Dopamine, Paranormal Belief, and the Detection of
Meaningful Stimuli
Peter Krummenacher1,2, Christine Mohr3, Helene Haker4,
and Peter Brugger1
Abstract
■ Dopamine (DA) is suggested to improve perceptual and
cognitive decisions by increasing the signal-to-noise ratio.
Somewhat paradoxically, a hyperdopaminergia (arguably more
accentuated in the right hemisphere) has also been implied in
the genesis of unusual experiences such as hallucinations and
paranormal thought. To test these opposing assumptions, we
used two lateralized decision tasks, one with lexical (tapping
left-hemisphere functions), the other with facial stimuli (tapping
right-hemisphere functions). Participants were 40 healthy right-
handed men, of whom 20 reported unusual, “paranormal”
experiences and beliefs (“believers”), whereas the remaining
participants were unexperienced and critical (“skeptics”). In a
between-subject design, levodopa (200 mg) or placebo adminis-
tration was balanced between belief groups (double-blind pro-
cedure). For each task and visual field, we calculated sensitivity
0) and response tendency (criterion) derived from signal detec-
(d
tion theory. Results showed the typical right visual field advan-
0 for verbal than
tage for the lexical decision task and a higher d
0 was lower in the levodopa than
facial stimuli. For the skeptics, d
in the placebo group. Criterion analyses revealed that believers
favored false alarms over misses, whereas skeptics displayed the
opposite preference. Unexpectedly, under levodopa, these de-
cision preferences were lower in both groups. We thus infer that
levodopa (1) decreases sensitivity in perceptual–cognitive deci-
sions, but only in skeptics, and (2) makes skeptics less and be-
lievers slightly more conservative. These results stand at odd to
the common view that DA generally improves signal-to-noise
ratios. Paranormal ideation seems an important personality di-
mension and should be assessed in investigations on the detec-
tion of signals in noise. ■
INTRODUCTION
Dopamine (DA) is a neurotransmitter in the extrapyramidal
motor system and a neuromodulator involved in motiva-
tion, emotion, and cognition (e.g., Gibbs, Naudts, Spencer,
& David, 2007; Smith, Li, Becker, & Kapur, 2006; Nieoullon,
2002; Middleton & Strick, 2000; Previc, 1999). It is assumed
to improve perceptual and cognitive decisions by increas-
ing the signal-to-noise ratio (SNR), that is, to modulate
neuronal activity by enhancing the ability of neurons to
transmit signals and reduce distortion by noise (Seamans
& Yang, 2004; Spitzer & Walter, 2003; Spitzer, 1997; Cohen
& Servan-Schreiber, 1993).
Links between DA and schizophrenia were originally
suggested by the observation that DA antagonistic treat-
ment can ameliorate positive symptoms in patients with
schizophrenia (Laruelle & Abi-Dargham, 1999; Davidson
et al., 1987; Carlsson & Lindqvist, 1963), whereas DA ago-
nistic treatment results in a deterioration of such symp-
toms (Sekine et al., 2001; Janowsky & Risch, 1979). In
healthy populations, DA agonists not only boost learning
1University Hospital Zurich, Switzerland, 2Collegium Helveticum,
Zurich, Switzerland, 3University of Bristol, Bristol, UK, 4Psychiatric
University Hospital, Zurich, Switzerland
(Breitenstein et al., 2006; Knecht et al., 2004) but have
also the potential to trigger psychotic symptoms (Sekine
et al., 2001; Abi-Dargham et al., 1998; Angrist & Gershon,
1970). Furthermore, DA overmedication can produce
psychosis in patients with Parkinsonʼs disease (Factor
& Molho, 2004; Kuzuhara, 2001). In a nutshell, acute
psychosis is generally considered a consequence of a
hyperdopaminergic state (Howes & Kapur, 2009; Kapur,
2003; Laruelle & Abi-Dargham, 1999; Davis, Kahn, Ko,
& Davidson, 1991; Carlsson & Lindqvist, 1963). In this
context, a number of behavioral DA-responsive studies
with unmedicated patients have specifically implicated
the right (RH) rather than the left hemisphere (LH)
(Mohr, Landis, Bracha, Fathi, & Brugger, 2003; Malaspina
et al., 2000; Bracha, Livingston, Clothier, Linington, &
Karson, 1993).
Against these clinical findings, the notion that DA would
increase the SNR in neural networks appears paradoxical.
Psychotic symptoms are associated with a poor discrimi-
nation between relevant and irrelevant stimuli, suggesting
impaired rather than improved perceptual sensitivity.
For example, positive psychotic symptoms in patients
with schizophrenia are often associated with different
kinds of source monitoring impairments (Anselmetti et al.,
2007) such as reality-monitoring deficits in terms of a
© 2009 Massachusetts Institute of Technology
Journal of Cognitive Neuroscience 22:8, pp. 1670–1681
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confusion between perception and imagination (Brebion,
Smith, Gorman, & Amador, 1997; Frith, 1992), disrupted
self–other differentiation (Lee, Kwon, Shin, Lee, & Park,
2007; Kircher & Leube, 2003; Hemsley, 1998), and the fail-
ure to distinguish between internal and external stimula-
tion (Brebion, David, Bressan, Ohlsen, & Pilowsky, 2009;
Henquet, Krabbendam, Dautzenberg, Jolles, & Merckelbach,
2005; Ford, Mathalon, Heinks, et al., 2001; Ford, Mathalon,
Kalba, et al., 2001). Source monitoring deficits have also
been observed in healthy individuals with high level of
psychotic-like experiences (Laroi, DʼArgembeau, Bredart,
& van der Linden, 2007; Allen, Freeman, Johns, & McGuire,
2006). Accordingly, signal detection parameter have un-
equivocally shown a decreased rather than increased
perceptual sensitivity in patients with auditory hallucina-
tions (Ishigaki & Tanno, 1999). Furthermore, a lowered
sensitivity in detecting a mismatch between expectancy
and experience was described. This reduction, accom-
panied by blood flow changes in right frontal cortex, corre-
lated with delusion severity in psychotic patients (Corlett
et al., 2007).
Paranormal belief in healthy participants (e.g., the be-
lief in extrasensory, psychokinetic and prophetic abilities)
has been associated with both creative and disordered
thought (Claridge, 2009; Leonhard & Brugger, 1998). For
instance, individuals with paranormal belief evidence a
marked willingness to perceive “patterns in noise” (Reed
et al., 2008; Bell, Reddy, Halligan, Kirov, & Ellis, 2007;
Brugger & Graves, 1997b) and are more inclined than
those skeptical about such abilities to attribute meaning
to random associations (Mohr, Landis, & Brugger, 2006;
Gianotti, Mohr, Pizzagalli, Lehmann, & Brugger, 2001;
Pizzagalli, Lehmann, & Brugger, 2001). Although the crea-
tive aspects of paranormal thought have been emphasized
by some authors (Folley & Park, 2005; Weinstein & Graves,
2002; Gianotti et al., 2001; Claridge, Pryor, & Watkins,
1990), others have equally stressed its conceptual similar-
ity to psychotic symptoms (Brugger & Graves, 1997b;
Kreweras, 1983). Thus, a reduced criterion to acknowledge
the presence of a signal not only increases the chances
to creatively detect a real stimulus but also bears the risk
of Type II errors, that is, hallucinatory perceptions and
delusional inferences. These aspects of paranormal belief
have been conceptualized by Eckblad and Chapman
(1983) as an indicator of the positive phenomenology of
schizotypy (i.e., a nonclinical analogue of positive psychotic
symptoms).
The link between positive symptoms of psychosis and
paranormal/positive schizotypal traits is further evidenced
by studies showing that psychometrically assessed posi-
tive schizotypal individuals, even if completely healthy,
perform similarly to patients with schizophrenia in sen-
sory (Mohr, Rohrenbach, Laska, & Brugger, 2001; Kwapil,
Chapman, Chapman, & Miller, 1996), cognitive (Tallent &
Gooding, 1999; Duchene, Graves, & Brugger, 1998), and
attentional tasks (Mohr, Bracha, & Brugger, 2003; Sarkin,
Dionisio, Hillix, & Granholm, 1998; Brugger & Graves,
1997a). Of particular relevance to the present study, these
similarities are also evident for lateralized neuropsychologi-
cal functions (Brugger, 2007; Mohr, Bracha, et al., 2003;
Barnett & Corballis, 2002; Pizzagalli et al., 2000; Chapman
& Chapman, 1987a). Furthermore, neuropharmacological
studies pointed to an enhanced dopaminergic respon-
sivity in both persons with schizotypy (Siever et al., 1993)
and schizophrenia (Davidson & Davis, 1988; Pickar et al.,
1984).
We here investigated whether DA indeed modulates
SNR and/or response bias (Reed et al., 2008; Brugger &
Graves, 1997b) in participants with paranormal experi-
ences and beliefs (“believers”) as compared to individuals
with a critical attitude toward paranormal phenomena
(“skeptics”). In order to account for a potential right
hemispheric shift of dopaminergic involvement in para-
normal belief, we used a double-blind levodopa/placebo
design and two lateralized tachistoscopic tasks, one with a
known advantage of the LH (lexical decision task [LDT])
and one with a processing advantage of the RH (facial de-
cision task [FDT]). Using classical signal detection analysis
(Gescheider, 1997; Green & Swets, 1966), we determined
0) and response tendencies (criterion C). Be-
sensitivity (d
cause we argued against the notion of DA increasing SNR,
we expected (1) an overall reduced detection sensitivity
0) in individuals in the levodopa as compared to
(lower d
the placebo group, especially for the believers. In addi-
tion, we predicted (2) a generally looser response criterion
(lower criterion C) in individuals in the levodopa as com-
pared to the placebo group, again, especially for the be-
lievers. Finally, we hypothesized that (3) this effect would
be more prominent for RH compared to LH processing for
both sensitivity and criterion measures.
METHODS
Participants
Forty healthy men were recruited by a classified advertise-
ment in a local newspaper and flyers posted in the Zurich
University district. The study was introduced as an “ex-
periment assessing the role of DA on cognitive func-
tions.” We mentioned that blood samples would be
taken, and announced an unspecified reimbursement.
Critically, the recruitment text for the believers contained
the phrase “You do not only consider extrasensory per-
ception a theoretical possibility but you think you are
using your own paranormal abilities in everyday situations”
(without further justification of why this was a require-
ment). A contact telephone number and electronic mail
address were included. Initially, via standardized telephone
interviews (Campbell, 2000), we excluded persons with
a personal or first-degree family history of neurological
and psychiatric disease, including serious learning dis-
abilities and drug abuse (casual tetrahydrocannabinol
consumption was not considered an abuse if the last
consumption was more than 2 months ago). Included
Krummenacher et al.
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participants were sent a brief information brochure (by
regular mail) about the role of DA in the central nervous
system, including potential side effects if it was taken
at high doses. It also mentioned the requirement to fill
in several questionnaires and stated the approximate
duration of the procedure (“1 to 2 hours”) and the finan-
cial reimbursement (Swiss Francs/CHF 50). After 20 be-
lievers were selected, 20 skeptics were recruited and
selected (same inclusion criterion as for believers) who re-
sponded to the same recruitment procedure, using an
identical advertisement to the first except that the pas-
sage referring to belief in the paranormal now stated:
“You have a skeptical attitude toward so-called paranormal
phenomena and do not generally believe in the existence
of extrasensory perceptions like telepathy, clairvoyance
and precognition.” Additionally, these skeptics were in-
dividually matched to the believers regarding age and
educational background (see Table 1). Right-handedness
(self-reported at the time of the telephone interview) was
later confirmed quantitatively with the 13-item scale by
Chapman and Chapman (1987b). Skeptics and believers
did not differ from one another in age [t(38) = 0.20, p =
.84], education [t(38) = 1.57, p = .13], and the strength of
right-handedness [t(38) = 1.23, p = .23].
All participants provided written informed consent to
participate in the experiment that had been approved
by the local Ethics committee.
Questionnaires
Australian Sheep–Goat Scale (ASGS)
This 18-item visual analog scale (Thalbourne & Delin,
1993) assesses both belief in paranormal phenomena (spe-
cifically telepathy, clairvoyance, precognition, and psycho-
kinesis) and self-perceived abilities. A sample item is: “I am
completely convinced that I have had at least one pre-
monition about the future that came true and which (I
believe) was not just a coincidence.” Each item is scored
from 0 to 13 reflecting the number of centimeters the par-
ticipantʼs answer mark was displaced from the end of the
line. Total scores on the ASGS may thus range from 0 to
234, with higher scores indicating stronger experiences
and self-perceived abilities.
Table 1. Descriptive Data of the Study Sample
Skeptics (n = 20)
Believers (n = 20)
Mean (SD)
Range Mean (SD)
Range
Age (years)
28.4 (4.5)
Education (years) 16.3 (2.6)
21–39
12–20
28.7 (4.8)
15.1 (2.2)
21–38
12–20
ASGS
MI
39.0 (25.4)
1–102 168.9 (30.4) 114–222
4.1 (2.9)
0–12
20.0 (3.79)
13–26
Handedness
13.9 (1.2)
14.5 (2.0)
Magical Ideation (MI) Scale
We assessed participantsʼ MI with a validated true–false
30-item questionnaire that includes items such as “I
sometimes have a feeling of gaining or losing energy
when people look at me or touch me,” or “Some people
can make me aware of them just by thinking about me.”
Scores range from 0 to 30, with higher scores indicating
more pronounced magical thinking. The scale is pub-
lished in full in Barnett and Corballis (2002) and in Eckblad
and Chapman (1983), and normative data can be found in
Garety and Wessely (1994).
Lateralized Tachistoscopic Tasks
Lexical Decision Task
Stimuli were pairs of four-letter strings, one string exposed
to the left visual field (LVF), the other simultaneously to
the right visual field (RVF) (exposure time = 140 msec,
horizontal eccentricity = 1.5° to 3.0° of visual angle; Re-
gard, Landis, & Graves, 1985; Graves, 1983). All letters were
written in black Helvetica font (18 points) and presented
on a computer screen (gray background). The following
LVF/RVF letter string combinations were presented in
a pseudorandomized sequence: 12 word/nonword pairs,
12 nonword/word pairs, and 24 nonword/nonword pairs.
Only one-syllable or two-syllable German function words
were used as targets. These have low pictorial and associa-
tive properties and are thus most likely to consistently re-
cruit LH network activation (Sabsevitz, Medler, Seidenberg,
& Binder, 2005). Sample stimuli are because (“weil”), since
(“seit”), or hardly (“kaum”) (see Figure 1, left).
Facial Decision Task
Stimuli were pairs of faces and “nonfaces” (Heider & Groner,
1996). The faces were photographs (black on gray back-
ground) of men and women, whereas the “nonfaces” had
scrambled eyes–nose–mouth configurations. All stimuli
appeared without face outlines or hairlines (“oval faces”).
One face or scrambled face was exposed to LVF, the other
simultaneously to RVF (exposure time = 140 msec, horizon-
tal eccentricity = 2.6° to 4.3° of visual angle). The follow-
ing LVF/RVF face/nonface combinations were presented
in a pseudorandomized sequence: 12 face/nonface pairs,
12 nonface/face pairs, and 24 nonface/nonface pairs (see
Figure 1, right).
During both decision tasks (counterbalanced order), in-
dividuals were asked to rest their head on a chin rest, and
to fixate their gaze on a cross in the center of the screen
(see Figure 1). On seeing a meaningful word (or face, re-
spectively) in LVF (RVF), they had to press a key with their
left (right) hand. On not seeing a meaningful word (face)
on either side, they had to press the space bar with both
thumbs (Figure 1). Participants were instructed to respond
as quickly and accurately as possible and to maintain cen-
tral fixation throughout the task. Participants were tested
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Volume 22, Number 8
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Figure 1. Time course of stimulations in the lexical (A) and facial (B) decision task. After 750 msec (fixation of a central cross), a stimulus
was exposed for 140 msec and participants had to press one of two lateral keys depending on the side where a meaningful word (A) or face
(B) had been exposed (space bar response for two nonsense words or scrambled faces; 50% of trials). Participants were instructed to respond
as quickly and accurately as possible and to fixate their gaze on a cross in the center of the screen.
individually, and light and contrast conditions were kept
constant across participants. We assessed accuracy and
reaction times (RTs) for correct responses.
Double-blind Procedure
L-Dopa Administration
The study was a randomized, double-blind between-
subject levodopa/placebo design. Creation of the randomi-
zation code and substance wrapping was performed by
a neurologist (H. H.). H. H. was not involved in the behav-
ioral testing, but was available in case of medical emer-
gencies and provided decoding information to the main
investigator (P. K.) after all participants had been tested.
A dual-release formulation of 200 mg levodopa/50 mg
benserazide (Madopar DR; Roche Pharma AG, Reinach,
Switzerland) with a fast absorption within the first hour
and sustained concentration levels thereafter (Gasser, Jorga,
Crevoisier, Hovens, & van Giersbergen, 1999) was admin-
istered. Each participant had been strictly instructed to
refrain from any food intake between noon and the be-
ginning of the experiment at 3:30 pm. Participants had also
been told not to consume alcohol or any other drugs for at
least 24 hr before testing. Compliance with these requests
were 100% by self-report.
At the end of behavioral testing, participants were asked
(questionnaire) which substance they thought they had
received, and what they expected the substance effects
to be.
L-Dopa Blood Serum Concentrations and Efficacy of
Double Blinding
In order to ensure that participants were under significant
levodopa concentration throughout the experiment, two
blood samples (5–7 ml each) were drawn. The first was col-
lected 30 min after drug administration, just before the
two detection tasks. Immediately after behavioral test-
ing, a second sample was drawn. Blood plasma was sepa-
rated by centrifugation, and the samples were immediately
frozen at −80°C for later high-pressure liquid chromatog-
raphy for electrochemical detection (see Mohr, Landis,
Sandor, Fathi, & Brugger, 2004 for further details). No
levodopa was detected in the placebo group. In the levo-
dopa group, the mean levodopa serum concentration
was 176.95 ± 163.16 ng/ml for the first blood sample
and 217.55 ± 106.28 ng/ml for the second blood sample
[t(19) = −0.852, p = .405].
Data Analysis
To test for a DA influence on SNR as a function of individ-
ualsʼ paranormal belief, we first determined the signal de-
0 (sensitivity) and (2) the
tection theory parameters (1) d
observerʼs response bias (criterion C) (Gescheider, 1997;
Green & Swets, 1966) for each visual field (LVF, RVF)
and task (LDT, FDT). Responses were scored as hits for
trials on which a real stimulus (a word in the LDT and a face
in the FDT) had on the side of the participantʼs keypress.
False positives were responses to nonwords or scrambled
faces, respectively. Trials on which a stimulus was pre-
sented but the response was made on the wrong side
(i.e., confusion errors) were planned to be counted as
misses (no such error occurred in the present dataset).
0 is a pure index of stimulus detection sensitivity unconta-
d
0
minated by location of the observerʼs criterion. Higher d
values indicate better stimulus detection sensitivity. Lower
C values reflect a looser response tendency (“YES answer
bias”), and higher C values indicate a stricter response
tendency (“NO answer bias”). These two main measures
were subjected to two major analyses, (1) and (2), that is,
two separate four-way repeated measures ANOVAs with
substance group (levodopa vs. placebo) and belief group
(believers vs. skeptics) as between-subject factors and task
(LDT, FDT) and visual field (LVF, RVF) as repeated within-
subject factors.
Krummenacher et al.
1673
In addition, to facilitate comparison of the results with
those of previous studies (e.g., Mohr, Krummenacher,
et al., 2005; Brugger, Gamma, Muri, Schafer, & Taylor,
1993), we also assessed (3) RTs for correct decisions, and
calculated laterality indices as previously used (RT correct
LVF − RT correct RVF)/(RT correct RVF + RT correct LVF)
(Mohr, Krummenacher, et al., 2005; Marshall, Caplan, &
Holmes, 1975). A positive value indicates an RVF/LH ad-
vantage and a negative value an LVF/RH advantage. These
indices were subjected to a three-way repeated measures
ANOVA with substance group (levodopa vs. placebo) and
belief group (believers vs. skeptics) as between-subject
factors and task (LDT, FDT) as repeated within-subject
factor. In case of significant interactions, single post hoc
comparisons (Newman–Keuls) were performed correct-
ing for multiple comparisons. Homogeneity of variances
in the signal detection measures was controlled using
Leveneʼs test.
Nonparametric (Spearman) correlation (rs) procedures
were used throughout in order (1) to be more conserva-
tive when running a large number of correlations on small
groups (n = 10) and because an interval level could not
be guaranteed for the questionnaire data.
If not stated otherwise, all p values are two-tailed, and
the significance level was set to α = .05.
One participant did not obey the LDT instruction (admin-
istered before the FDT in his case) and pressed solely the
nontarget key in a stereotyped manner. LDT data were thus
available from only 39 participants. All other comparisons
are based on the originally recruited 40 participants.
RESULTS
Double-blind Procedure
None of the participants reported any remarkable side
effects of placebo/levodopa intake. Chi-square analyses
of the number of individuals with correct versus incorrect
guesses in the postexperimental assessment revealed that
neither skeptics (11 correct substance detections consist-
ing of 3 hits and 8 correct rejections and 9 false substance
detections consisting of 7 miss and 2 false alarms; χ2 = 0.20,
p = .66) nor believers (12 correct substance detections
consisting of 5 hits and 7 correct rejections and 8 false
substance detections consisting of 5 miss and 3 false alarms;
χ2 = 0.80, p = .37) could reliably guess what kind of treat-
ment they had received.
Questionnaire Data
Two separate two-way ANOVAs with substance group (levo-
dopa vs. placebo) and belief group (believers vs. skeptics)
as between-subject factors, once on MI scores and once
on ASGS scores, revealed significant main effects for be-
lief group in both cases [MI: F(1, 36) = 229.85, p = .000;
ASGS: F(1, 36) = 215.30, p = .000]. Believersʼ ASGS and
MI scores were significantly larger than the respective
values for the skeptics (see Table 1). As indicated by non-
significant main effects in both ANOVAs, mean MI scores
of the levodopa (11.1 ± 8.5) and placebo (13.0 ± 8.9)
groups were comparable [F(1, 36) = 3.32, p = .08] as were
ASGS scores [levodopa: 99.6 ± 75.5; placebo: 108.3 ± 68.7;
F(1, 36) = 0.98, p = .33]. Interactions were not significant
[F(1, 36) < 1.02, p > .32].
A split at the median MI scale score (12.5) into a high
and low MI group revealed that all participants with ASGS
scores above the median (ASGS = 108) belonged to the
high MI group, and all participants with ASGS scores below
the median to the low MI group. Despite only moderate
item-overlap, raw scores on the MI and the ASGS inven-
tories were positively correlated (rs = .92, p < .000).
Lateralized Stimulus Detection: (1) Sensitivity Data
0 measures revealed a significant
The four-way ANOVA for d
main effect for substance group [F(1, 35) = 6.13, p = .02];
0 was lower in the levodopa (1.53 ± 0.49) than placebo
d
(2.03 ± 0.72) group. There was also a main effect for task
0 being higher in the LDT
[F(1, 35) = 6.19, p = .02] with d
(1.97 ± 0.12) than in the FDT (1.54 ± 0.13). Among the
two-way interactions, those between visual field and task
[F(1, 35) = 4.96, p = .03], and belief and substance group
[F(1, 35) = 8.68, p = .006; see Figure 2A] were significant.
As to the first interaction (visual field and task), post hoc
0 for RVF than LVF in
comparisons revealed a higher d
0 between visual
the LDT ( p = .01), and no difference in d
fields in the FDT ( p = .69). Moreover, for RVF presenta-
0 was higher in the LDT than in the FDT ( p =
tions, d
.006), with no task difference being observed for LVF pre-
sentations ( p = .66). As to the second interaction (belief
and substance group), post hoc comparisons showed that
0 was comparable between belief groups in the levodopa
d
0 in skeptics as
group ( p = .61), but there was a higher d
compared to believers in the placebo group ( p = .008).
0 was comparable in the sub-
Moreover, for believers, d
0 was lower in
stance groups ( p = .74), but for skeptics, d
the levodopa than in the placebo group ( p = .003; see
Figure 2A).
The three-way interaction between paranormal belief,
task, and visual field [F(1, 35) = 3.7, p = .06] just failed
to reach conventional significance level. It indicated that
for RVF presentations, the believersʼ sensitivity was lower
for face than word detections ( p = .042), and skepticsʼ
sensitivity was higher for RVF compared to LVF presen-
tations in the LDT ( p < .01; see Supplemental Figure
at http://neuro-psychologie.net/SF.pdf ). No other main
effects or interactions were significant (all post hoc tests
revealed that this trend indicates F values ≤ 2.85, corre-
sponding p values ≥ .10).
Correlations analyses between belief and sensitivity
0 in the LDT was signifi-
revealed that skepticsʼ LVF/RH d
cantly positively correlated to raw scores on the ASGS
(rs = .78, p < .001; Figure 3A) in both substance groups
(placebo group: rs = .72, p = .02; levodopa group: rs =
1674
Journal of Cognitive Neuroscience
Volume 22, Number 8
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Figure 2. (A) d
tasks and hemispheres, but displayed separately for the two belief and substance groups.
0 values of detection sensitivity (mean ± SEM ). (B) Values of response criterion C (mean ± SEM ). Data are averaged over
.86, p = .001). For neither substance group, a comparable
correlation was found for RVF/LH sensitivity (rs < −.12,
p > .73), nor were believersʼ ASGS raw scores (calculated
separately for both substance groups) correlated with
0 measures in the LDT (rs < −.10, p > .78 for LVF/RH
d
stimulations; rs > .08, p > .85 for RVF/LH stimulations).
Analogous correlation analyses for the FDT revealed no
significant results: rs < −.55, p > .10).
Lateralized Stimulus Detection:
(2) Response Criterion
The four-way ANOVA for criterion C revealed a significant
main effect for belief group [F(1, 35) = 11.12, p = .002],
indicating that believersʼ criterion was lower than skepticsʼ
(see Figure 2B). The two-way interaction between belief
group and substance group was as predicted, but signifi-
cant only at the one-tailed level [F(1, 35) = 2,98, p <
.05]. In more detail, believersʼ criterion was lower than
the one of skeptics in the placebo group ( p = .006),
but comparable between belief groups in the levodopa
group ( p > .10). Moreover, skepticsʼ ( p = .38) and believ-
ersʼ ( p = .13) criterions were comparable in the levodopa
and placebo groups. No other main effects or interac-
tions were significant (all F values ≤ 2.28, corresponding
p values ≥ .14).
Correlation between the C values and raw scores on the
ASGS revealed a negative association between these param-
eters for skeptics in the placebo group and exclusively with
respect to LVF/RH stimulations in the LDT (rs = −.68, p =
.029; see Figure 3B). For believers in the placebo group,
ASGS raw scores were significantly positively correlated to
Figure 3. (A) Spearman
correlation between skepticsʼ
LVF/RH sensitivity for lexical
decisions and paranormal belief
scores for the two substance
groups separately. No
comparable correlation
pattern was found for RVF/LH
stimulations. (B) Analogous
correlation as in A, but for the
response criterion C.
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Krummenacher et al.
1675
C values regarding RVF/LH stimulations in the FDT (rs = .63,
p = .049).
Lateralized Stimulus Detection: (3) Reaction
Time Analyses
The three-way ANOVA on RT indices revealed no signifi-
cant main effects or interactions [F(1, 35) < 0.87, p > .36].
Correlation analyses in the two belief groups between
the RT index score in the two decision tasks and partici-
pantsʼ raw scores on the ASGS revealed that increasing
ASGS scores were negatively correlated to the RT lateral-
ity index in skeptics in the levodopa group in both the
FDT (rs = −.69, p = .026) and marginally significantly
the LDT task (rs = −.62, p = .056), again indicating an
association between paranormal belief and attenuated
LH superiority.
DISCUSSION
Healthy participants preselected for their attitudes toward
paranormal phenomena were administered two lateral-
ized decision tasks in a double-blind, levodopa, placebo-
controlled between-subject design. We aimed to investigate
relationships between increased DA availability, the person-
ality trait paranormal belief (notably based on self-perceived
own paranormal abilities) and hemisphere-specific signal
detection performance. “Signal” was defined here as a
meaningful constellation of verbal or facial elements against
a “noisy,” meaningless configuration of the same elements.
We set out to test two main hypotheses concerning the
influence of DA on perceptual decisions, that is, decreased
sensitivity (lower SNR) and reduced response criterion. In
addition, we hypothesized an accentuation of these pre-
dicted effects in the believer group and prominently so for
RH processing.
On a methodological note, we emphasize that a genuine
double-blind procedure could be realized in the present
study; neither believers nor skeptics could reliably guess
the type of substance they did ingest nor were there any
conspicuous behavioral signs that would have informed
the experimenter about the type of pharmacological treat-
ment a particular participant had received.
a consequence of an increase in “internal noise” (Kapur,
2003) that may have interfered with the sensory input.
However, the pharmacological treatment per se was not
related to overall decision performance. The lower sen-
sitivity in the levodopa than placebo group was dependent
on participantsʼ paranormal belief. Links between the
emergence of paranormal belief and the DA system have
been suggested (Brugger & Graves, 1997a) and tested
(Mohr et al., 2004; Kumari et al., 1999). Here, contrary to
our expectation, a reduced sensitivity in the levodopa as
compared to the placebo group was only evident in the
skeptics. Previous findings (Mohr, Krummenacher, et al.,
2005) would have suggested a modulating effect of DA
for both believers and skeptics and an improvement of
specifically RVF/LH decisions. This could not be replicated
in the present study. One reason for this discrepancy may
be differences in the study populations. In the previous
study, participants had been recruited randomly from the
general population and participants with pronounced para-
normal beliefs were excluded. Also, Mohr, Krummenacher
et al. (2005) reported only hit rates, whereas the SDT anal-
yses in the present study considered both hits and false
alarms. Alternatively, one may speculate that the lack of
an effect in the group of believers reflected a plateau ef-
fect caused by high cerebral baseline DA levels arguably
0 values. A single dose of levodopa
responsible for low d
0 values because
would then not further have lowered d
of a possibly saturated DA receptor activity.
These results further decrease the plausibility that DA
generally improves SNR, and instead propose the oppo-
site, in particular, in individuals with a presumably hypo-
dopaminergic system (i.e., the skeptics). A shift in low MI
participantsʼ neuropsychological profile toward the psycho-
tic range has repeatedly been shown subsequent to DA
agonistic consumption (Mohr et al., 2006; Mohr, Landis,
Bracha, Fathi, & Brugger, 2005; Mohr et al., 2004) and seems
to be a consistent pattern. Therefore, the present finding is
more in accordance with the hypothesis that enhanced
dopaminergic transmission produces less constrained firing
patterns in mesolimbic neurons. This would generate more
internal noise and promote, on the phenomenal level, loos-
ened associations and superstitious beliefs (Shaner, 1999).
Response Criterion C
Perceptual Sensitivity: d0
In line with our hypothesis, participants having received
levodopa—irrespective of paranormal belief, task, and vi-
sual field/hemispheres—had generally lower sensitivities
than participants having received placebo. These results
indicate that the notion of a general enhancement of the
SNR by DA (Seamans & Yang, 2004; Spitzer, 1997; Cohen
& Servan-Schreiber, 1993) may have to be revised. At
least in our two classical decision tasks, a decrease rather
than an increase in sensitivity was apparent. We interpret
this reduced perceptual sensitivity caused by levodopa as
Irrespective of substance, task, and visual field/hemispheres,
believers had a lower response criterion than skeptics. This
means that they evidenced a tendency to say “yes” rather
than “no,” or in more general terms, favored a Type I error
over a Type II error strategy. Such an inclination is in ac-
cordance with previous research with healthy persons en-
dorsing paranormal beliefs (Reed et al., 2008; Tsakanikos
& Reed, 2005; Wiseman, Greening, & Smith, 2003; Weinstein
& Graves, 2002; Brugger & Graves, 1997b). Although a re-
duced criterion may be adaptive in detecting real associa-
tions and develop novel links between concepts (Folley &
Park, 2005; Folley, Doop, & Park, 2003; Weinstein & Graves,
1676
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Volume 22, Number 8
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2001, 2002; Gianotti et al., 2001; Claridge et al., 1990), it
simultaneously bears the risk of false causal attributions
and lowers the threshold for hallucinatory perceptual ex-
periences (Bell et al., 2007; Brugger & Graves, 1997b).
This general belief effect interacted with DA administra-
tion. More specifically, the modulation of response criterion
by paranormal belief was only significant for participants in
the placebo group. This is in contrast to the sensitivity find-
ings and indicates a balancing effect of levodopa; believersʼ
criterion turned more conservative and skepticsʼ slightly
looser. This interaction effect further supports the pro-
posed links between a participantʼs baseline dopaminergic
activity, and the emergence and maintenance of paranor-
mal ideation. It is also compatible with the finding that, in
believers, compensation from higher than normal levels
of dopaminergic activity may contribute to psychosis pro-
tection (Mohr et al., 2004).
There is evidence both from studies with animals
(Arnsten, 1997; Arnsten, Cai, Steere, & Goldman-Rakic,
1995; Williams & Goldman-Rakic, 1995; Arnsten, Cai,
Murphy, & Goldman-Rakic, 1994) and humans (Mehta
et al., 2000) that with respect to a specific task or task
demands (e.g., Floresco & Magyar, 2006) dopaminergic
actions might follow the trajectory of an inverted U-shape
function, with an improvement of cognitive performance
from low to medium doses, but a decline from medium
to high doses (Goto, Otani, & Grace, 2007). Applying this
idea to the present results, it could be speculated that
believers already have a high baseline DA activity, which,
when further stimulated by an additional levodopa sup-
plementation, results in a reversal of performance.
Although these conjectures may be promising, caution
has to be expressed with regard to the possibly weakest
point of the present study, that is, use of a between-subject
design. Not only did we have to deal with two different
baselines in behavioral performance but also with po-
tentially different baselines in DA responsivity (see Mehta
(2002) for the importance of initial physiological states or
baseline values). No doubt that future research in DA ef-
fects on signal detection and their modulation by paranor-
mal belief must employ a within-subject design, even at the
cost of running into problems of learning (Gescheider,
1997) and confounding effects of motivational demands.
In order to trace participants at a point of the above-
mentioned hypothesized inverted U-curve, in future within-
subject experiments, baseline assessment (e.g., Muller, von
Cramon, & Pollmann, 1998) should not only include cog-
nitive measures but also direct serum or spinal DA meta-
bolic markers such as homovanillin acid (HVA).
Laterality Aspects of Perceptual Sensitivity and
Response Criterion
The visual field main effects in the two lateralized tasks con-
firm the RVF/LH superiority for lexical decisions (Hugdahl,
2000; Regard et al., 1985; Graves, 1983) and a similar (al-
though not statistically significant) superiority of the RH
for face detections (Heider & Groner, 1996). This is in line
with previous reports showing that RH dominance for face
recognition is less pronounced than LH dominance for
lexical decisions (e.g., Leehey & Cahn, 1979). This may
be due to verbal strategies used by some participants for
face recognition leading to activation of LH processes.
With respect to hemispheric processing of verbal mate-
rial, we found a strikingly different pattern of results for the
two belief groups. Interestingly, no lexical decision lateral-
ity effects emerged for those participants who claimed to
have paranormal experiences and abilities in everyday life.
In contrast, in the skeptics, paranormal belief was highly
positively correlated in both substance groups to detection
sensitivity for words presented to LVF. In other words, as
long as paranormal belief remains within a modest range,
RH linguistic capacity increases with increasing belief. This
is exactly what has been described in numerous previous
experiments (e.g., Weinstein & Graves, 2001; Leonhard
& Brugger, 1998; Brugger, Regard, et al., 1993) including
one recent study that employed an identical LDT para-
digm (but no facial decisions, no signal detection analysis,
and no preselection with respect to belief groups; Mohr,
Krummenacher, et al., 2005).
We have previously argued (Brugger & Mohr, 2008)
that the association between paranormal belief and RH
linguistic competence may be crucial for an understanding
of any laterality aspects of psychotic symptoms. Similar to
the situation in acute schizophrenia (Crow, 1997), there
seems to be a breakdown of the regular LH dominance
for language in extreme paranormal believers whose belief
is notably based on apparently paranormal experiences. Up
to a certain degree, RH participation in verbal tasks may
foster creative thinking, but an overreliance on the less
focused associative processing characteristics of the RH
may bring about undue claims about perceptual and in-
ferential thoughts. We note that not only skepticsʼ lexical
decision sensitivity in the LVF/RH was correlated to para-
normal belief but also their tendency to favor “yes” over
“no” responses. The shift in the response criterion was,
however, confined to the placebo group. Thus, again
within the moderate range of paranormal ideation typical
for unselected participants, the higher the belief, the more
liberal the response criterion based on verbal information
displayed in LVF.
Divided visual field performance did not interact sub-
stantially with levodopa administration. This may cast
doubt on theories that have inconsistently and even
antithetically proposed hemispheric asymmetries in the
responsivity to dopaminergic treatment. (e.g., Bracha
et al., 1993; Bracha, 1989 for RH > LH, but see Reynolds,
1983 for opposite predictions based on a postmortem
investigation). Previous inconsistencies in this admittedly
controversial clinical literature may have been due to
(1) differences in illness duration (acute vs. chronic),
(2) medication effects, and/or (3) methodological dif-
ferences (e.g., behavioral paradigms vs. receptor density
determinations).
Krummenacher et al.
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The influence of DA may be more marked in the do-
main of verbal learning (e.g., Breitenstein et al., 2006),
and our simple detection tasks may not have been ideal
in uncovering differential hemispheric responses to a
single dose of levodopa. Rather, indirect evidence for a
modulation of laterality patterns by L-dopa comes from
the observation that most of the correlations between
hemifield performances and paranormal belief were only
significant in the placebo group. An exception is the as-
sociation of belief and the laterality index based on RTs
of correct (lexical and facial) decisions (in disfavor of an
LH superiority). This was exclusively present for partici-
pants (notably skeptics) in the levodopa group. We ten-
tatively conclude that the effects of DA on lateralized
performance, if present at all, may target consolidation
and motor output processes, rather than the low-level
perceptual side of signal detection.
Further Issues
The personality-dependent pharmacological effects de-
scribed in the present report highlight the need of taking
individual personality and capacity differences into ac-
count (Mehta, 2002). When ignored, these could obscure
meaningful correlations and blur subgroup differences
in task performance (White, Lott, & de Wit, 2006; Mohr,
Krummenacher, et al., 2005; Mohr, Landis, et al., 2005;
Kosslyn et al., 2002; Mattay et al., 2000; Mehta et al., 2000;
Kimberg, DʼEsposito, & Farah, 1997; Fleming, Bigelow,
Weinberger, & Goldberg, 1995). Furthermore, as Hassler
and Thadewald (2003) pointed out, pooling data from
heterogeneous samples bears the risk to produce mislead-
ing results due to “mean value artifacts” (see also the re-
cent illustration by Hadzi-Pavlovic (2007). One source of
individual variability in DA responsiveness may be geneti-
cally determined (Goldberg et al., 2003; Egan et al., 2001;
Weinberger et al., 2001), and potential interactions be-
tween such genetic predispositions and an individualʼs
attitude toward the paranormal are currently explored (Raz,
Hines, Fossella, & Castro, 2008).
As to the modulation of performance by positive schizo-
typal personality, we may mention previous findings com-
municated in the psychiatric literature. It was argued that,
contrasted to patients with schizophrenia, individuals with
schizotypal personality disorder might have an attenuated
risk for psychosis and functional impairments because of
a more efficient fronto-cortical buffering of subcortical
dopaminergic tone (Koenigsberg et al., 2001; Kirrane et al.,
2000; for a similar view with respect to healthy participantsʼ
MI and paranormal belief, see Mohr et al., 2004).
General Implications
The main findings of the present study could have implica-
tions for the development of novel pharmacotherapeutic
treatment strategies for cognitive neuropsychiatric dys-
function. As Floresco and Magyar (2006) pointed out,
dopaminergic agents must take into account both the spe-
cific types of function that are impaired in patients and the
beneficial or deleterious effects that these drugs may have
on different cognitive functions.
Further research on the specific DA receptor mechanism
that contribute to different types of neuropsychological
functions is thus likely to facilitate the development of
more selective and effective treatments for specific do-
mains of cognitive dysfunction. To further advance this line
of research, it would be challenging to focus more on the
specificity of dopaminergic activity by analyzing the effects
of (1) dose–response associations, (2) repeated dose ad-
ministration, and (3) DA antagonists. (4) Monitoring re-
ceptor activity in a within-subject design with ligand-PET
methodologies may likewise be indispensable.
Apart from their clinical relevance, our findings may have
broadest implications for general cognitive psychology and
applied psychology. Over the past decade, evidence has
been accumulated that paranormal belief is an important
personality dimension that should never be left unassessed
in the framework of tasks that investigate the detection
of signals in noise and the subsequent processes of de-
ductive reasoning. Paranormal ideation may profoundly
modulate recognition memory, pain evaluation, social be-
havior (e.g., reward sensitivity), risk assessments, and gen-
eral decision making.
Acknowledgments
We thank Marianne Wackermann for blood serum preparation,
Lotti Batschelet and the nurses on floor HAL C of the Department
of Neurology for the collection of, and Marc Fathi (Geneva) for
the analysis of, the blood samples. This research was funded by
a grant from the “Institut für Grenzgebiete der Psychologie und
Psychohygiene,” Freiburg i. Br., Germany (#690610) to P. B. and
C. M. Preparation of this report was further supported by a grant
from the OPO Stiftung to the first author. Parts of this work were
presented at the FENS 2002 (Abstr. vol 1, A074.14).
Reprint requests should be sent to Peter Krummenacher,
Collegium Helveticum, University of Zurich and ETH Zurich,
Schmelzbergstrasse 25, 8092 Zurich, Switzerland, or via e-mail:
krummenacher@collegium.ethz.ch.
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