Looking for the agent: an investigation into consciousness
of action and self-consciousness in schizophrenic patients
, N. Francka,
*, N. Georgieffa,b
, J. Proustc
, J. Daleryd
, M. Jeanneroda
Institut des Sciences Cognitives, CNRS UPR 9075, 8 Avenue Rockefeller, 69373 Lyon, France
Jeune Equipe 1882, Universite´ Lyon-1, Lyon, France
CREA, Paris, France
Hopital du Vinatier, Lyon, France
Institute of Human Physiology, University of Parma, Via Gramsci 14, 43100 Parma, Italy
Received 21 July 1997; accepted 8 October 1997
The abilities to attribute an action to its proper agent and to understand its meaning when it
is produced by someone else are basic aspects of human social communication. Several
psychiatric syndromes, such as schizophrenia, seem to lead to a dysfunction of the awareness
of one's own action as well as of recognition of actions performed by others. Such syndromes
offer a framework for studying the determinants of agency, the ability to correctly attribute
actions to their veridical source. Thirty normal subjects and 30 schizophrenic patients with
and without hallucinations and/or delusional experiences were required to execute simple
finger and wrist movements, without direct visual control of their hand. The image of either
their own hand or an alien hand executing the same or a different movement was presented on
a TV-screen in real time. The task for the subjects was to discriminate whether the hand
presented on the screen was their own or not. Hallucinating and deluded schizophrenic
patients were more impaired in discriminating their own hand from the alien one than the
non-hallucinating ones, and tended to misattribute the alien hand to themselves. Results are
discussed according to a model of action control. A tentative description of the mechanisms
leading to action consciousness is proposed. © 1997 Elsevier Science B.V.
Keywords: Schizophrenia; Hallucinations; Consciousness; Representation of action; Motor
0010-0277/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved
C O G N I T I O N
Cognition 65 (1997) 71-86
* Corresponding author. Fax: +33 478 777286; e-mail: email@example.com
A fundamental problem is raised by the mechanism of how one becomes aware of
one's own actions, and how one's actions are distinguished from those of other
people. This mechanism is critical for a number of reasons. First, the ability to
make a distinction between the acting self and the acting other is one way by
which the self feels distinct from other individuals and self-consciousness is built.
Second, the ability to recognise the meaning of an action when it is produced by
someone else and to attribute it to its proper agent is a basic function underlying
human social communication.
Research on these points has been undertaken within several different frameworks. The study of mental and cognitive development is one of those. Researchers
in this field have set the stage for a renewal of developmental studies, by demonstrating the appearance, in early childhood, of a special ability of the individual to
attribute to others mental states differing from his/her own, and to infer the content
of these mental states from his/her own mental content (the so-called `theory
of mind', Leslie, 1987). The ability to imitate and to learn by observation is
another critical developmental function, which might relate to the same basic
mechanism (Meltzoff, 1995). This highly influential research has recently merged
with other work in normal adult subjects, aimed at determining the neural mechanisms of mental representations, particularly in the context of action generation. The
notions of internal models and internal simulation borrowed from computational
neuroscience (see Wolpert et al., 1995) have now become a conceptual tool for
studying the early stages of action generation, such as intending, for example.
Experiments using brain mapping techniques have shown that merely observing
an action performed by someone else is no less a powerful way of eliciting brain
activity than other cognitive motor states, such as mentally simulating or preparing to execute that same action. These results (see Section 4) raise the point of
the differences and similarities of motor representations activated during these
The study of mental disorders is another possible framework for understanding the problems of consciousness of action and self-consciousness. Some of the
symptoms observed in schizophrenia, specifically the positive symptoms typical
of the acute stage of the disease, seem to be relevant to a dysfunction of the awareness of one's own action and to a dysfunction of recognition of actions performed
by others. One possible, general explanation for this difficulty would arise from
a disability in attributing meaning to external events, itself due to a weakening
of inhibitory processes crucial for consciously focusing attention (see Gray et al.,
1991). Another, more specific, hypothesis is that of Frith, implying that schizophrenics fail in monitoring their willed intentions, including those related to the
expression of thought (Frith, 1987, 1992). They fail to attribute elements arising
from their long-term memory, from which they form their goals and plans, to their
real origin. The consequence of this failure in monitoring makes them unable to
disentangle `intentions' arising from external stimuli, from those generated as a
consequence of their own cognitive functioning. Positive symptoms such as inser-
72 E. Daprati et al. / Cognition 65 (1997) 71-86
tion of thought, hallucinations and delusion of control would directly derive from
Verbal hallucinations seem to be liable to this explanation. According to Frith
(1996), the normal mechanism for attributing thought to its internal origin would be
a comparison between the executive commands leading to speech and the anticipated sensory consequences of these commands. Normally, execution of the speech
motor commands implies that the related sensory signals will be inhibited (Creutzfeldt et al., 1989). During verbal hallucinations in schizophrenic patients, by contrast, the sensory areas for language remain active, which suggests that the
cancellation process does not operate. The nervous system in these patients behaves
as if it were actually processing the speech of an external speaker. Hence their
perception of their own thinking as originating from the outside world (Silbersweig
et al., 1995; McGuire et al., 1996).
In the above situation, the patients attributed their own action to an external
source. This tendency, however, only corresponds to part of the positive symptomatology in schizophrenia. The same patients may experience the reverse as well,
i.e. they may experience the action of others as a consequence of their own intentions.
In the present study we compared groups of normal controls and of patients with
and without positive symptoms, when required to make agency judgements about a
hand movement originating either from themselves or from an experimenter. Our
results showed that patients with hallucinations or delusional control, when faced
with hand movements of uncertain origin (e.g. an alien hand performing the same
movement as them), systematically considered the movements as theirs. This striking result raises the point of which cues are normally used for producing conscious
judgements about the origin of an action.
The purpose of the study was approved by the local ethical committee (CCPPRBLyon). All subjects gave informed consent for participation in the experiment.
Sixty subjects participated, 30 schizophrenic patients and 30 normal control subjects.
2.1.1. Schizophrenic patients
The patients (24 males and 6 females, mean age 34.5) were diagnosed as
schizophrenics according to the DSM-IV. Eighteen received diagnosis of paranoid
schizophrenia, seven of undifferentiated schizophrenia, and five of residual schizophrenia. According to clinical evaluation (Andreasen's scales for assessment
of positive and negative symptoms; Andreasen, 1983, 1984), a clear pattern of
hallucinations was present in 13 patients. Delusion of control was reported
73E. Daprati et al. / Cognition 65 (1997) 71-86
by seven patients. In six patients delusion of control was associated with presence
of hallucinations. All patients were under medication during the period of the
study, and medication averaged 427.2 (SD 320.4) chlorpromazine equivalents. Estimated mean illness duration was 12.3 years. A summary of patients' data is given in
All but four patients were right-handed according to the Edinburgh Inventory
(Oldfield, 1971). Vision was normal or corrected to normal. Patients' mean IQ, as
estimated by the score at the Raven Progressive Matrices PM47, was 91.7. Shortterm memory span was 6.4 and 3.5, forward and backwards, respectively (Wechsler,
1981). Language comprehension, as assessed by the Token Test (De Renzi and
Vignolo, 1962) was good (mean score 33.2/36).
Name Sex Age Handedness Classification
VAC M 23 Mixed Paranoid 6 700 +++ -
BOU M 29 Right-hander Paranoid 2 260 +++ -
VEU M 22 Mixed Residual 1 170 - -
DIF M 21 Right-hander Residual 3 50 - -
PEG M 21 Right-hander Residual 2 360 - -
SEQ M 20 Right-hander Paranoid 4 1500 - +
MAT M 39 Left-hander Paranoid 17 600 ++ -
LAI M 34 Right-hander Paranoid 12 250 ++ -
CAT M 41 Right-hander Paranoid 14 1080 ++ +
MAR F 26 Right-hander Paranoid 9 533 + -
KIZ F 27 Right-hander Undifferentiated 5 150 - -
BUA F 58 Right-hander Undifferentiated 23 170 - -
OUR M 24 Right-hander Paranoid 5 800 - -
ROU M 52 Right-hander Paranoid 32 150 - -
CHA M 37 Left-hander Residual 21 400 - -
THE M 57 Right-hander Undifferentiated 32 700 ++ -
BEN M 44 Right-hander Undifferentiated 17 90 - -
BAR M 42 Right-hander Paranoid 26 500 + +
TRA M 25 Right-hander Undifferentiated 5 400 - -
FER M 35 Right-hander Paranoid 13 400 +++ +
NOU M 21 Right-hander Residual 4 300 - -
CAM F 43 Right-hander Undifferentiated 18 150 - -
OLL F 20 Right-hander Paranoid 5 600 ++ +
TIB M 24 Right-hander Paranoid 1 250 - -
BOJ M 48 Right-hander Paranoid 15 420 +++ +
AUA F 33 Right-hander Paranoid 4 300 - -
MES M 32 Right-hander Paranoid 9 250 + +
BEI M 60 Right-hander Paranoid 27 130 - -
MOU M 41 Right-hander Paranoid 21 825 ++ -
POL M 36 Right-hander Undifferentiated 17 330 - -
74 E. Daprati et al. / Cognition 65 (1997) 71-86
From the experimental group, 17 patients (six with hallucinations, five with
hallucinations and delusion of control, and six without hallucinations nor delusion
of control) could be recruited for a further session in which the integrity of visual
perception was assessed by means of a subset of the Birmingham Object Recognition Battery (BORB; Riddoch and Humphreys, 1993). Four tests investigated the
ability of the patients to process basic properties of visual stimuli, such as length
(line length match test A), size (circle size match test A), orientation (line orientation
match test A) and location (position of gap match test A). In each test a samedifferent judgement on a pair of stimuli is required. Two tests assessed the capacity
of the patients to derive viewpoint-invariant visual information, and to access stored
knowledge about specific items from vision. In the first test, the patient had to
choose which of two pictures represents the same object of a target picture despite
being depicted from a different viewpoint (Minimal Feature View Task), whereas in
the second test the patient had to find which picture belongs to the same category of
the target (Item Match Task). The patients performance on the six tests was well
within the normal range (line match test A = 26,7/30, cut-off point 22; circle match
test A = 27,4/30, cut-off point 19; line orientation match test A = 26,6/30, cut-off
point 18; position of gap match test A = 34/40, cut-off point 24; Minimal Feature
View Task = 24,8/25, cut-off point 18,5; Item Match Task = 31,9/32, cut-off point
2.1.2. Control subjects
Thirty normal subjects (19 males and 11 females, mean age 28.8 years) volunteered for this study. None of them reported evidence of neurological diseases nor
psychiatric problems, as assessed by the Mini International Neuropsychiatric Interview (Lecrubier et al., 1997). All but four subjects were right-handed according to
the Edinburgh Inventory (Oldfield, 1971). Vision was normal or corrected to normal.
Their mean IQ, as estimated by the score at the Raven Progressive Matrices PM47,
was 97.4. Short-term memory span was 6.6 and 4.6, forward and backwards, respectively (Wechsler, 1981). Language comprehension, as assessed by the Token Test
(De Renzi and Vignolo, 1962) was normal (mean score 35.4/36).
2.1.3. Comparison between patients and controls
A t-test for independent samples was carried out in order to verify that control
subjects and schizophrenic patients were sufficiently matched. No difference was
found between the two groups regarding age and IQ. However, patients performed
more poorly than controls at the Token Test (t = 6.909, P 0.0001), and showed a
reduced backward short-term memory span (t = +3.047, P 0.004).
2.1.4. Comparisons within the schizophrenics group
The 13 patients reporting hallucinations did not differ from the rest of the group
regarding age, illness duration, therapy and neuropsychological variables. The seven
patients with experience of delusion of control were significantly more medicated
than the other patients (t = -2.593, P 0.01), their IQ was lower (t = 2.264,
P 0.03) and performance at the Token Test poorer (t = 2.330, P 0.02). Finally,
75E. Daprati et al. / Cognition 65 (1997) 71-86
patients with experience of delusion of control showed a reduced forward short-term
memory span (t = 2.500, P 0.02) with respect to the other ones.
2.2. Apparatus and procedure
Both schizophrenic patients and control subjects were naive as to the purpose of
the experiment. An apparatus inspired from Nielsen (1963) was built, a schematic
representation of which is shown in Fig. 1. Two cabins were located in a dimly-lit
room. Subjects entered the apparatus by one side of the largest cabin
(200 × 200 × 70 cm). The technical devices were hidden to them. Subjects sat on
a comfortable chair in front of a table (height 80 cm). A rectangular mirror (30 × 30
cm) was placed at 40 cm from the plane of the table, and at 35 cm from the subjects'
frontal plane. This mirror (M1 on Fig. 1) was oriented with an inclination of 30° on
the vertical plane. Subjects positioned their right hand below the mirror. The hand
rested either on a support located at approximately 40 cm from the subjects' frontal
plane, or grasped a joystick located at the same position. A camera (distance 150 cm
from the table) connected to a closed-circuit television system filmed the subject's
hand through another mirror (M2). Its image appeared on a TV-screen located on a
shelf, 15 cm over the subject's head, and was reflected by mirror M1. Thus, looking
Fig. 1. Schematic representation of the experimental apparatus. S, subject; E, experimenter; M1, mirror
one; M2, mirror two. Video cameras 1 and 2 were located in separate cabins. They were not visible to the
subjects. Further details in text.
76 E. Daprati et al. / Cognition 65 (1997) 71-86
at the mirror, subjects got the impression that they watched their own hand as
through a window.
Close to the subject's cabin, a second cabin (170 × 170 × 70 cm) was devised
for the experimenter, whose hand was filmed by a second camera. The cabin
was identical to the subjects' one, apart from the fact that no mirror was
present. Two control screens were present in the cabin, one showing the image
of the subject's hand, the other, of the experimenter's hand. This display allowed
the experimenter to exactly match the image of her hand with that of the
subject's hand before the beginning of each trial. A special switch allowed the
TV-screen to present either the image of the subject's hand or that of the
experimenter. The experimenter's and the subject's hands were covered with
identical gloves, in order to minimise the effects of gross morphological differences.
The task for the subjects was to perform a requested movement with their
right hand, and to monitor its execution by looking at the image on the mirror.
The head was free to move during the experiment, and no constraints were imposed on eye-movements. Before the experiment, participants were allowed
to freely move the hand below the mirror, until they were familiar with the apparatus. Next, six practice trials were run before the experimental session itself started.
The procedure was as follows: at the beginning of each trial the screen was dark
and no image was presented. Then, an advance instruction was given as to which
movement to execute, and the image of a hand appeared on the screen. At the same
time, a beep produced by a PC was the signal for the subject and the experimenter to
execute the requested movement. Once the movement was performed, the screen
returned to darkness, within about 1 s, and a question was asked to the subject. The
question said: `You have just seen the image of a moving hand. Was it your own
hand? Answer YES if you saw your own hand performing the movement you have
been executing. Answer NO in any other case, that is if you doubt that it was your
own hand or your own movement.'
Two sets of movements were tested: finger set and wrist set. In the finger set the
hand was resting on the support, fist closed. One of four possible movements of the
fingers was required in each trial: 1. extend thumb, 2. extend index finger, 3. extend
index and middle finger, 4. open hand wide. In the wrist set the hand grasped a
joystick, holding it in the vertical position. One of four possible wrist rotations was
required in each trial: 1. forward, 2. backward, 3. left, 4. right.
In each set of movement, one of three possible images of the hand could be
presented to the subjects in each trial: 1. their own hand (condition: Subject), 2.
the experimenter's hand performing the same movement (condition: Experimenter
Same), 3. the experimenter's hand performing a different movement (condition:
Twelve trials were run for each hand condition, that is 3 trials for each movement
type. All together, 36 trials were run for each set of movements. Order of set
presentation was counterbalanced across subjects, whereas movement type and
hand presented were randomised in each set.
77E. Daprati et al. / Cognition 65 (1997) 71-86
2.3. Data analysis
Verbal responses were recorded for analysis. Each time subjects and patients did
not recognise their own hand, or mis-attributed the alien hand to themselves, an error
was recorded. The number of erroneous responses was submitted to analysis. Due to
the peculiar nature of the tested variable and to the small number of trials, nonparametric analysis was used to test the interactions between subjects in the different
experimental conditions. The Kruskal-Wallis analysis of ranks was used for between
groups comparisons, whereas the Friedman's two-way analysis of variance, and the
Wilcoxon's matched pairs test were used to test differences between conditions.
Pairwise comparisons were assessed by means of the Mann-Whitney U-test for
comparisons of groups and Wilcoxon's matched pairs test for comparisons of conditions.
Both control subjects and patients well understood the task requests. Errors in the
procedure were not frequent (controls 0.37%; patients 1.02%), and included execution of a movement different from the required one, movement start before the gosignal, additional movements or submovements during, or immediately after the
required one. The trial was repeated each time one of these errors occurred.
3.1. Presence/absence of hallucinations
Descriptive analyses showed that overall hallucinating patients made more recognition errors (median value: 24) than non-hallucinating patients (11), and controls
(5). The Kruskal-Wallis test on total error number revealed that the difference
between groups was significant (P = 0.00001). Subsequent pairwise comparisons
performed using the Mann-Whitney U-test showed that both groups of patients
produced significantly more errors than the control group (control vs. non-hallucinating P = 0.0135; control vs. hallucinating P = 0.000009). The two groups of
patients differed from each other (P = 0.031).
At a descriptive level, no difference was evident in distribution of errors in either
the finger or the wrist set for each group of subjects (median value - controls: finger
set 3, wrist set 3; non-hallucinating patients: finger set 6, wrist set 6; hallucinating
patients: finger set 11, wrist set 12). A Wilcoxon's matched pairs test confirmed that
errors occurred in the same proportion in the two sets of movements in all subject
populations (comparison finger set vs. wrist set, controls P = 0.338; non-hallucinating P = 0.816; hallucinating P = 0.102). Therefore, the number of errors recorded in
the two sets of movements were pooled for subsequent analysis.
Errors occurred especially in the condition Experimenter Same, in all groups
(controls P = 0.000001; non-hallucinating P = 0.000001; hallucinating P =
0.00002 in the Friedman test). In the trials when the hand on the screen performed
the same movement that was required from the subject but was not his/her own, the
78 E. Daprati et al. / Cognition 65 (1997) 71-86
median for error rate was 5 in the control group, 11 in the non-hallucinating group
and 22 in the hallucinating group, whereas virtually no judgement errors occurred
when subjects saw their own hand, or a hand performing a different movement.
Differences between the three groups for the condition Experimenter Same showed
a strong trend to significance (comparison controls vs. non-hallucinating P = 0.06;
controls vs. hallucinating P = 0.00006; non-hallucinating vs. hallucinating P = 0.05
in the Mann-Whitney U test). Results are summarised in Table 2a and Fig. 2a.
3.2. Presence/absence of delusion of control
Descriptive analyses showed that overall patients experiencing delusion of control made more recognition errors (median value: 24) than the rest of the patients
(18), and controls (five). The Kruskal-Wallis test on total error number revealed that
the difference between groups was significant (P = 0.0001). Subsequent pairwise
comparisons performed using the Mann-Whitney U-test showed that both groups of
patients produced significantly more errors than the control group (control vs. nondelusional P = 0.0006; control vs. delusional P = 0.0006). The two groups of
patients did not differ from each other (P = 0.3).
At a descriptive level, no difference was evident in distribution of errors in either
the finger or the wrist set for each group of subjects (median value - controls: finger
set 3, wrist set 3; non-delusional patients: finger set 9, wrist set 8; delusional
patients: finger set 11, wrist set 12). A Wilcoxon's matched pairs test confirmed
that errors occurred in the same proportion in the two sets of movements in all
subject populations (comparison finger set vs. wrist set, controls P = 0.338; nondelusional P = 0.903; delusional P = 0.338). Therefore, the number of errors recorded in the two sets of movements were pooled for subsequent analysis.
Errors occurred especially in the condition Experimenter Same, in all groups
(controls P = 0.000001; non-delusional P = 0.000001; delusional P = 0.002 in the
Friedman test). In the trials when the hand on the screen performed the same movement that was required from the subject but was not his/her own, the median for
Mean values (standard deviations) for error rate in control subjects and schizophrenic patients. Patients
have been divided in two groups according to symptoms
Subject Exp. same Exp. diff. Subject Exp. same Exp. Diff.
Controls 0.07(0.36) 3.83(3.23) 0.00(0.00) 0.10(0.30) 3.50(3.49) 0.00(0.00)
Non-hallucinating 0.65(1.27) 6.53(4.27) 0.12(0.48) 0.29(0.77) 5.65(4.73) 1.23(2.54)
Hallucinating 0.69(1.70) 9.23(3.96) 0.23(0.83) 0.61(1.12) 9.31(2.98) 1.61(2.81)
(b) Delusion of control
Controls 0.07(0.36) 3.83(3.23) 0.00(0.00) 0.10(0.30) 3.50(3.49) 0.00(0.00)
Non-delusional 0.74(1.60) 7.30(4.26) 0.22(0.74) 0.43(1.04) 6.30(4.46) 1.74(2.89)
Delusional 0.43(0.79) 9.00(4.47) 0.00(0.00) 0.43(0.53) 10.29(2.63) 0.29(0.76)
79E. Daprati et al. / Cognition 65 (1997) 71-86
Fig. 2. Number of errors reported for control subjects and schizophrenic patients (median values). a.
Schizophrenic patients have been divided in two groups according to the presence (n = 13) or absence
(n = 17) of hallucinations. b. Schizophrenic patients have been divided in two groups according to the
presence (n = 7) or absence (n = 23) of delusion of control.
80 E. Daprati et al. / Cognition 65 (1997) 71-86
error rate was 5 in the control group, 17 in the non-delusional group and 23 in the
delusional group, whereas virtually no judgement errors occurred when subjects saw
their own hand, or a hand performing a different movement. Differences between
the three groups for the condition Experimenter Same showed a strong trend to
significance (comparison controls vs. non-delusional P = 0.006; controls vs. delusional P = 0.001; non-delusional vs. delusional P = 0.07 in the Mann-Whitney Utest). Results are summarised in Table 2b and Fig. 2b.
In the present experiment, normal control subjects were able to unambiguously
determine whether the moving hand seen on the screen was theirs or not, in two
conditions. First, when they saw their own hand (trials from the condition Subject),
they correctly attributed the movement to themselves. Second, when they saw the
experimenter's hand performing a movement which departed from the instruction
they had received (condition Experimenter Different), they denied seeing their own
hand. By contrast, their performance degraded in the condition Experimenter Same,
that is, in trials where they saw the experimenter's hand performing the same movement as required by the instruction: in this condition, they misjudged the hand as
theirs in about 30% of cases.
It is also in this particular condition that the rate of incorrect responses increased
in schizophrenic patients. The error rate amounted to 77% in the group of patients
with hallucinations or 80% in the group with delusional experiences, whereas in the
non-hallucinating group, it was around 50%. The fact that all patients gave nearly
correct responses in the other two conditions (the error rate remained within 1-7%)
shows that the effect observed in trials Experimenter Same was not due to factors
unrelated to the task, such as lack of attention, perceptual deficits or a perseverative
bias when task demands increase. Seventeen patients accepted to undergo a further
session of testing in which their basic perceptual abilities as well as their capacity to
make same-different judgements was examined. In a subset of the BORB (Riddoch
and Humphreys, 1993), the patients were asked to perform fine-grained analysis of
the physical features of the stimulus in order to accomplish the task. All the patients
performed well within the normal range in these tests. That is, their ability to process
basic properties of visual stimuli, as well as to derive viewpoint-invariant information was unimpaired. Moreover, even when the task was of a remarkable complexity, no bias towards one type of response was found. Therefore the explanation for
the peculiar distribution of the responses in the agency attribution experiment should
be found in a deficit of the mechanism which is normally used for controlling and
recognising one's own movements.
The discussion below examines several hypotheses for explaining the specific
increase in error rate observed in the condition Experimenter Same. The first hypothesis relates to the corollary discharge model already alluded to in the introduction.
According to this model (Sperry, 1950; von Holst, 1954), a comparison is made
within the central nervous system, between an internally generated signal (which
81E. Daprati et al. / Cognition 65 (1997) 71-86
reflects the desired movement) and other central signals, anticipating the consequences of this movement. The study of visuomotor behaviour in normal subjects
clearly supports the existence of such a mechanism for accounting for the generation
of fast and automatic correcting movements during reaching at small targets, for
example (see Jeannerod, 1988 for review).
The pattern of responses that we recorded in our condition Experimenter Same
(in both normal controls and schizophrenics) could be explained by the paucity
of movement related cues available to the comparator mechanism. As the subject's (invisible) hand and the experimenter's (visible) hand both executed the
same movement, no mismatch occurred between the anticipated and the perceived final hand postures. The only available cues were the dynamic signals
generated during the movements themselves. Slight differences in timing and kinematic pattern between the intended movement and that perceived by visual and
kinaesthetic channels had to be used by the comparator in order to give the correct response. A decrease in sensitivity of this mechanism would explain the greater
difficulties met by the schizophrenic patients. If this explanation were correct,
however, the same patients should have difficulties in other visuomotor tasks involving a comparison between an intended movement and reafferent visual cues.
Frith and Done (1989) reported absence of corrections in schizophrenics in tracking tasks, but only when visual feedback from the manipulandum was precluded
(see also Malenka et al., 1982). This result shows that the patients were unable
to match internal cues about movement direction with the anticipated outcome,
but not that they were unable to use actual reafferent signals from the movement.
In fact, a recent experiment performed by Fourneret et al. (unpublished data)
shows that schizophrenic patients are not impaired in making accurate corrections
in a difficult visuomotor task involving a conflict between the visual and
the kinaesthetic reafferences from their movement (see also Pitblado et al.,
The Frith model of schizophrenia (Frith, 1987, 1992) places the main emphasis on
the disturbance of automatic processes as responsible for the failure of patients in
making a conscious distinction between their own actions and those performed by
others. In fact, there is evidence that automatic processes, such as implicit memory,
priming etc., are usually left intact in schizophrenic patients (Callaway and Naghdi,
1982; Gras-Vincendon et al., 1994), whereas conscious cognitive processes (explicit
memory, action planning) are known to be more severely affected (e.g. Gray et al.,
1991). Accordingly, the present experiment was deliberately undertaken within a
different conceptual framework, that of a dissociation between different behavioural
modalities. There is abundant evidence in normal subjects that actions can be adequately performed without being consciously monitored. Subjects may correct
visuomotor errors toward targets that they have not perceived (e.g. Bridgeman et
al., 1981; Goodale et al., 1986). They may also make perceptual judgements on
visual objects, that are in contradiction with their movements directed towards the
same objects (e.g. Gentilucci and Negrotti, 1994; Gentilucci et al., 1996). More
recently, Fourneret and Jeannerod (unpublished data) reported in normal subjects
a similar dissociation between correct automatic tracking in a visuomotor task and
82 E. Daprati et al. / Cognition 65 (1997) 71-86
erroneous conscious reports on what the subjects thought their hand had done to
achieve these corrections.
These two levels of processing action-related events refer to widely different
behavioural modalities (Jeannerod, 1997). Generating an action in the visuomotor
context (reaching a predetermined visual goal, for example) implies that the representation of the action is transferred into execution mechanisms. Thus, the spatial
(extrinsic) co-ordinates of the goal have to be transformed into a set of commands
coded into a body-centred (intrinsic) frame of reference. This operation is the condition for the effector (e.g. the arm) to reach the goal. Such an operation is likely to
result from a comparison, within the subject, between the internal model of the
action and its sensory outcome. According to the above corollary discharge
model, it is sufficient for the motor system to monitor the degree of match between
the motor commands (representing the internal model of the goal) and peripheral
signals arising from the movement, and to set a threshold beyond which a correction
has to be produced. Note that this mechanism has to remain `private' to be effective,
for the reason that the reafferent signals can only be adequately used by the comparison mechanism if they refer to a movement which has been produced by the
Generating a judgement on an action, on the other hand, is distinct from execution
mechanisms. It relies on a comparison between the internal model of the goal and
the perceptual (mostly visual) configuration of the environment. This comparison
therefore does not necessitate transforming the co-ordinates of the goal into an
intrinsic, motor code. Indeed, the visual configuration may arise from several
sources and can result from an action performed by the observer, but also by
other people. The problem is therefore, not only to determine whether the intended
action was correctly achieved or not (which is the role of the above automatic
mechanism), it is to determine where the configuration resulting from the action
originated from. This perceptual judgement is likely to be made on the basis of
central representations coded in a set of co-ordinates used for perception, that is, coordinates that relate to the external environment. At variance with what happens
with the above `private' mechanism, this set of co-ordinates will be shared by other
agents and/or observers attending the same visual scene (see Frith, 1995).
This notion of shared representations therefore raises the key problem of the
present experiment: how are representations pertaining to actions from different
origins (and from different agents) distinguished from one another? And how are
actions attributed to their veridical source? These critical issues have been raised in
recent PET experiments in normal subjects. Conditions where subjects were intending actions and preparing for execution, mentally simulating actions, or observing
actions performed by other people were compared (Decety et al., 1994, 1997;
Grafton et al., 1996; Rizzolatti et al., 1996a; Stephan et al., 1995). The outcome
of these studies is two-fold: First, there exists a cortical network common to all
conditions, to which the inferior parietal lobule (area 40), the ventral premotor area
(ventral area 6), and part of SMA contribute; second, motor representations for each
individual condition are clearly specified by the activation of cortical zones which
do not overlap between conditions. Area 4 is one of those: it is activated during
83E. Daprati et al. / Cognition 65 (1997) 71-86
motor execution and motor imagination, not during observation. Conversely, prefrontal areas in the middle and inferior frontal gyri (areas 45 and 46) are strongly
activated during simulation and observation, but not during execution. In addition,
even within overlapping zones, there are indications that both the extent and the
precise topography of the activated areas may differ between conditions: This seems
to be the case at least for SMA, where the activation during imagination is more
rostral than during execution (Picard and Strick, 1996).
One of these cortical zones, located in the posterior and inferior part of the frontal
lobe at the junction between the precentral and the inferior frontal gyri (the frontal
operculum) deserves special attention. According to Rizzolatti et al. (1996b), this
area, at the border between the ventral part of area 6 and areas 44 and 45, is
homologous to a monkey area (area F5) involved in a particular form of movement
representation. Neurones herein are activated whether the monkey executes a given
hand movement or watches the same movement performed by another monkey. This
property illustrates the close relationships of the two modalities of motor representation for intending and recognising actions, respectively. In humans, the ability of
making agency judgements could rely on a similar mechanism, provided additional
activity from other cortical areas would signal the intentional state of the agent. This
activity would be the cue for referring the perceptual configuration resulting from
the action to an internal origin, when it is present, or to an external source, when it is
It remains to be determined how the difficulties met by schizophrenic patients in
recognising movements of uncertain origin is compatible with this framework. In
our condition Experimenter Same, normal subjects misjudged the ownership of the
experimenter's hand in about 30% of the trials. This finding suggests that the
mechanism for recognising actions and attributing them to their true origin operates
with a relatively narrow safety margin: in conditions where the visual cues are
degraded or ambiguous, it is barely sufficient for making correct judgements
about the origin of action, although it remains compatible with correct determination
of agency in everyday life.
Deterioration of this mechanism leads to a situation where agency is systematically misjudged, as was the case in our patients with hallucinations and delusion
of control. The consequence of this situation is attributing to oneself actions that
pertain to someone else. Different degrees of deterioration would account for different degrees of severity of the symptoms created by mis-attributions of action. In
the present case, patients failed to detect presentation of the alien hand only in the
condition Experimenter Same. Virtually no errors were recorded when the patients
saw their own hand. Clinicians report that most hallucinating patients show a tendency to incorporate external events in their own experience, or to interpret environmental cues as specifically directed to themselves. The peculiar error pattern that
emerged from the present experiment is consistent with such a behaviour.
At present there is no direct evidence as to the nature of the mechanism, the
alteration of which would create these problems in schizophrenic patients. If the
above hypothesis that attribution of an action to the self or to another person is made
on the basis of specific cortical activation patterns, then a form of comparison by
84 E. Daprati et al. / Cognition 65 (1997) 71-86
`subtraction' between activation patterns in different conditions could be used to
detect neural signals originating from non-overlapping areas. In schizophrenia, by
contrast, the cortical activation related to cognitive motor states would become less
selective, with the consequence that intending, observing or imagining an action
would lead to indistinguishable patterns.
We thank Professor M. Marie-Cardine and Drs. T. d'Amato, J. Furtos, G. Burloux, J. Dubuis, A. Parriaud and B. Stalloni for referring the patients to us. Mr. P.
Monjaud and M. Dittmar were involved in building the apparatus. J.L. Borach
provided photographic assistance. We also gratefully acknowledge the thoughtful
comments of G. Rizzolatti (Parma) and P. Livet (Aix-en-Provence) on an earlier
version of this paper. Work supported by a grant from GIS Sciences de la Cognition,
Paris. E.D. was supported by a fellowship from ESF, Strasbourg. N.G. is associated
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