of M values
(
86 ms) are 'corrected' by subtracting the average of S values
( 47 ms),
a
small average net M of about —40 ms still remains. (Even for subject
B.D.,
subtracting the average of his mean Ss, +
83
ms, from the average of mean Ms
of +51 ms
in
the same
4
sessions, produces
a
net 'corrected' average
M
of about
30 ms.) This produces the unexpected result that reported time of awareness of
'actually moving' generally preceded the activation of the muscle at EMG zero time!
(See Discussion.)
It is important to note that mean W values were consistently negative to mean M
values in the respective session for each subject (Table
1A),
in spite of the frequently
negative values for M. The average of all mean Ws (about —200 ms) indicated that
632 BENJAMIN LIBET AND OTHERS
awareness of wanting to move preceded average awareness of actually moving
(about —85 ms) by more than 100 ms. When only those W values obtained in the
same 20 sessions with M values are included, the average of mean Ws was —216
instead of
204 ms (Table
2D).
Mean Ws obtained in sessions when an M series was
carried out before the W series appeared to be significantly more negative than Ws
obtained when a W series was carried out before an M series {see separate averages
in Table
2D).
A Wilcoxon test for this ranking order gave a one-sided P = 0.038, and
a two-sided P = 0.076. (This ranking was not related to the use of (A)
vs
(O) mode of
recall in the session.) However, the actual differences of about 50 ms between the
two sets of Ws are relatively unimportant when comparing W times to onset times of
the corresponding readiness potentials {see Section III, below).
II. Onset Times of Readiness-potentials {RP)
RPs associated with the freely voluntary, self-initiated movements employed in
this study have been described (Libet et al., 1982). They can be categorized into two
or three types, based on their form and the time of the main negative (MN) shift
{see
Tables
1B
and
2B).
Type II (and III) RPs are obtained when all 40 self-initiated
movements in the averaged series are reported by the subject to have originated
'spontaneously' and 'capriciously', with no recollections of preplanning experiences
for any of the 40 events in the series. Additional experiences of a 'preplanning' phase
are associated with type I RPs (Libet et al., 1982). No significant association could
be detected between mode of recall for W (that is (A) or (O)) and type of RP
obtained.
Onset times of RPs listed in Table
1B
are for RPs recorded in the same series of 40
self-initiated movements for which the reports of W times are given, in each
respective session for each subject. This simultaneity, for RP and W observations, is
important because there can be considerable variations of RP onsets in different
series even in the same session (Libet et al., 1982). The actual RPs for each W series
listed in Table
1
for the Group 2 subjects
(S.B.,
G.L. and B.D.) are presented in fig. 2.
RPs were also obtained with each M series in the session, but onset times for these
are not listed in Table 1. Onset times for RPs in M series were actually, on average,
similar to those for RPs in the W series {see also Libet et al., 1982).
Two values for onset time are given for each RP (W series) in Table
1B.
(1) Onset
time of the main negative (MN) shift was determined by 'eye-ball inspection',
checked independently by a second investigator. (2) Onset time was also com-
puted for the point at which 90 per cent of the area under the RP tracing preceded
EMG zero time.
Onset time based on RP area was determined as follows. On an enlarged projected image, the area
under the RP was measured for each interval of 50
ms,
starting from EMG zero time and progressing to
successive intervals in the negative (pre-EMG) direction until
600
ms; between
600 and —1400 ms,
areas for 100 ms intervals were measured. Within each time interval, any areas below the baseline were
subtracted from those above. In estimating total area, however, it was considered advisable to exclude
any early brief shifts of potential that did not continue progressively into the main RP, as some of these
CEREBRAL AND CONSCIOUS TIMES OF VOLITION
633
S.B.
G.L.
B.D.
5a
5b
6a
6b500 ms
FIG.
2.
Readiness-potentials (RPs) recorded at the vertex and averaged for each
(W)
series of
40
trials for subjects
S.B., G.L.
and
B.D. Each RP corresponds
to
the respective (W) series as listed by session number in Table
1.
The
solid vertical
line
indicates the EMG
zero
time,
marking
the
end of the
RP.
Dashed horizontal
lines
represent
the
d.c.
baseline drift for the
2 s
of that
tracing,
as
estimated from the total voltage compensation for shift
in
d.c.
level
during
the total time between beginning and end
of
that series of 40 trials.
could have been artefactual
in
nature. Therefore,
the
rule was adopted that any 200 ms segment having
a total area
< 4
mm2 (actually equivalent
to
50
/tV-ms) was
to be
regarded as zero,
and
that any
and all
areas preceding that segment were also regarded
as
zero. (Making
the
rule even more stringent,
by
reducing the excluding low level segment
to
100 ms, rarely changed the results significantly and, when
it
did,
the
final estimates
of
time
of
onset were very little different.)
It was
also recognized that
measurements
of the
beginning
of the
negative potential shift
are
subject
to
some possible error
in
judging
the
d.c. baseline, especially
in a
somewhat noisy/bumpy tracing. Therefore, after arriving
at a
total area
for a
given
RP
under
the
rule above,
the
time interval that included only
90 per
cent
of
this
area
was
computed.
The 90 per
cent values also
fit the
range
of
differences between
the
independent
measurements
of
the areas
by two
different investigators.
634 BENJAMIN LIBET AND OTHERS
Averages of the onset times for RPMN and RP90% area, respectively, for the Ws
series in Table
1B
are given in Table
2.
The initially slower but progressive ramp-like
rise of type I RPs accounts for these onset times being more negative for RPMN than
for RP^ area. On the other hand, in types II and III RPs some definitely
distinguishable negativity is often present even before the main (MN) shift. Such
negativities tend to have a relatively irregular, low, amplitude but there was no
reason to regard them as other than actual RP components in these self-initiated
acts (see Libet et al., 1982). Their inclusion in the measurements of total area makes
it possible for onset of RP^ area to precede onset of RPMN in some cases. Averages
of the onsets for RPMN and RP,^ were in fact not very dissimilar for type II RPs,
although individual values for the difference (RPMN
RIV/J) were in a range between
- 207 and + 526 ms; but for type III RPs average RP^ preceded RPMN by -
247
ms
(range of
RPMN
RP,^ was between
115 and +556 ms).
///. Differences between Onset time ofRP and Time of Awareness of Wanting to Move
The data comparisons given in Table lc are central to the objective of this study;
they relate the time of appearance of the conscious intention to act, on the one hand,
to the time of onset of the cerebral processes before the act (as evidenced in the RP),
on the other. The difference between RP onset time and each W awareness-time is
given for each respective series of self-initiated voluntary acts. Differences are
presented when utilizing the W times as actually reported (W 'uncorrected'), giving
(onset RP) minus (W); or the W times 'corrected' by subtracting the reported mean
time for the S obtained in the same session, giving (onset RP) minus (W-S).
'Correcting' the W value by subtracting the S value of each subject's 'bias' in
reporting, did not qualitatively change the relation of RP onset-time to W; rather it
generally increased the difference by which onset ofRP precedes W (as 'corrected').
For subject B.D., his positive values for S have the oposite effect; but even for him,
the only qualitatively important change in the difference is introduced in session 1,
which had a large positive S (+135 ms).
It may be seen (Table lc) that, with few exceptions, onset ofRP occurred before
reported awareness time by substantial amounts of
time.
This was true irrespective
of which measure of RP-onset or of W
is
employed to obtain the difference. The sizes
and consistency of these differences, between onset of RP and W, indicate they are
highly significant. However, it is difficult to produce a rigorous quantitative value
for significance of the large differences between onset of RP and W. An SD
(standard deviation) for variability among individual RPs within each series of 40 is
not available, as only the average RP for the whole series could be meaningfully
recorded. Consequently, only the mean W value and the averaged RP obtained