Brain 1983 Libet

7/27/2019 Brain 1983 Libet

1/20

Brain (1983), 106, 623-642

TIME OF CONSCIOUS INTENTION TO ACTIN RELATION TO ONSET OF CEREBRAL

ACTIVITY (READINESS-POTENTIAL)THE UNCONSCIOUS INITIATION OF A FREELYVOL UNT AR Y ACT

by BENJAMIN LIBET, CURTIS A. GLEASON, ELWOOD W. WRIGHT andDENNIS K. PEARL1

{From the Neurological Institute, Departmen t of Neuroscience, Mount Zion Hospital and MedicalCenter, the Department of Physiology, School of Medicine, University ofCalifornia, San Francisco,CA 94143 and the Department of Statistics, University of California, Berkeley, CA)

S U M M A R YThe recordable cerebral activity (readiness-potential, RP) that precedes a freely voluntary, fullyendogenous motor act was directly compared with the reportable time (W) for appearance of thesubjective experience of'wanting ' o r intending to act. The onset of cerebral activity clearly p receded byat least several hundred milliseconds the reported time of conscious intention to act. This relationshipheld even for those series (with 'type I I' RPs) in which subjects reported that all of the 40 self-initiatedmovements in the series appeared 'spontaneously' and capriciously.

Data were obtained in at least 6 different experimental sessions w ith each of 5 subjects. In series withtype II RPs, onset of the main negative shift in each RP preceded the corresponding mean W value byan average of about 350 ms, and by a minimum of about 150 ms. In series with type I RP s, in which anexperience of preplanning occurred in some of the 40 self-initiated acts, onset of RP preceded W by anaverage of abou t 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 revolvingspot at the time of his initial awareness of wanting or intending to move. Two different modes of recallproduced similar values. Subjects distinguished awareness of wanting to move (W) from awareness ofactually moving (M). W times were consistently and substantially negative to, in advance of, meantimes reported for M and also those for S, the sensation elicited by a task-related skin stimulusdelivered at irregular times that were unknown to the subject.

It is concluded that cerebral initiation of a spontaneous, freely voluntary act can beginunconsciously, that is, before there is any (at least recallable) subjective awareness that a 'decision ' toact has already been initiated cerebrally. This introduces certain constraints on the potentiality forconscious initiation and control of voluntary acts.

1 Present address : Departm ent ofStatistics, Ohio State University, Columbus, Ohio.Reprint requests to Dr B. Libet , Department of Physiology, University of California, San Francisco,CA 94143, USA.

at

UniversityofPlymouthonJanuary13,2012

http://brain.oxfordjournals.org/

Downloadedf

rom
http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/

7/27/2019 Brain 1983 Libet

2/20

624 B E N J A M I N L I B E T A N D O T H E R S

I N T RO D U CT I O NThe 'readiness-potential' (RP ), a scalp-recorded slow negative potential shift thatbegins 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 whichare no t only fully endogenous but even spontaneously capricious in origin (L ibet etal., 1982). The appearance of preparatory cerebral processes at such surprisinglylong times before a freely voluntary act raises the question of whether consciousawareness of the voluntary urge or intention to act also appears with such similaradvance timings. The present study attempts to answer this question experimentally.In the present study, the experience of the time of thefirstawareness of the urge tomove was related by the subject to his observed 'clock-position' of a spot of lightrevolving in a circle; the subject subsequently recalled and reported this position ofthe spot. Thus the experience of timing of the awareness was converted to areportable, visually related spatial image, analogous to reading and recalling theclock-time for any experience. (The reliability and validity of this operationalcriterion are further considered below.) This indicator of the time of the consciousexperience could then be related (1) to the actu al time of the voluntary m otor ac t, asindicated by the electromyogram (EMG) recorded from the appropriate muscle,and (2) to the time of appearance of the simultaneously recorded RP that isgenerated by the brain in advance of each act. The voluntary mo tor acts under studywere those produced with minimal or no restrictions on the subject's independentchoice of when to act, and under instructions tha t encouraged spontaneity of eachvolitional urge to act (Libet et al., 1982).The present findings thus provide experimental evidence on the timing of theconscious inten tion to act relative to the onset of cerebral activity prepa ratory to theact, and on the roles of conscious processes in the initiation of a freely voluntarymo tor act.

METHODS AND PROCEDURESSubjects

Six right-handed college students were studied as two separate groups of three each. Group 1comprised 3 females (S.S., C M . and M.B.), but the quality of the EEGs and the minimal amplitude ofthe 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 m onths after completing the study of G roup 1.Recording

The d.c. recording and averaging of the EEG has been described (Libet et al., 1982). For presentpurposes, analysis of RPs is made for those recorded at the vertex, where they were all maximal. (Fo rthe first 4 experimental sessions with Group 1, only the contralateral precentral recording site isavailable.) Linked m astoid electrodes served as the reference lead, with a ground electrode on the leftear lobe. Controls excluded the electro-oculogram as a source of the slow potentials. In eachexperimental series, 40 trials were performed and averaged by a computer of average transients (CAT400B). The 2 s period of EEG stored by the CAT with each trial included a 1.4 s period already on the

at

UniversityofPlymouthonJanuary13

,2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

3/20

C E R E B R A L AND C O N S C I O U S T I M E S OF V O L I T I O N 625recording tape before 'O-time' . The latter was signified by the EMG, recorded with bipolar electrodeson 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 theroom. Each tr ial was started only when the subject considered himself comfortably ready. The trialbegan with a brief 'ge t-read y' tone . This signalled tha t durin g the next 1 -3 s the subject should relax hismuscles, especially those of the head, neck and forearm, blink his eyelids if he wished, and fix his gazeon the centre of the 5 inch circular screen of a cathod e rayoscil loscope (CRO) that was posit ioned atabou t 1.95 m away in his direct line of vision. At the end of these irregular get-ready periods theoperator activated the PDP-12 computer to initiate circular revolution of the beam of t he CR O . TheCRO spo t of light revolved in a clockwise circle near the circumference of the screen starting fromth e' 12-o'clock' posit ion; this mo tion sim ulated a sweep secondh and of a clock but each revolution wascompleted in2.56 rather than 60 s. Acircular scale, with n um bers at each '5 s' posi t ion, was mounted atthe external edge of the CRO screen, and a plastic grille on the peripheral portion of the screendisplayed i l luminated radial l ines spaced at '2.5s. ' intervals (each equal to 107 ms of actual t ime).Subjects were asked to maintain their gaze fixed on the centre of the CR O screen and not to follow theCR O spot aroun d, even though they were to report information relating the 'clock-pos it ion' of the spotto theevents (see below). The visual angle subtended between the centre and theperipheral posit ion ofthe moving spot was small enough (1.8 deg) to present no difficulty from loss of visual acuity. The'clock-time' of the C R O spo t at each event, namely EM G with moto r act orstimu lator synch pulse withstimulus to skin, was recorded by the PDP-12 computer. Subjects were trained to make their self-initiated mo vem ent sufficiently brisk so tha t within nomo re than 10-20 ms from the start of any EMGpotentials they achieved the amplitude pre-set to trigger the computer .

The subject was asked not toblink from the t ime the CR O spot started revolving until after the event.To minimize the possibility that the need to blink might become a controll ing 'external ' factor thatcompels or impels him to act, the subject was told that he may blink du ring the trial if the need arose;but that, if he did blink (or made some other ext ranaeous mot ion) , he should then simply wait for theCR O spot to make at least ano ther full revolution before performing the quick volun tary mo vem ent, asat the star t of the tr ial .

Two different kinds of series were studied.(1) Self-initiated voluntary acts. The subject was asked to wait for one comp lete revolut ion of theCRO spo t and then, at any t ime thereafter when he felt like doing so, to perform the quick, abruptflexion of the fingers and /or the wrist of his right ha nd (see Libet etai, 1982). An additional instructionto encourage ' spontanei ty ' of th e act wasgiven routinely to subjects in G r o u p 2 and only in the latterhalf to two-thirds of sessions with Group 1. For this, the subject was instructed 'to let the urge to actappear on its own at any t ime without any preplanning or concentrat ion onwhen to act ' , that is, to tryto be ' spontaneous ' in deciding when to perform each act; this instruction was designed to elicit

volun tary acts that were freely c apricious in origin.(2) Skin-stimuli 'at unknown times'. For such a series the subject e xpected to receive a near-thresholdstimulus pulse on the back of the right hand. Delivery of the pulse was made b y the opera tor at irregulartimes 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 thetimes of the self-initiated movements. These conditions closely paralleled the attentive and otherrequirements associated with performing and recalling the CRO clock t ime for ' s pon t aneous ' self-init iated voluntary acts (see also Libet et al., 1982).Subjects' reports of the time of an event. The 'clock posi t ion' of the revolving CR O sp ot at the t ime ofthe subject 's aw areness of an event was observed by the subject for later recall. Within a few secondsafter the event, the subject was asked for his repor t of that t iming, as in recalling a spatial image ofordin ary clock time inconjunction with another event. It was emphasized that only an after-the-event

at



Downloadedf

rom

7/27/2019 Brain 1983 Libet

4/20

626 BE N JA M I N LI BET A N D O T H E RSrecall of the experience was required, and that the subject should n ot wo rry abou t the task in advanceof each event. Subjects became rapidly accustomed to this task during the training runs and did notfind 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 therevolving CRO spot and visually noting, to himself, the position of the spot at the actual time of hisawarene ss (of the event under study see below), two different modes were employed for his after-the-event recall of that spot posit ion . With the (A) or 'abso lute' mo de, the subject was asked to look backon the circular t ime scale mounted on the CRO and report the 'clock-time' of the spot posit ion in'seconds' . (Each 'second' on this scale corresponded to an actual t ime of 2560/60 or about 42.7 ms.)W ith the (O) or 'orde r ' mo de, the subject was asked to report the orde r of the final stopped p osit ion ofthe C R O s pot, at the end of the trial, relative to his recalled po sition of the mo ving sp ot at the time of hisawaren ess. Fo r this, the subjects simply reported ' C R O sp ot (stop-posit ion) f irst ', at an earlier clock-posit ion th an the event-aw areness; or 'awareness first' , or ' togeth er ' (same posit ion for both , insofar asthe subject could discriminate). Th e (O) mo de of recall was found by mo st subjects to be som ewhat lessdemanding than the (A) mode.Th e final stop posit ion for the CR O spot following each event was arrived at in a complex m ann er

tha t differed for (A) or (O) m od e of recall. Wh en either of the mo des of recall was to be used, the co m pu tercon tinued the clockwise m otio n of the spot for a period bey ond the t ime tr iggered by the event; this wascalled the 'contina tion in terval ' . The continuation interval could have one of 20 different valu es, all inthe range between +5 0 0 an d + 80 0 ms (approxima tely 12 to 19 'second s' of clock-dial). One of theseco ntin ua tio n va lues was selected by the comp uter, from a ran dom ized series of the 20, for use in a giventrial . N o violations of indepen dence of answer were found in relation to the rando mized contin uationintervals.For the (O)-order-mode only, however, the CRO spot did not stop after reaching i ts continuationinterval; instead the spot jum ped discontinuously, to stop at clock posit ions tha t were both before andafter the subject 's recalled posit ions for his awareness. (1) 'Stopping range' . The clock-times withinwhich 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 foreach even t (EM G or S-synch pulse ), were chosen so as to spa n the entire ran ge of times (relative to O) inthe reports for a given awareness (W, M or S, defined below). We usually succeeded in setting theposit ive end of the stopping range well beyond the stop-times of the CRO spot for which all reportswere 'W -first ' (or M- or S-first), tha t is, earlier tha n the stop ped spo t position ; and the negative end ofthe s top pin g rang e well beyo nd the stop-times for which all repor ts were 'spo t first ' (as in fig. 1B ).Ac tual distr ibutions va ried with the subject. The beginning and end points of the 'stoppin g ran ge' weretherefore set individually before each series of trials, depending on previous results with the kind ofawareness to be reported and with that subject (see exam ples in fig. 1); the training series of 10 trialsthat routinely preceded each regular series of 40 trials was useful for this purpose. (2) 'Stop-times'.W ithin each selected stop ping ran ge tha t spanned 600 ms, one of 40 different actual stopp ing t imes forthe C R O spot, at intervals of 15 ms , was rand om ly selected by the co m pu ter for use after each of the 40events. A different sequence of these randomized stop times could be preselected for the successive 40trials in given series, so that a given sequence of stop-times was not repeated in a given session. Thelength of the 'con tinuatio n-interv al ' , tha t precedes the final jum p of the spot to i ts stop-time (see above),was randomly varied in a fashion that was independent of the randomized sequences selected for thefinal stop-times. Th e objective of all this was of cou rse to av oid pro vidin g any clues tha t might rela te thestop-time posit ion 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 byG ar ne r (1954). In the latte r 's case, subjects were asked to judg e 'half-loudness ' referred to a stan dardacoustic st imulus; each subject gave reliably consistent judgements, but these turned out to beap pro pria te only with reference to each different ran ge of st imulus intensit ies presented rath er than tothe standard stimulus. In our case, the stopping range of reference t imes was determined for eachsubject from his own range of reported W times, as indicated by initial trials, rather than vice versa.

atUniversityofPlymouthonJanuary13,

2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

5/20

C E R E B R A L A N D C O N S C I O U S T I M E S O F V O L I T I O N 627There were some series for which the adopted stopping range did not ap propriately span the full rangeof poten tial repo rts by the subject; that is , the numbers of'W first' and 'spot first' reports were far fromequ al, with few or no rep orts for one of these possible respo nses. This indicates tha t the subject was notshaping his reports to correspond to the adopted range of stop-times for the CRO spot. In suchinstances, the series had to be repeated, in the same or a later session, with a more suitable stoppingrange 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 thatwere used. In a usual given experimental session, each separate series for W, M and S awareness timesrequired a different stopping ran ge. This mea nt th at the subject did no t have any consistent stop pingrange with which he was repeatedly presented in successive series.

Type o f 'awareness' to be timed by the subject. Th ree different sub jective experiences were to be time d.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 hisconscious awaren ess of'wanting' to perform a given self-initiated movement. The experience was alsodescribed as an 'urg e' or ' intentio n' or 'decision' to move , though subjects usually settled for the word s'wan ting' or 'u rge ' . Subjects were told there were no right or w rong or inaccu rate answers possible inthe W and M series; that w e simply wanted their best recollection and r epo rt of their own experience.Subjects were also asked, after completing a series of 40 self-initiated acts, whether any of the actsoccurred witho ut their experiencing any prior awareness of an urge or intention to act, that is , whetherthey were 'surprised' to realize a voluntary act had occurred without a conscious prior intention. Inthree series, subjects (3 of the 5 studied) did repo rt tha t som e, usually a small num ber, of the 40 acts'surprised' them (Libet et al., 1982), although the mean W time for such series of 40 was negative toEMG zero time, as usual. Asking subjects about 'surprise' acts should have indicated to them that itwas acceptable even to have and to report the absence of a conscious urge or intention to act prior to aself-initiated act. The fact that instances of 'surprises' were reported increases confidence that thereports of timing prior to the act represented endogenous experiences not defined or induced by theinstructions.

It was emphasized that the reported time should refer to the earliest appearan ce of the W awarenessthat might precede an actual movement; the subjects easily distinguished this specific urge to act fromany awareness of a mo re general preplan ning n atur e that m ight precede W, as in association with type IRPs (see Libet et al., 1982), and also from an occasional urge to move that was not followed by anactual 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 anexperience related t o but different from tha t of W , and t hus to act as a partial test of the validity of theW timings. Som e subjects stated, on their own, th at their m ental set differed somew hat in W M Mseries. In a W series there wa s a feeling of active attent ion to or 'w atch ing for' th e awarenes s of wan tingto move, so as to be able to note the time of its appearance, although the urge to move arosespontaneously with no preplanning; but M series proceeded without such advance 'watching' for theevent to occur. It should be emphasized that any such difference in mental set did not appear to affectthe associated RP s, which had similar forms and onset times for W a nd M series (see Results, below).

(S). In S series, time of awaren ess of the sensation elicited by the nea r-threshold stimulus pulse to theback of the hand, delivered at rando mly irregular times unk now n to the subject, was to be noted andreported after each trial. The attentive and o ther con ditions of this task closely paralleled those for theW and M series, except that the event was an externally-induced sensory instead of a self-initiatedmo tor one. After each S series, whether for train ing or exp erimental p urpos es, the subject was given arough indication of ho w close he was to the actual times for the stimuli, as an aid in improving accuracyin all the experim ents. If the awaren ess times rep orte d in an S series were to differ significantly from theactua l times of the stimulu s pulse in each respective trial, the m ean difference b etween the rep orted andactual stimulus times may be regarded as a measure of that subject 's 'bias ' , when observing andreporting awareness times under the experimental conditions employed (see Libet et al., 1979). Th is

at


,2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

6/20

62 8 BENJAMIN LIBET AND OTHERSwould include errors in making s imultaneous judgements of the times for a mental event (sensationhere) and a visual event (position of the CRO spot) (see Discussion). There were in fact meandifferences 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 w as purelyfor 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 skinstimuli intended as a retraining of report ing the recalled times of a subjective experience. This seriesdiffered from the experimental S series (of 40 trials) in tha t, after each 5 trials of this series, the subjectwas told what the actual 'clock-times' were for the skin stimuli, whose randomly irregular deliverytimes were not known before the trials. Following this there were two 40-trial series of self-initiatedacts, one W an d one M , each preceded by a briefer 10-trial series for retraining purposes, and then one40-trial series of skin stimuli delivered at unknown times (S). Theorder of W and M series within asession was altern ated for each successive session. A given m ode for subjects to recall clock-position s ofthe revolving spot, (A)vs (O), was used for all series in a given session, but the modes were alternatedfor successive sessions. In sessions after the first 4, few M or S series were studie d.

2

I +1 2 8 ( m s )' + 2 . 5 s'

(W) time reportedW-lst

TSpot-1st IT I

- 4 0 0 - 2 5 0 - 1 0 0 50 200 (ms)Stop position of CRO spot

FIG. 1. Examples of plots of a subject's repo rts of (W) time after each trial in series of 40 self-initiated, voluntarymovements. A, series in session 3 for subject S.B. mode of recall is 'absolute' (A); i.e. subject reported 'clock-time' ofCR O spot at time offirstawareness of'w antin g' to move. From this value, the 'clock-time' of the EM G-trigger wassubtracted; the resultant net (W) time, relative to EM G 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 subjectS.B. mode of recall is by 'order' (O). The subject reported one of three alternatives for his recalled position of therevolving spot at the initial awareness of W. As indicated on the ordinate these were 'W-first' (W time earlier thanthefinalstop-position of the CRO spot); or 'Spot-first'; or 'togethe r', T (W time indistinguishable from or same asfor thefinalstop-position of the spot). The 'stopping range', within which lay all the 40 different stop-positions ofthe spot, randomly sequenced in the series of 40 trials, was set between 200 ms positive to (i.e., later than) and 400 msnegative to the EMG zero time in each trial. (Since both ends of the 600 ms stopping range were included, thecomputer actually designated 41 stop-positions at 15 ms intervals, but the subject did not repo rt W time for the 41 sttrial of the series. This accounts for an absence of a report at the stop-time of 115 ms in the figure,which happenedto be the randomly sequenced stop-time of the CR O-spot in the 41st trial of this series.)

atUniversityofPlymouthonJanuary13,2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

7/20

C E R E B R A L A N D C O N S C I O U S T I M E S O F V O L I T I O N 629Statistical handling of the response times. This is described in relation to the da ta in tw o actua l 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' ofthe 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 wan ting to move, relative to E M G zero time) are plottedfor that series. (Each 60 s of 'clock-time' = 2560 ms actual time.) For example, in fig. 1 A we see thatsubject S.B. reported W times of 43 ms tw ice, O m s 4 times, 43 ms 8 times, etc. (as converted fromnet reported clock-times of 1, 0, 1 'seconds', respectively). Averaging these values for the wholeseries gives a mean shift for W, relative to EM G -O time, of 2.1 'seco nds ' of clock-time o r 90 msof real time.For the (O) mode of recall we have extended to trinomial data the idea presented by Church andCo bb (1971). With this technique, the me an W shift was calculated as:

(upper, positive end of 'stopp ing range') (time interval between 'stop times') x (num ber of points 1 /2)'Stoppin g ran ge' and 'stop -times' are defined above. 'Nu mb er of points ' is calculated by giving 1 pointfor each time the subject says 'W first' (i.e. spo t position at time of awareness has an earlier 'clo ck-tim e'than the final 'stop-time' of the CRO spot), and 1/2 point for each response of 'together' (i.e. W timeand s top-tim e of spot ap pea r the same to the subject). (In some series a trial was 'abo rte d' o r a subject'sreport was not available, for some technical reason. In such a case that trial was considered tocontribute a number of points equal to our estimate of the probability that the subject would say 'Wfirst' for that particular 'stop-time' of the CRO spot.) In the example shown in fig. 1 B , the respon ses foreach of the different 'stop-times' are plotted. In that series, the upper end of the 'stopping range' was200 ms after EM G-tr igge r time (lower end was 400 ms), with the usual minim um 15 ms time intervalbetween 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 amissing report in this series, at the CRO stop-time of 115 ms; we estimate the p roba bility of saying'W first' as 1/2 in this case. Putting this together, our estimate of the mean shift for W is

2 0 0 - 1 5 ( 1 9 + 8/2 + 1/2-1/2)= - 1 4 5 m sThe mean shifts for the awareness in an M or S series were computed in an analogous manner for A orO mode of recall, respectively, using the stimulator-synch trigger for zero time in the S series.

R E S U L T S/ . Subjective Timings

Th e mean v alues of the 40 reported times of awareness (whether for W , M o r S),for each series in a given study session, are presen ted in Table 1, A. Each value is fornet time relative to 'zero-tim e' for each event, that is, W or M relative to E M G zerotime for activation of muscle in a self-initiated mo vem ent, o r S relative to stim ulus-pulse time in the case of skin stimuli delivered at irregular times. There were noobvious or consistent differences between sessions in which mode of recall of timewas 'absolute' (A) or by order (O) relative to final 'clock-position' of the spot (seeMethods ) .Th e mea n va lue of W in each series, th at is of the recalled times for be ing aw are of'wan ting' to m ove, was invariably in advance of or negative to the E M G zero t ime.The average of all such mean W s was ab ou t 200 ms (Table 2D). Except for the

nature of the event, the basic procedures for attentive monitoring of the revolving



Downloadedfrom

7/27/2019 Brain 1983 Libet

8/20

630 BENJAMIN LIBET AND OTHERS

spot and of noting visually and later recalling theclock-position of the spot, inconnection with appearance of an awareness, were the same for M ('actuallymoved') or for S (skin sensation produced by irregularly timed, stimulus pulse) asthey were for W. Reported times forS might be expected on the average to be closeTABLE 1

Column A. Awareness times (W) and, column B , RP-onset times, for each 'Wseries' 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 negativeshift ' (MN) and for time at which 90 per cent of total area under the RP begins (see text for definitions of W, M andS) . C o l u m n c. Differences (ms) between RP onset-times and W times ('uncorrected' , and ' corrected' for S), takingRP o nset ei ther for the main negat ive (M N) comp onent 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 ofRP are givenafter RPMN onset for the respective series.)

Sub-jectS.B.

G.L.

B. D.

S.S.

C M .

Ses-sion12345ab6ab

12345ab6ab

12345ab6a

12347abc

1

ModecallAOAOAAAA

OAOA000O

AOA0O00

000AAAA

O(3 0 trials)23467ab8

00AAAAA

X- 54- 1 4 5- 90- 1 8 8- 1 2 3- 1 1 9- 1 1 8- 1 6 1- 2 0 8- 4 2 2- 3 7 7- 2 5 8- 2 1 3- 2 8 3- 2 2 1- 2 7 1

- 2 2 5- 1 4 5- 1 5 2- 1 4 2- 1 4 5- 1 0 8- 1 4 6

- 2 3 5- 2 5 3- 2 5 5- 2 8 3- 2 4 8- 2 3 6- 2 0 9

- 2 8 7- 2 2 3- 2 4 5- 1 3 2- 2 1 1- 2 6 0- 2 5 1- 2 0 4

A.

0SE111191134161013152824272142343440

19241418294630

31282619201716

25251713171716

Awareness times (ms)(M )

It- 21- 48- 72- 95

- 2 1 3- 1 7 2- 2 2 0- 2 0 1

+ 92- 3+ 76+ 40

- 1 6 8- 33- 1 5 3- 1 1 3

- 1 3 8- 1 2 3- 83- 69

SE15201220

28222625

I110251221

2519249

212010

(S)It- 12- 53- 42- 53

+157

- 1 4 7- 1 8 4- 2 1 7- 1 2 0

164

+ 135+ 45+ 61+ 90

- 1 3 0- 83- 75- 1 5 7

- 63- 23- 23+ 29

SE11151218

28303025

20

1326920

27172413

202710

B. Onset of RP (Wseries)TypeRfIIIIIIIIIIII

IIIIIIIIIII

IIIIIIIIIIIIIIIII

Ic11ncmcvi

" c

MN-- 550- 900- 1 1 0 0- 1 1 5 0- 800- 950- 900- 600

- 500- 1 2 0 0- 900- 800- 900- 1 2 0 0- 600- 1 4 0 0

- 400- 225- 500- 425- 250- 325- 650

- 250- 400- 400- 300- 900- 400- 1 1 0 0

- 500- 400

90 % area- 1 0 7 6- 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)

IIIc

" c" c

- 600- 400- 400- 1 0 0 0- 1 0 5 0- 450- 475

- 520- 781- 227- 703- 694- 368- 479

c. (Onset RP) minus (W),i.e. (M N

- 496- 755- 1 0 1 0- 977- 677- 831- 782- 439

- 292- 778- 523- 542- 687- 917- 379- 1 1 2 9

- 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 ) ,

toMN00000000

00000000

31000|0

30217040

00110021

usingKr90 %- 1 0 2 2- 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

(OnsetRl(W-S),RP MN- 508- 808- 1 0 5 2- 1 0 3 0

- 625- 282

- 439- 962- 740- 662

- 543- 1 2 9 3

- 40- 35- 287- 195

- 145- 230- 220- 174

- 274- 200- 439

f FninususingRP90 %- 1 0 3 4- 637- 815- 637

- 410- 166

- 319- 517- 475- 455

- 506- 658

+ 65+ 33- 188- 239

- 691- 112- 102- 569

- 184- 289- 359

* All va lues for subjects S.B., G.L. and B.D . are for RPs recorded at the vertex. For subjects S.S. and C M . the relevant RPs were recorded only at thecontralateral prccentral area for the hand, as designated by subscript c, except for some vertex recordings noted by subscript v. Simultaneous values for vand c ar e given for sessions 6 and 7a of subject C M . f SE = standard error for our estim ate of the mean value.



Downloadedfrom

7/27/2019 Brain 1983 Libet

9/20

CEREBRAL AND CONSCIOUS TIMES OF VOLITION 631to zero (actual stimu lus time) or possibly delayed slightly. But the actual m ean valuesfor S were usually negative rath er th an p ositive or delayed, except for subject B .D.,and they differed for each subject an d w ith each session. Th e value obtain ed for S ina given session could be regarded as at least a par tial me asure of th e way the subjectis handling those reporting factors th at S and W series do have in com mo n. As anapp roxim ation, o ne may 'correct ' W for the subject's 'bia s ' in reporting awarenesstime by ou r meth od s, by sub tracting S from W for tha t given session. Th e average ofall W values (about 200 ms) would be changed to abo ut 150 ms by subtractin gthe average of ab ou t 50 ms for all S values (see Table 2D).

TABLE 2. GRAND AVERAGES (MS) FOR ALL SERIES INEA CH COLU M N OFTABLE 1,A CCO R DI N G TO TY P E OFA S S OCI A TED RPA N D TO OR DER OFW A N D MSERIES INA SESSIONType of RP,

for Wseriesn

I 12II 20III 5

For all series

A. AwarenesstimesW- 2 3 3- 1 9 2- 1 8 3

In sessions whenW series done before M seriesIn sessions whenM series done before W series

B . Onset of RP(in W series)RP M N RP 9O X- 1 0 2 5 -784- 5 3 5 - 527- 2 7 0 -517

Wn X37 - 204

10 -191

10 - 24 0

c . B-A; i.e.,(Onset RP)minus ( W), using

- 8 2 5- 3 4 3- 8 7

np90 %- 5 2 2- 3 3 3- 3 3 4

D. Awareness timesM

n20

10

10

X- 8 6

- 9 2

- 8 0

n6143

(Onset RP)minus ( W-S),using

RP M N RP 90 %- 9 5 0 -585- 3 6 6 -323- 1 1 8 -409

S1

n X22 - 4 7

10 - 4 1

10 - 5 3

Mean values for M series were also mostly negative (except for subject B.D.),averaging about 85 ms for all mea n M s (Table 2D). M was also slightly nega tive toS in almost every individu al stud y session (see Table 1, colum n A); so that even if theaverage of M values ( 86 ms) are 'corrected ' by sub tractin g th e average of S values( 47 ms), a small average net M of ab ou t 40 ms still rem ains. (Even for subjectB.D ., sub tracting the average of his mean Ss, + 83 m s, from the average of me an M sof +51 ms in the same 4 sessions, produces a net 'corrected' average Mof about 30 ms.) This produces the unexpected result that reported time of awareness of'actually m ov ing' generally preceded the activation of the muscle at E M G zero time!(See Discussion.)

It is important to note that mean W values were consistently negative to mean Mvalues in the respective session for each subject (Table 1A), in spite of the frequentlynegative values for M . Th e average of all me an W s (ab ou t 200 ms) indicated th at

at


,2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

10/20

632 B E N J A M I N L I B E T A N D O T H E R Sawareness 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 thesame 20 sessions with M values are included, the average of mean Ws was 216instead of 204 ms (Table 2D). M ean W s obtained in sessions when an M series wascarried out before the W series appeared to be significantly more negative than Wsob taine d when a W series was carried out before an M series {see separate averagesin Tab le 2D). A W ilcoxon test for this rankin g ord er gave a one-sided P = 0.038, anda two-sided P = 0.076. (This rank ing w as not related to the use of (A) vs (O) mo de ofrecall in the session.) However, the actual differences of about 50 ms between thetwo sets of W s are relatively unim po rtan t w hen com parin g W times to onset times ofthe correspond ing readiness potentials {see Section III, below).II. Onset Times of Readiness-potentials {RP)

RPs associated with the freely voluntary, self-initiated movements employed inthis study have been described (L ibet et al., 1982). They can be categorized into tw oor three types, based on their form and the time of the main n egative (M N ) shift {seeTables 1 B and 2B). Type II (and III) RPs are obtained when all 40 self-initiatedmovements in the averaged series are reported by the subject to have originated'spontaneously' and 'capriciously' , with no recollections of preplanning experiencesfor any of the 40 events in the series. Ad ditiona l experiences of a 'prepl ann ing ' p haseare associated with type I RPs (Libet et al., 1982). N o significant asso ciation cou ldbe detected between mode of recall for W (that is (A) or (O)) and type of RPobta ined.On set times of R Ps listed in Table 1 B are for R Ps reco rded in the sam e series of 40self-initiated movements for which the reports of W times are given, in eachrespective session for each sub ject. This simu ltaneity, for R P an d W ob servatio ns, isimportant because there can be considerable variations of RP onsets in differentseries even in the s am e session (Libet et al., 1982). Th e actu al R Ps for each W serieslisted in Tab le 1 for the Gr o up 2 subjects (S.B., G.L . and B.D .) are presented in fig. 2.RP s were also o btain ed with each M series in the session, bu t onset times for theseare 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 1 B . (1) Onsettime 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 precededEMG zero t ime.Onset time based on RP area was determined as follows. On an enlarged projected image, the areaund er the R P was me asured for each interval of 50 m s, starting from E M G zero time and progressing tosuccessive intervals in the negative (pre-EM G) direction u ntil 600 ms; between 600 and 1400 ms,area s for 100 ms intervals w ere me asure d. Within each time interval, any areas below the baseline weresubtracted from those above. In estimating total area, however, it was considered advisable to excludeany early brief shifts of pote ntial th at did no t continue progressively into the ma in R P , as some of these

at



Downloadedfrom

7/27/2019 Brain 1983 Libet

11/20

CEREBRAL AND CONSCIOUS TIMES OF VOLITION 633

S.B. G.L. B.D.

5a

5b

6a

6b500 ms

FIG. 2. Readiness-po tentials (RPs) recorded at the vertex and averaged for each (W) series of 40 trials for subjectsS.B., G.L. and B.D. Each R P corresponds to the respective (W) series as listed by session number in Table 1. Thesolid vertical line indicates the EM G zero time, marking the end of the RP. Dashed h orizontal 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 duringthe total time between beginning and end of that series of 40 trials.

could have been artefactual in natur e. Therefore, the rule was adopted that any 200 ms segment havinga total area < 4 m m 2 (actually equivalent to 50 /tV-ms) was to be regarded as zero, and that any and allareas preceding that segment were also regarded as zero. (Making the rule even more stringent, byreducing th e excluding low level segmen t to 100 ms, rarely chang ed the results significantly a nd , when itdid, the final estimates of time of onset were very little different.) It was also recognized thatmeasurements of the beginning of the negative potential shift are subject to some possible error injudging thed.c. ba seline, especially in a somewhat noisy/bumpy tracing. Therefore, after arriving at atotal area for a given RP under the rule above , the time interval that included only 90 per cent of thisarea wascomputed . The 90 per cent values also fit the range of differences between the independentmeasurements of the areas by twodifferent investigators.

at


,2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

12/20

634 B E N J A M I N L I B E T A N D O T H E R SAverages of the onset times for RP M N and RP90 % area, respectively, for the Wsseries in T abl e 1 B are given in Ta ble 2. The initially slower but progressive ramp -likerise of type I RP s accou nts for these onset times being more neg ative for R P M N thanfor R P ^ area. On the other han d, in types II and III R Ps some definitely

distinguishable negativity is often present even before the main (MN) shift. Suchnegativities tend to have a relatively irregular, low, amplitude but there was noreason to regard them as other than actual RP components in these self-initiatedacts (see Libet et al., 1982). Th eir inclusion in the me asure me nts of total area ma kesit possible for onset of R P ^ area to precede onset of RP M N in som e cases. Averagesof the onsets for RP M N and R P , ^ were in fact no t very dissimilar for type II RP s,alth ou gh individual values for the difference (RP M N RIV/J) were in a ran ge between- 207 and + 526 ms; but for type III RP s average R P ^ preceded RP M N by - 247 m s(range of RPM N R P ,^ was between 115 and +5 56 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 app eara nce of the conscious intention to act , on the one ha nd,to th e 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 isgiven for each respective series of self-initiated voluntary acts. Differences arepresented when utilizing the W times as actually reported (W 'uncorrected'), giving(onset R P) minus (W); or the W times 'corrected' by subtracting the reported meantime for the S obtained in the same session, giving (onset RP) minus (W-S).'Co rrec tin g' the W value by subtractin g the S value of each subject's 'b ias' inreporting, did not qualitatively change the relation of RP onset-time to W; rather itgenerally increased the difference by which onset o f R P precedes W (as 'corr ecte d').For subject B.D., his positive values for S have the oposite effect; but even for him,the only qu alitatively i m po rtan t change in the difference is introd uced in session 1,which had a large posit ive S (+ 1 3 5 m s).

It may be seen (Table lc) that, with few exceptions, onset ofRP occurred beforereported awareness time by substantial amounts of time. This was true irrespectiveof which m easure of RP-o nse t or of W is employed to obtain the difference. The sizesan d con sistency of these differences, between on set of RP an d W , indicate they arehighly significant. However, it is difficult to produce a rigorous quantitative valuefor significance of the large differences between onset of RP and W. An SD(stan da rd deviation) for variability amo ng individual RP s within each series of 40 isnot available, as only the average RP for the whole series could be meaningfullyrecorded. C onsequ ently, only the mean W value and the averaged R P ob tained in agiven series can be compared for statistical purposes. Confidence in the significanceof the differences (in Table lc) is further raised by the fact that they were almostinvariably very large when compared to the SEs (standard errors) for the meanvalues of W (Table 1A). In add ition , each W series of 40 trials was examined for theincidence of individual W values tha t may have deviated sufficiently from the m ean



Downloadedfr

om

7/27/2019 Brain 1983 Libet

13/20

CEREBR AL AND CONSCIOU S TIMES OF VOLITION 635W so as to be negative to (precede), rather than positive to (follow) the onset ofaverage R P in tha t series {see Table 1 c) . F or 36 W series of 40 trials each, instan ces inwhich individual W time preceded onset time of averaged RP numbered zero in 26series an d 1 to 3 in 8 series! (Of the rem ainin g 2 series, in both of which R Ps w ererecorded at contra latera l precen tral sites where R P onse ts are often less steep tha n a tthe vertex, 1 had 4 an d 1 had 17 instances. Th e latter large value holds only inrelat ion to onset of M N in a type III RP b ut not if onset R P ^ is used.)

The SE of each mean value for W, as given in Table 1A, is more meaningful in relation to difference of(onset of averaged RP)-(mean W), than is the SD for the distribution of the individual W values in eachseries of 40 trials. (The SD for each series of 40 W values may be calculated using the respective SE,given in Table 1A; SD = V4 0 x [SE].) If the individual RPs were available, the difference between eachRP onset and the respective individual W value could be determined for each event, and a meaningfulevaluation of variability for such individual differences could be made. However, in the absence ofindividual RP v alues, it is reasonable to assume tha t the W and RP-onset for each individual act areprobab ly related in a dependent and positively correlated m anner, in which onset of each R P tends tobe negative to (before) each W . This condition seems likely because (1) W and RP are features of thesame underlying process, (2) observed differences between averaged RP and mean W were consistentlylarge, and (3) there was a nearly complete absence of individual W values that were negative to theaveraged R P {see above).

For reporting each W time, subjects were asked to note the earliest awareness ofthe specific urge o r intention to act which mig ht occ ur prio r to the act. All subjectsreported that they could distinguish readily between this awareness and anyexperience of 'pre-planning' that sometimes occurred in acts associated with type IRPs (Libet et al., 1982). Awarenesses of 'preplanning' were completely absent inseries associated with type II (or III) RPs, in which all 40 self-initiated movementswere 'spo nta ne ou s' in origin. Therefore, it is useful to con sider the values for type II(and III) R P series separately from those for type I RP series, as summ arized in Tab le2A-C. AS might be expected, series with type I RPs generally exhibit an earlier onsetof R P , relative to W , than do those with type II (or III), especially for R P M N onsets.However, even for the series of 'spontaneous' acts with types II and III RPs, onsetsof RP s generally preceded W by subs tantial a m ou nts . The average of the differences[(onset RPM N) minu s (W)] for the 20 series with type II RP s was - 343 ms (Table 2c) .The relatively few sessions in which these differences were possibly not significantoccurred ma inly in association w ith type III RP s, for which onsets of RP M N averagedonly 270 ms. For the 5 sessions with type III RPs, the average difference (onsetRP M N W ) was 87 ms , al though when R P ^ is used, the difference increasedto - 334 m s.

DISCUSSIONIt is clear that n euro nal processes tha t precede a self-initiated volu ntary action , asreflected in the readiness-potential, generally begin substantially before the reportedappearance of conscious intention to perform that specific act. This temporal

difference of several hundreds of milliseconds appeared fairly consistently regardless

atUniversityofPlymouthonJanuary13,

2012


Downloadedfrom

7/27/2019 Brain 1983 Libet

14/20

636 B E N J A M I N L I B E T A N D O T H E R Sof which of the available criteria for onset of RP or for the time of awareness areadopted. Series with type II RPs are of especial interest as all of the 40 self-initiatedacts arise spontaneously; on this and other evidence, the main negative (M N) shiftwith average onset abou t 550 ms was postulated to reflect the cerebral volitionalprocess uniquely involved in initiating a freely voluntary, fully endogenous act(Libet et al., 1982). Even for such series, with type II RPs, onset of RP preceded W byabo ut 350 ms on the average. In series with type I RPs the earlier M N shift (averageonset abou t 1025 ms) appears to reflect a more general preparation or intention toact that can be either endogenous or cued externally; it is not necessarily associatedwith freedom of choosing when to act (Libet et al., 1982). However, actualexperiences of 'prep lanning' were reported for only a minority of self-initiated actsin series with type I RPs. Consequently, the much larger differences between onset oftype I RP and W, on the average as much as 800 ms, may also reflect advancecerebral preparation that is generally accomplished before conscious intentionalityarises. Only in the case of a small number of series with type III RPs was thedifference between RP-onset and W less negative than 100 ms, when onset of M Nshift (average 270 ms) is adopted. But if start of 90 per cent of RP area (average 517 ms) rather than MN shift is taken as the criterion for onset of RP, eventhese type III RP's precede W by an average of more than 300 ms (Table 2c). Inseries with type III RPs, all self-initiated acts were also spontaneous, as in type II(Libet etai, 1982).The validity of the RP as an indicator of cerebral activity had already beenestablished (Kornhuber and Deecke, 1965; Deecke et al., 1976; Pieper et al., 1980).Actually, the onset of R P provides only a minimum timing for initiation of cerebralactivity, as the recorded RP probably represents neuronal activity in a limitedportion of the brain, possibly that of the supplementary motor area in the mesialneocortex (Deecke and Kornhuber, 1978; Eccles, 1982; Libet et al., 1982). It ispossible that cerebral activity is initiated at times earlier than the onset of therecorded RP in some other regions (see Groll-Knapp et al., 1977). For the presentissue, the requirement of averaging 40 individual R Ps for each recordable R P mightraise the question whether this obscures an actually randomized group of widelydifferent onset times. Even in such a case, however, there are several considerationsthat work mathematically against an interpretation that W times are consistentlyequal to or precede onset times of individual RPs. Some of these have been discussedabove (Section III, R esults). An example might assume that the averaged RPs arecontaminated by a small number of unusual individual RPs having very earlyonsets, compared to onsets of most RPs in each series of 40. However, if this weretrue, we would expect that such unusual RPs should not appear in an appreciablenum ber of the 40-trial series, causing the order of times for W and onset of averagedRP to be reversed; but, in fact, the onset of averaged RP preceded mean W in all ofthe 37 W series (Table lc, and see above). Furthermore, in type II RPs the mainnegative shift (MN) has a relatively abrupt onset and rises rapidly; any smallnumber of individual RPs with unusually early onsets could not appreciably affect

at



Downloadedfrom

7/27/2019 Brain 1983 Libet

15/20

C E R E B R A L A N D C O N S C I O U S T I M E S O F V O L I T I O N 637the onset time of RP M N , as measured in type II RPs. Yet, onset of RP M N precededmea n W , usually by large time interv als, in all the 20 W series that ex hibited type IIRPs . To circumvent all these points of evidence one would have to introduceunsu ppo rted assum ptions th at there exist specially biased distr ibutions of R Ps .

As another possibility, it might be proposed that neural activities, represented by individual RPswith randomly variable amplitudes and onset times, must achieve some threshold, whether integrativeor other, before the brain 'decides' to act voluntarily; for this one might apply a kind of 'ra nd om -w alk 'model for sequential decision making (see Audley, 1973). If applied to our case, a delayed appearanceof a 'neural threshold' might coincide in time with the W time, and thus nullify any apparentdiscrepancy between time of cerebral decision and time of conscious intention to act. But a consistentbias or change in the random-walk distribution of the variable neural functions would have to beinitiated at the onset of averaged RP, preceding the achievement of threshold for the decision. Thiswould be analogous to the required initiation and presence of a sensory signal to produce thedistribution of sensory rando m variables that may lead to a yes-no decision in that ran dom -walk model(Green and Luce, 1973). Therefore, such a proposal could, at best, only separate the cerebral initiatingprocess into two stages. An earlier stage, which would precede a final decision stage, would start withsome initiating endogenous trigger. Such a model would not fundamentally affect our conclusion, thatcerebral activity initiates the voluntary act before rep ortable conscious intention appe ars.The Criteria for Time of Con scious Intention to Act

Th e reliability and validity of these opera tion al criteria are of course crucial to theissue of the temporal order of cerebral processes vs conscious intention. Thereliability of the subjects ' reports of'clock-position' for the revolving CRO spot atthe time of awareness appe ars to be fully ad equ ate (see discussion of SEs of mean Wvalues and incidence of individual W values, relative to onset of averaged RP, inSection III of Results).Consideration of validity of our criteria begins with the premise that thesubjective event in question is only introspectively accessible to the subject himself,and that this requires a report by the subject (see Libet, 1965, 1966; Nagel, 1974;Creutzfeldt an d R ager, 1978). An y behav ioura l response th at is no t a direct functionof such a report could not be used as a primary indicator of the subjective event(Libet, 1981a, b), although it might be found to be associated with the subjectiveevent as studied by suitably valid reports. Acceptance of this premise, and of ourspecific operational precedure for the required introspective report, introducesseveral issues that may affect the validity of the reported W time. In particular,factors that may affect the transmission between the subject's introspectiveexperience and his verbal report must be considered.

(1) Simultaneity of judgem ents. Our method requires the subject to observesimultaneo usly, for later repor t, the app eara nce of a mental event (conscious u rge tomove) and the visual clock-position of the revolving spot of light at that time.Reports of simultaneous events have long been known to be subject to potentialerrors, depending on the circumstances (differential attentiveness, in the 'priorentry' phenomenon) and on the individual subject (see Boring, 1957; Efron, 1973;Sternbe rg and K noll, 1973). O ur S expe rimen tal series, in which subjects reportedawareness times for skin stimuli, were designed to serve as controls for potential



Downloadedfrom

7/27/2019 Brain 1983 Libet

16/20

638 B E N J A M I N L I B E T A N D O T H E R Serrors in such 'simu ltaneity' as well as for other individual biases and errors in theentire reporting procedure. Procedures and requirements for subject's attentiveness,observations and later recall, of clock-positions of the revolving CRO spot at thetime of awareness of a random ly appearing skin sensation, were the same as in the Wand M series. But in the S series the actual time of the stimulus was later known tothe investigator, and the error in the subject's reports could be determinedobjectively. The bias or error found in the S series did not qualitatively alter thedifference between onset of RP and W, as determined in a given session for the samesubject; in fact, they generally enlarged th e differences (Table 2c ).(2) Timing of an endogenous mental event. The subject's reported time forspontaneously arising awareness of the urge/intention to move cann ot be directly orobjectively validated in the manner possible for skin stimuli in the S series. Thesubject's report constitutes the prim ary evidence of his introspective experience. Noother independent measure of such subjective timing is available, although some ofthe other available evidence can affect confidence in the validity of the observedtimings; this is summarized below in point (3).

It is of course possible to conceive or postulate conditions that might introduce discrepanciesbetween the actual and the reported initial times of such an awareness. For example, what if it werepossible to judge accurately only the end of the mental event, the conscious urge to move; the actualtime of its on set, in relation to a perceived clock-time, would then be unknown and in doubt. In relationto such a suggestion wenote,first, hat each subject was instructed to 'watch for' and report the earliestappea rance of the awareness in question, and subjects did not raise any difficulties abou t doing this.Secondly, perceptua l timings of onset and offset found for a peripherally induced sensation, at least, donot support the suggestion that only the end is accurately judged. Using a method of cross-modalitysimultaneity judgements, Efron (1973) found that (a) there was no difficulty in distinguishing onsetsfrom offsets; (b) the perceptual onset latency was constant regardless of large changes in duration of thestimulus; whereas (c) perceptual offset latency could in fact vary when stimulus durations wereshortened to < 150 ms, with the change in offset latency probab ly originating, in part, in theperipheral sensory structures. Somewhat related findings on perceptual onset latencies are available fora cerebrally induced men tal event, elicited by a stimulus in the somatosensory system in man (Libet etal., 1979). The m ean differences between perceptual timings, for a brief skin stimulus relative to onsetof a medial lemniscus stimulus lasting 200 ms, were only within a few tens of milliseconds. One mightsuggest that timing of an endogenous mental event, the spontaneously arising conscious intention toact, may be more difficult subjectively to pinpoint with accuracy than the timing even for the sensationelicited by an intracerebral stimulus in the medial lemniscus. The individual W values reported by oursubjects in a given series of 40 events did show variability; but the difference between (onset of RP ) and(mean W), in Table lc, was consistently and considerably greater than the SE of mean W for therespective series. Actually, the converse possibility, of W times reported earlier than the actual time ofawareness of the urge to move, presented a more real difficulty. A 'preawareness' that one is preparing toperform the voluntary act, sometime within the next second or so, does in fact accompany at least someof the events in those series that produce a type I RP, as noted above (Libet et al., 1982). If such apreawareness were to have affected the report, it would mean tha t the reported W times were morenegative, earlier, than they should have been; the difference between onset of RP and 'rea l' W shouldthen be even greater than indicated by our results. However, in series giving type II RPs , all of the self-initiated acts were described as 'spontaneous'; the subjects reported that each urge or wish to actappeared suddenly 'out of nowhere', with no specific preplanning or preawareness that it was about tohappen.

at



Downloadedfrom

7/27/2019 Brain 1983 Libet

17/20

CEREBRAL AND CONSCIOUS TIMES OF VOLITION 639Finally, the possibility could be raised that an earlier nonrecallable phase of theconscious urge exists, one that is not storable as a short-term memory. If it isfurther assumed that the subject's report of W time requires short-term memory ofthe mental event, then the reported time would apply only to a later, recallable phaseof awareness, given such assumptions. First, one should note that, to report W time,the subject needed to recall only the clock-position of the revolving spot at the timehe became aware of the u rge/intention to move, and not necessarily the consciousmental event itself. (Actually, the latter was at least often recallable, as the subjectswere able to describe it, even in relation to experiences just preceding it during thetrial.) Secondly, the proposal of a nonrecallable initial phase of the conscious urge tomove is a hypothetical construct which, like some other potential uncertainties intiming an endogenous mental event, is at present not directly testable. Thus,although it cannot be definitively excluded, it also lacks experimental support.(3) Additional evidence bearing on the validity of the reported timings. One way totest and improve confidence in the validity of the reported timings lies in usingdifferent but converging modes of observing and reporting, with each mode havingindependent validity without further assumptions {see Garner, 1954, for discussionof this approach in connection with a related issue). Two quite different modes wereemployed for reporting the 'clock-positions' of the CRO spot at the time ofawareness, that is, the absolute (A) reading vs the order (O) relative to final spotpositions. Yet both modes produced values for W in the same range and wereessentially indistinguishable. (This also held for reported timings in the M and Sseries.)Subjects definitively distinguished the experience and time of awareness ofwanting to move (W) not only from those of a skin sensation (S) but also fromawareness of actually moving (M). Mean values for W times were consistentlynegative (by > 100 ms on average) to those for M times (Tables 1A, 2D). This wastrue in spite of the unexpectedfinding hat mean M values were themselves generallynegative toEM G zero time, although they were only slightly negative toS values inwhich nomovement was involved. M has some features of an endogenous mentalevent, rather than simply of a sensation elicited by input from peripheral sensorysources (see below, for discussion of M preceding the EMG). On the other hand,

onset-times for type I or II RPs in series asking for W reports were generally similarto those asking forM reports, or even to those in series when no reports of awarenesswere requested (see also Libet et al., 1982). This indicates that the somewhatdifferent mental sets associated with each kind of reporting (or absence of reporting)did not affect the RP side of the time differential with respect to W.It might be argued that subjects' reports of W times could be distorted by theirawareness of the time of the actual movement; this might induce them to report Wtimes that are later than the actual time of appearance of the conscious intention tomove. But the subjects confirmed that for W reports they concentrated on notingtheir earliest awareness of any urge/intention to move. They further stated that theirmental set for 'observing' W time was also different from that for M time.

at



Downloadedfrom

7/27/2019 Brain 1983 Libet

18/20

640 BEN JA M I N LI BET A N D O T H E RSFurthermore, the available evidence indicates that the subjects' experience withattending to the awareness of actually moving (M) may have induced them to reportW times that were somewhat m ore, not less, negative relative to E M G zero time {seeResults, end of Section I). The mean W values were on the average about 50 ms morenegative when, in a given session, the W series was performed after instead of beforean M series {see Table 2 D , etc.).

Awa reness of 'actu ally mo ving' (M) preceded the EM G . Mean M values were generally negative toEMG zero time for most subjects, and consistently though slightly negative (average about 40 ms)relative to S values for all subjects. Tim ing of M so as to precede the activation of muscle contractionindicates that M was not reflecting awareness of proprioceptive sensory impulses elicited by themovement. It suggests the possibility that M reflected an awareness associated more immediately withinitiation of efferent cerebral output for the movement. Components that follow the main negativeRP shift are recordable just prior to movement, including a negative 'motor potential ' that beginsabou t 50 ms before the EM G {see Deecke et al., 1976; Shibasaki et al., 1980).Unconscious and Conscious Initiation or Control of Voluntary Acts

Since onset of RP regularly begins at least several hundreds of milliseconds beforethe appearance of a reportable time for awareness of any subjective intention orwish to act, it would appear that some neuronal activity associated with the eventualperformance of the act has started well before any (recallable) conscious initiation orintervention could be possible. Put another way, the brain evidently 'decides' toinitiate or, at the least, prepare to initiate the act at a time before there is anyreportable subjective awareness tha t such a decision has taken place. It is concludedtha t cerebral initiation even of a spontaneous voluntary act, of the kind studied here,can and usually does begin unconsciously. The term 'unconscious' refers here simplyto all processes that are not expressed as a conscious experience; this may includeand does not distinguish among preconscious, subconscious or other possiblenonrep ortable unconscious processes.A general hypothesis had already been proposed that some substantial timeperiod of appropriate cerebral activity may be required for eliciting all specificconscious experiences (Libet, 1965). This developed out of experimentally basedfindings tha t cortical activities must persist for up to 500 ms or m ore before'neuro nal adequacy ' for a conscious sensory experience is achieved {see Libet, 1966,1973, 1981a; Libet et al., 1972). In that hypothesis, those cerebral activities that didnot persist sufficiently would remain at unconscious levels. The present evidenceappears to provide support for that more general hypothesis. It suggests that asimilar substantial period of cerebral activity may also be required to achieve'neuro nal adequacy ' for an experience of conscious intention or desire to perform avoluntary act.The present evidence for the unconscious initiation of a voluntary act of courseapplies to one very limited form of such acts. However, the simple voluntary motoract studied here has in fact often been regarded as an incontrovertible and idealexample of a fully endogenous and 'freely volun tary ' act. The absence of any largermeaning in the simple quick flexion of hand or fingers, and the possibility of

at

UniversityofPlymouthonJanuary13,

2012


Downloadedf

rom
http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.org/http://brain.oxfordjournals.or

Brain 1983 Libet

Documents

Transcript of Brain 1983 Libet