Electrodermically Nonresponsive Schizophrenia
Patients Make More Errors in the Stroop
Color Word Test, Indicating Selective
Attention Deficit
by Eleomar Ziglia Lopes'Machado, Jose Alexandre de Souza Crippa,
Jaime Eduardo Cecdio Hcdlak, Francisco Silveira Quimarcies,
and Antonio Waldo Zuardi
identify homogeneous subgroups based on biological
parameters (Buchsbaum and Haier 1983). In this regard,
the study of attention and information processing may be
useful, because neurocognitive deficits have been consistently described in schizophrenia patients (Bellak 1994;
Mirsky et al. 1995).
The electrodermal response to sensory stimulation has
been used for the study of attention, orientation, and information processing (Bernstein 1981; Ohman 1981; Dawson
and Nuechterlein 1984). Exteroceptive stimuli induce a
sharp increase in skin conductance as part of the orienting
response; that increase is due to sympathetic activation of
the sweat glands. It is well known that 40 to 50 percent of
schizophrenia patients do not show a significant electrodermal response to an innocuous auditory stimulus
(Ohman 1981; Hazlett et al. 1997; Perry et al. 1998; Lim
et al. 1999). The remaining patients do react to tones and
present a higher level of autonomic activity. As a result,
schizophrenia patients may be classified into electrodermally nonresponsive and responsive, respectively (Gruzelier and Venables 1972; Straube 1979; Ohman 1981; Dawson etal. 1994).
The SCWT (Stroop 1935) is a cognitive test often
used for the study of selective attention (Barch et al. 1999;
Grapperon and Delage 1999). It has been shown that
schizophrenia patients perform less well than healthy controls on the SCWT (Abramczyk et al. 1983; Hepp et al.
1996; Schooler et al. 1997; Velligan et al. 1997). This has
been attributed either to a deficit of inhibitory processes
underlying selective attention (Everett et al. 1989) or to a
working memory deficit due to prefrontal dysfunction
(Goldman-Rakic and Selemon 1997; Cohen et al. 1999).
Abstract
This study investigated whether skin conductance
responsivity is associated with selective attention
assessed by the Stroop Color Word Test (SCWT) in
schizophrenia patients. The subjects (31 schizophrenia
patients and 20 patients with other psychotic diagnoses) were selected from among inpatients of a general hospital psychiatric ward or day hospital attendees. They were matched with 31 healthy volunteers.
The patients began experimental sessions immediately
after remission of an acute episode. The three groups
of participants were subdivided according to electrodermal responsivity into nonresponsive (NR) and
responsive (R) groups. After the psychophysiological
recording, the SCWT was applied. Results indicated
that on the SCWT, the error interference of the NR
schizophrenia group was significantly higher than that
of all the other groups. Furthermore, the NR schizophrenia patients had significantly more negative symptoms than the R schizophrenia patients. These results
suggest that there is a homogeneous subgroup of schizophrenia patients characterized by low neurovegetative responsiveness to external stimuli, predominance
of negative symptoms, and selective attention deficit.
Keywords: Electrodermal activity, skin conductance, Stroop Color Word Test, selective attention,
schizophrenia, negative symptoms.
Schizophrenia Bulletin, 28(3):459-466,2002.
Current diagnosis of schizophrenia relies on many different symptoms. Although adequate for diagnostic purposes,
this comprehensive view may hamper etiological research,
because the variety of symptoms joined together under the
label of schizophrenia may have different underlying
mechanisms (Andreasen 2000). An alternative outlook is
to assume the heterogeneity of schizophrenia and try to
Send reprint requests to Dr. E.Z. Lopes-Machado, Departamento de
Neurologia, Psiquiatria e Psicologia Mddica, Faculdade de Medicina de
Ribeirao Preto, USP, Av. Nove de julho 980, Ribeirao Preto, Sao Paulo,
Brazil CEP: 14025-000; e-mail: [email protected].
459
Schizophrenia Bulletin, Vol. 28, No. 3, 2002
E.Z. Lopes-Machado et al.
The former hypothesis may be questioned, because schizophrenia patients are slow responders in simple word
reading or color naming, which do not involve stimulus
inhibition (Everett et al. 1989; Liddle and Morris 1991).
This observation suggests lessened responsitivity to external stimuli in general rather than impairment of inhibitory
mechanisms.
Studies with magnetic resonance imaging (MRI) in
healthy subjects have demonstrated that the decrease in
volume of the prefrontal cortex is associated with reduction in the skin conductance orienting response (Raine et
al. 1991; Tranel and Damasio 1994). These studies are
consistent with earlier studies about the effect of frontal
lesions on the electrodermal activity in both human
patients and laboratory animals (Bagshaw et al. 1965;
Grueninger et al. 1965).
Using MRI, Lencz et al. (1996) showed that the volume of the prefrontal cortex is reduced in electrodermally
nonresponsive schizophrenia patients, when compared to
either healthy subjects or controls with other psychiatric
diagnoses. Because lower performance on the SCWT has
been related to prefrontal impairment (Goldman-Rakic
and Selemon 1997; Cohen et al. 1999), nonresponsive
schizophrenia patients may be hampered in this cognitive
test. To verify this hypothesis, the present study evaluates
nonresponsive schizophrenia patients on the SCWT, comparing their performance to that of responsive schizophrenia patients, healthy subjects, and patients with other psychiatric diagnoses under antipsychotic medication
(medication control).
Method
Subjects. The following groups took part in this study:
31 schizophrenia patients, 20 patients with other psychiatric diagnoses, and 31 healthy controls. They were
assessed through the Structured Clinical Interview for
DSM-IV (First et al. 1997), patient (SCID-P) or nonpatient (SCID-NP) version, which has been translated into
Portuguese and evaluated by Del-Ben et al. (2001). The
patients were under care as inpatients, semi-inpatients, or
outpatients. All of them were taking neuroleptics. The
diagnoses for nonschizophrenia patients were depressive
disorder with psychotic symptoms (6), bipolar disorder—
mania episode (5), schizoaffective disorder (4), brief psychotic disorder (1), delusional disorder (1), somatomorphic disorder (1), and borderline type personality disorder
(2). For the healthy control group, volunteers were
recruited out of the hospital staff. The two control groups
matched the schizophrenia group in sex, age, hand dominance, and schooling. The subjects' demographic and
clinical characteristics are shown in table 1.
The following were the exclusion criteria: having a
suspicion of brain lesion, having a history of serious drug
or alcohol abuse, having a hearing deficit, stammering,
being unable to understand instructions on the SCWT, and
having first degree relatives with a history of psychiatric
disorder (in the healthy controls).
The Research Ethical Committee from the Clinical
Hospital of Ribeirao Preto approved the study. The subjects and the relatives responsible for the patients signed a
form of consent after being informed about the research
procedure.
General Procedure. The patients included in the study
were interviewed weekly by two psychiatrists. They used
the Brief Psychiatric Rating Scale (BPRS, Overall and
Gorham 1962) in the version by Bech et al. (1986), translated to Portuguese and validated by Zuardi et al. (1994).
In this version the items are graded in five severity levels,
from 0 (absent) to 4 (present in severe or extreme degree).
The patients were submitted to the experimental session
only when their score was lower than 2 on all the BPRS
items, except for negative symptoms (emotional withdrawal, psychomotor retardation, affective bluntness, lack
of cooperation, disorientation, and confusion). The interviews were performed using a structured interview guide
(Crippa et al. 2001) that has been shown to enhance
Table 1. Demographic and clinical characteristics of experimental subjects
Schizophrenia
patients (n = 31)
Other psychotics
(n = 20)
Healthy controls
(n = 31)
Statistics
20/11
11/9
20/11
X2 = 0.56, ns
Handedness (r/l)
29/2
19/1
29/2
X2 = 0.05, ns
Age, mean (SD)
30.4(10.1)
27.8 (8.2)
30.1 (9.7)
F = 0.53, ns
7.8 (2.7)
7.2 (2.5)
8.6 (2.2)
F = 2.00, ns
400.4(172.4)
301.0(216.4)
—
t= 1.80, ns
1.87 (2.4)
1.2(1.9)
—
f = 1.10, ns
Parameter
Gender (m/f)
Schooling, mean (SD)
Antipsychotic (CPZ, mg), mean (SD)
Biperiden (mg), mean (SD)
Note.—CPZ = chlorpromazine equivalents; ns = nonsignificant; SD = standard deviation.
460
Selective Attention Deficit
Schizophrenia Bulletin, Vol. 28, No. 3, 2002
BPRS test-retest reliability. This precaution was taken
because in the acute stage some nonresponsive schizophrenia patients become electrodermally responsive
(Dawson et al. 1994; Hazlett et al. 1997).
At the beginning of the experimental session, patients
were reassessed by the BPRS and evaluated by the negative subscale from the Positive and Negative Syndrome
Scale (PANSS, Kay et al. 1987). After that, the electrodermal recording was performed and the SCWT applied.
subtests: reading black printed words (Card 1) and naming colors of printed words with nonmatching colors and
words (Card 2), according to the procedure described by
Liddle and Morris (1991). A short practice period preceded the testing for each card to ensure that participants
understood the instructions and were performing the task
appropriately. Error criteria were breaking the sequence of
words or trying to correct him- or herself in Card 1; and
reading words, breaking the sequence of colors, naming a
different color, or trying to correct him- or herself in Card
2. Each card consisted of 100 stimuli distributed in 20
lines and 5 columns. Subject answers were recorded and
assessed independently by two evaluators (J.E.C.H. and
E.Z.L-M.) who were unaware of the symptom scores,
diagnostic status, and experimental groups. The number
of errors made and the time to complete each part of the
test were recorded. The intraclass correlation coefficient
between the two evaluators was 0.99 for Card 1 and 0.96
for Card 2. When there was disagreement between the
evaluators, they replayed the subjects' answers and came
to a consensus.
Electrodermal Activity. The experimental session was
carried out in a sound-attenuated room under dim light.
Researcher observed the subjects through a unidirectional
mirror. Subjects were asked to sit still in a comfortable
armchair and told that they would listen to some tones
through headphones. After a rest period of 5 minutes,
stimulus presentation started. This consisted of ten 1-second tones of 80 dB, 800 Hz, and 10 ms rise time, which
were presented binaurally at pseudorandom intervals
(40-80 seconds, mean 60 seconds). Following Venables
and Christie's (1980) recommendation, skin conductance
was measured between the medial phalanges of the index
and middle fingers of the right hand in right-handed subjects and of the left hand in left-handed subjects, by
means of standard silver/silver chloride electrodes.
Recording was made through a computerized physiological recording system.
The number of skin conductance responses (SCRs)
elicited by the stimuli was recorded, as well as the skin
conductance level (SCL) between adjacent stimuli and the
number of spontaneous fluctuations (SFs) of skin conductance. An SCR was defined as occurring within a latency
window of 1-5 seconds poststimulus (Gruzelier and Venables 1972) and having at least 0.05 JJLS amplitude (Venables and Christie 1980). Similar deflections beyond this
interval were recorded as SFs. An SCR nonresponder was
defined according to a strict criterion (Dawson et al.
1994): one who did not give an SCR to any innocuous
tones.
Analysis of Results. Each diagnostic group was subdivided according to the electrodermal response to tones
into two subgroups, namely NR (no SCR to any of the ten
sound stimuli) and R (at least one SCR). This resulted in
six experimental groups, as follows: schizophrenia
patients (SCHIZ-R and SCHIZ-NR), other psychotics
(OTHERS-R and OTHERS-NR), and healthy controls
(HEALTHY-R and HEALTHY-NR).
To assess the Stroop effect, the error interference
(number of errors on Card 2 minus number of errors on
Card 1) and the time interference (time spent reading Card
2 minus time spent reading Card 1) were analyzed (Barch
et al. 1999).
Comparisons of the demographic and clinical data
among groups were done with the y} test for nominal variables, or with one-way analysis of variance (ANOVA).
Comparison between two groups was done with the Student t test. The SCWT and the skin conductance data were
analyzed by two-way ANOVA (factors: diagnosis and skin
conductance responsivity), then Duncan's test. Correlation
coefficients between SCWT error interference and the
number of elicited SCRs were calculated using Spearman's rank correlation analysis. The statistical analysis
was performed through the Statistical Package for Social
Science (SPSS) program, version 6.0, and the significance
level was/? = 0.05.
SCWT. The test was administered in a standardized fashion in a single session of about 20 minutes by one of the
authors (J.E.C.H. or E.Z.L-M.) who was blind to the SCR
data. Subjects were tested individually in a sound-free
room. The procedure was similar to that described by
Abramczyk et al. (1983). They sat in front of a table,
where the card was displayed. Care was taken to ensure
that there was optimal lighting with no glare. Subjects sat
at a distance where the words could easily be read.
Subjects were asked to maintain visual scanning of the
lists, switching from item to item and from column to column and selecting the stimulus to be named from the card
(Perlstein et al. 1998). The SCWT was divided into two
Results
Table 2 shows the demographic and clinical characteristics of schizophrenia patients, patients with other psy-
461
Schizophrenia Bulletin, Vol. 28, No. 3, 2002
E.Z. Lopes-Machado et al.
Table 2. Demographic and clinical characteristics of experimental subjects divided according to
electrodermic responsivity
Schizophrenia Patients
(n = 31)
Other Psychotics
07 = 20)
Healthy Controls
(n == 31)
Statistics
R
NR
R
NR
R
NR
12
(38.7)
19
(61.3)
10
(50.0)
10
(50.0)
22
(71.0)
9
(29.0)
X2 = 6.70
p < 0.05
Gender (m/f)
10/2
10/9
7/3
4/6
16/6
4/5
X2 = 7.70
ns
Age, mean (SD)
32.3
(7.4)
29.2
(11.5)
26.1
(7.6)
29.4
(8.8)
29.3
(8.7)
32.0
(12.3)
F 5 7 6 =0.58
ns
Schooling, mean (SD)
8.8
(2.8)
7.2
(2.4)
7.1
(2.0)
7.3
(3.0)
8.6
(2.5)
8.7
(1.4)
F 5 7 6 =1.46
ns
449.4
(204.3)
371.0
(142.6)
347.5
(283.5)
254.5
(116.8)
—
—
F 3 4 8 =2.00
ns
Biperiden (mg), mean (SD)
1.8
(2.3)
1.9
(2.5)
0.8
(1.7)
1.6
(2.0)
—
—
F3 48 =0.59
ns
Years of illness, mean (SD)
13.1
(7.7)
9.1
(8.2)
—
—
—
—
f = 1.36
ns
Years on drug, mean (SD)
12.5
(8.2)
8.8
(8.3)
—
—
—
—
f = 1.19
ns
No. of hospitalizations,
mean (SD)
3.6
(3.1)
3.4
(2.7)
—
—
—
—
r= 0.15
ns
Age of onset, mean (SD)
19.1
(5.0)
20.0
(8.0)
—
—
—
—
f = -0.35
ns
PANSS, negative scale,
mean (SD)
18.6
(6.8)
25.2
(7.6)
—
—
—
—
f=-2.43
p = 0.02
Parameter
n (%)
Antipsychotic (CPZ, mg),
mean (SD)
'
Note.— NR = nonresponsive; ns = nonsignificant; R = responsive; SD = standard deviation.
chotic disorders, and healthy controls, divided according
to their electrodermal responsiveness. It can be seen that
electrodermal nonresponsiveness among schizophrenia
patients and patients with other psychotic disorders was
significantly higher than among healthy controls. Even
though there were not statistically significant differences,
it can be seen that in the three responsive groups, male
subjects were predominant. In contrast, there were more
females than males in two nonresponsive groups. The six
experimental groups did not significantly differ in age and
schooling. The dose of antipsychotic medication, in equivalents of chlorpromazine, and of the anticholinergic
biperiden did not differ significantly among the four
groups of patients. It can also be seen that the two groups
of schizophrenia patients did not differ in age of onset and
duration of the disease, number of years on antipsychotic
medication, and number of previous hospitalizations.
However, the SCHIZ-NR group presented significantly
higher scores in the PANSS negative subscale than the
SCHIZ-R group.
The average parameters of the electrodermal skin
conductance and the performance on the SCWT are presented in table 3. The number of SCRs did not differ significantly among the three responsive groups. The SCL
was significantly lower in the two groups of schizophrenia
patients, in relation to the HEALTHY-R group, and lower
in the SCHIZ-NR group than in the SCHIZ-R group. The
SF number was significantly lower in the nonresponsive
groups, when compared to the responsive groups with the
same diagnosis.
In the SCWT, the error interference made by the subjects of the SCHIZ-NR group was significantly higher
than that of all the other groups, including the SCHIZ-R
group. Time interference was greater in the groups of
schizophrenia patients and other psychotics than in the
HEALTHY-R group, irrespective of electrodermal sensitivity. SCWT error interference was significantly correlated with the number of elicited SCRs in the schizophrenia patient groups (Spearman's rho = -0.45, p = 0.012) but
not in the other two groups.
462
Selective Attention Deficit
Schizophrenia Bulletin, Vol. 28, No. 3, 2002
Table 3. SCWT and skin conductance parameters
Schizophrenia
Patients
Skin conductance,
mean (SD)
SCL
SF
SCWT, mean (SD)
Time
Interference
Error
Interference
Other
Psychotics
Healthy
Controls
2-way ANOVA
Diagnosis x
Diagnosis Responsivity responsivity
R
NR
R
NR
R
15.91
(3.5)
42.8
(11.1)
9.81'2
(2.2)
4.2 1 ' 2
(2.9)
20.3
(2.8)
53.7
(10.9)
21.0
(0.2)
2.9 1 ' 2
(1.1)
23.6
(0.5)
29.9
(5.5)
123.1 1
(19.1)
8.1 (3.5)
100.21
(12.4)
33.41'2
(7.7)
133.61
(24.2)
11.2
(4.3)
172.41
56.5
58.4 F 2 7 6 = 15.18
(30.9)
(5.9)
(9.9)
p = 0.0001
15.8 5.6(1.4) 3 .7(1.1) F 2 7 6 =5.29
p = 0.007
(3.6)
NR
21.5 F2 7 6 =13.28 F 1 7 6 =2.16 F 2 7 6 =1.32
(0.3)
p = 0.0001
ns
ns
2.81-2 F 2 7 6 =1.44 ^,76 = 47.44 F 2 7 6 =1.36
(1.7)
ns
ns
p= 0.0001
F U 6 =0.19
ns
1=1,76=4.74
p = 0.03
R>,76=1-63
ns
F 2 7 6 =4.13
p = 0.02
Note.—ANOVA = analysis of variance; NR = nonresponsive; ns = nonsignificant; R = responsive; SCL = skin conductance level; SCWT =
Stroop Color Word Test; SD = standard deviation.; SF = spontaneous fluctuation.
1
Significant differences (p< 0.05) with Duncan's test in respect to healthy controls R.
2
Significant differences (p< 0.05) with Duncan's test in respect to R of same diagnosis.
that the higher frequency of errors observed in schizophrenia
patients was not simply due to a speed/accuracy trade-off.
Our study employed a card version of the SCWT in
which all stimuli are presented on the same card, while in
computerized versions only one stimulus is presented at a
time (Perlstein et al. 1998). Thus, it is difficult to compare
these results with those using computerized versions of the
SCWT.
The present results further show that nonresponsive
schizophrenia patients manifest more negative symptoms
than responsive schizophrenia patients, a finding that has
been reported before (Zahn 1986; Kim et al. 1993).
Changes in the frontal lobe have also been associated with
deficient performance on the SCWT (Perret 1974; Vendrell et al. 1995) and to the predominance of negative
symptoms (Liddle et al. 1992). In addition, electrodermal
hyporesponsiveness has been linked to reduction in size of
the prefrontal cortex (Lencz et al. 1996) and metabolic
reduction in the lateral and medial frontal regions, as well
as in the thalamus, hippocampus, and amygdala (Hazlett et
al. 1993). Thus, frontal lobe abnormalities could be the
common biological substrate for the schizophrenia subgroup characterized by electrodermal hyporesponsiveness,
poor performance on the SCWT, and predominance of
negative symptoms. However, other possibilities should
also be considered. For example, other brain areas—such
as the cingulate gyrus—have been associated with deficient performance on the SCWT (Nordahl et al. 2001).
In this study the number of electrodermally nonresponsive subjects in the healthy control group (29%) is
greater than the 5 to 10 percent reported in most studies
Discussion
The main contribution of this study is the finding that low
electrodermal responsiveness in schizophrenia patients is
associated with a deficit in selective attention, as indicated
by higher error interference (Card 2 minus Card 1) on the
SCWT. In healthy controls and in other psychotics the error
interference was similar in both responsive and nonresponsive subgroups. Consistent with the present results, Straube
(1979) found a higher number of errors in the Dichotic Listening Shadowing Task made by nonresponsive schizophrenia patients, when compared to responsive schizophrenia
patients, neurotic patients, and healthy controls. The latter
test requires selective attention, because different words are
simultaneously presented to each of the subject's ears, but
the subject is told to listen to and repeat the words from only
one side, ignoring the other one. Therefore, although in
Straube's study the higher nonresponsive errors may be
more an effect of withdrawl from external stimuli than a
problem of interference, the results of both these studies are
compatible with the conclusion that schizophrenia patients
have a deficit in selective attention.
The higher time interference shown by all patient
groups compared to healthy controls could be attributed to
medication or to slowness in psychosis, although there is
evidence that medication has minimal effects on response
speed and many cognitive functions (Spohn and Strauss
1989; Goldberg et al. 1993). The very specific deficit in
errors, but not time, in the schizophrenia nonresponsive
group may be due to the fact that these subjects sacrificed
accuracy for speed. However, Nordahl et al. (2001) found
463
Schizophrenia Bulletin, Vol. 28, No. 3, 2002
E.Z. Lopes-Machado et al.
Andreasen, N.C. Schizophrenia: The fundamental questions. Brain Research Reviews, 31:106-112, 2000.
(Ohman 1981). Nevertheless, some studies have obtained
results comparable to the present ones (Levinson 1991;
Dawson et al. 1994). Such a high percentage of nonresponsive healthy subjects has been reported in groups
demographically matched with samples of schizophrenia
patients, to the recruitment of controls out of the hospital
staff, or to both, as happened in the present study (Iacono
et al. 1993). This finding has also been attributed to the
intensity and frequency of the tones that elicit the SCR
(Iacono et al. 1993; Dawson et al. 1994). Also, the medial
phalanges of the fingers—the recording site used in this
study—are known to exhibit less electrodermal responding than the distal ones (Scerbo et al. 1992). On the other
hand, the high occurrence of nonresponsiveness in the
group of patients with other psychotic disorders may be
due to the predominance of patients with affective disorder, who are known to have a high incidence of nonresponsiveness (Bernstein et al. 1988; Iacono et al. 1993).
The finding that male subjects were predominant in
responsive groups, while more females than males were nonresponsive, is compatible with previous studies (Boucsein
1992). The observed gender-related differences in skin conductance are probably related to the fact that women display
more delayed sweating and, in total, less sweating than men
(Edelberg 1971; Morimoto 1978). The menstrual cycle may
contribute to gender differences as well (Edelberg 1972).
The lower SCL in this study's schizophrenia groups is
unlikely to be due to medication; there were no significant
differences in drug regimen between the schizophrenia
group and other psychotic groups, who showed SCL similar to healthy controls. The SCL results from schizophrenia patients reported in the literature are inconsistent.
Some studies report higher SCL in schizophrenia patients
than in healthy controls, whereas others report the opposite (Dawson et al. 1994; Perry et al. 1998). Given that the
SCL rises during episodes of increased symptoms (Dawson et al. 1994; Hazlett et al. 1997), the lower values found
in the present study may be related to the low level of positive symptoms shown by the patients in this study.
In conclusion, the present results support the earlier
suggestion (Bernstein 1987; Perry et al. 1998) that electrodermal responsiveness may identify a subgroup of schizophrenia patients with predominantly negative symptoms
and impaired performance in tasks that require selective
attention. Further studies are needed to better define the
cognitive deficits of these patients, as well as to explore
the underlying structural and functional changes.
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Acknowledgments
This work was supported by a grant from Fundagao de
Amparo a Pesquisa do Estado de Sao Paulo (FAPESP).
A.W.Z. is the recipient of a Conselho National de Desenvolvimento Cientffico e Tecnol6gico (CNPQ) fellowship. J.A.S.C.
is the recipient of an FAPESP fellowship. We also thank Sandra Aparecida Bernardo and Geraldo Cdssio dos Reis for help
in the statistical analysis. Finally, we thank Frederico G. Graeff for comments and suggestions on the manuscript.
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Eleomar Ziglia Lopes-Machado, M.D., Ph.D., and Jose"
Alexandra de Souza Crippa, M.D., Ph.D., are Researcher
Collaborators in Psychiatry, Ribeirao Preto Medical
School, Sao Paulo, Brazil. Jaime Eduardo Cecflio Hallak,
M.D., Ph.D., is Assistant Doctor, Clinical Hospital of
Ribeirao Preto, Sao Paulo, Brazil. Francisco Silveira
Guimaraes, M.D., Ph.D., is Professor of Pharmacology,
and Antonio Waldo Zuardi, M.D., Ph.D., is Full Professor
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