Jenzabar University

Neural circuitry underlying pain
modulation: expectation, hypnosis,
Alexander Ploghaus, Lino Becerra, Cristina Borras and David Borsook
Harvard Medical School, Massachusetts General Hospital, Martinos NMR Center, 149 Thirteenth Street, Charlestown,
The ability to predict the likelihood of an aversive event
is an important adaptive capacity. Certainty and uncertainty
regarding pain cause different adaptive behavior,
emotional states, attentional focus, and perceptual
changes. Recent functional neuroimaging studies indicate
that certain and uncertain expectation are mediated
by different neural pathways–the former having
been associated with activity in the rostral anterior cingulate
cortex and posterior cerebellum, the latter with
activation changes in the ventromedial prefrontal cortex,
mid-cingulate cortex and hippocampus. Expectation
plays an important role not only in its modulation of
acute and chronic pain, but also in other disorders
which are characterized by certain expectation (specific
phobias) or uncertain expectation (generalized anxiety
disorder) of aversive events.
Millions of people worldwide suffer from chronic pain and
pharmaceutical expenditure for its treatment is enormous,
but treatment efficacy remains low for many chronic pain
states. Patients with chronic pain frequently report that
they suffer more from the cognitive and emotional consequences
of chronic pain than from the pain itself [1], and
pain, in turn, is subject to considerable modulation by
these processes [2]. Current treatments do not target these
interactions, but with the advent of functional neuroimaging,
their neural basis can now be studied.
One important cognitive factor is expectation regarding
pain. Although the ability to predict the likelihood of pain
or other unpleasant events by learning from prior experience
is an important adaptive behavior in healthy organisms
[3], it can cause disabling fear and avoidance in
patients with chronic pain [1]. Functional neuroimaging
studies of pain expectation have now provided important
information regarding the underlying neural circuitry.
The present article argues that the variability between
results found in these studies reveals the effect of a
biologically important mediating variable: the degree of
certainty associated with an expectation. Behavioral
studies have shown that different degrees of certainty
are associated with different emotional, physiological and
behavioral consequences [4].
Subjective certainty that a particular aversive event is
impending (‘certain expectation’) is associated with the
emotional state of fear. Fear mobilizes the organism to take
action (fight or flight), or, if these options are not available,
to minimize the impact of the aversive event (e.g. by
cognitive distraction) [5,6]. Furthermore, fear has an
impact on pain perception: numerous studies in experimental
animals (and some in humans) have shown that
fear leads to decreased pain sensitivity or hypoalgesia [7].
In contrast, uncertainty about the nature of the impending
event (‘uncertain expectation’), has very different consequences.
It is associated with the emotional state of
anxiety (rather than fear) which is characterized by risk
assessment behavior or behavioral inhibition, and by
increased somatic and environmental attention (rather
than by distraction as in the case of fear) [5,6]. Compared
with fear, anxiety has the opposite effect on pain perception:
it has been shown to lead to increased pain sensitivity
or hyperalgesia [7,8].
In the following, we identify brain areas involved in
certain expectation/fear and uncertain expectation/anxiety,
respectively (see also Table 1), and to derive a possible
explanation for the involvement of these brain regions in
pain modulation by hypnosis and placebo.
Fear and certain expectation of pain
The functional neuroanatomy of certain expectation of
aversive stimulation has been examined in experiments by
Buchel [9], Chua [10], Ploghaus [11], and their colleagues.
In these experiments, visual signals served as reliable
Table 1. Imaging expectation of aversive eventsa
Uncertain expectation
Certain
expectation Outcome type Intensity
vmPFC – " #
RACC " – –
MCC – " #
SI – " #
AINS " – –
pCEREB " – –
aThe effect of certain expectation and two types of uncertain expectation of
an aversive event on activation change in the ventromedial prefrontal cortex
(vmPFC), rostral anterior cingulate cortex (rACC), mid-cingulate cortex (mCC),
primary somatosensory cortex (SI), anterior insula (aINS) and posterior cerebellum
(pCEREB). " signifies increased activity, # signifies decreased activity.
Corresponding author: Alexander Ploghaus
(alexander_ploghaus@mckinsey.com).
Opinion TRENDS in Cognitive Sciences Vol.7 No.5 May 2003 197
http://tics.trends.com 1364-6613/03/$ - see front matter q 2003 Published by Elsevier Science Ltd. doi:10.1016/S1364-6613(03)00061-5
predictors of the type of impending stimulation, thereby
allowing subjects to learn by experience to anticipate the
characteristics of the stimulation.
In the experiment by Chua et al. [10] electric shocks
were administered to subjects in the context of a red signal,
but not in the context of a blue signal. No shocks were
given during the red signal while a scan was performed, so
scanning could have become a safety signal. Using PET,
the authors found increased activation during the red
relative to the blue signal in the rostral anterior cingulate
cortex (rACC) and in the insula, and the magnitude of
the activation was positively correlated with self-rated
fear. Using fMRI, Buchel and colleagues [9] demonstrated
activity in rACC during visual stimuli predicting
unpleasant loud noise.
Ploghaus et al. [11] presented subjects with painfully
hot as well as innocuous warm stimulation. Color cues
signaled in advance the type of impending stimulation
(color–temperature associations were randomized across
subjects). Steady anticipation throughout the signal
presentation was achieved by randomizing signal–
temperature intervals, and color cues were the only
reliable predictors due to randomization of trial order
and inter-trial intervals. As subjects learned to anticipate
the type of impending stimulation, fMRI responses
increased in rACC, anterior insula and posterior cerebellum
to the colored light signaling pain, but not to the
color signaling warm stimulation. A direct comparison
of painful heat and certain expectation of painful heat
(Ref. [11], Note 13) revealed close but separate activation
sites: painful heat activated the mid-cingulate, mid-insula
and anterior cerebellum around the vermis.
Taken together, the studies by Buchel, Chua, and
Ploghaus suggest that rACC, anterior insula and posterior
cerebellum play an important role during certain expectation
of unpleasant events. In the next section, we derive
a more specific hypothesis by taking into account conditioning
studies in animals, and studies of human brain
activity during pain modulation by hypnosis and placebo,
which also induce certain expectation.
Certain expectation in hypnotic and placebo effects
Research in experimental animals has firmly established
that light or tone stimuli reliably predicting specific
noxious stimulation acquire the ability to decrease pain
sensitivity [12,13,14,15]. This effect is mediated by fear
[16], which triggers descending opioidergic and nonopioidergic
analgesic systems [17]. Similar effects of certain
expectation of pain have been demonstrated in humans: it
has been shown in experimental [4], as well as clinical
settings [18], that pain sensitivity is decreased in subjects
who obtained reliable information about the characteristics
of impending noxious stimulation.
What predictions about brain activity during certain
expectation of pain can be derived from these observations?
Increased activity should be found in brain areas
controlling descending analgesic systems, but not in brain
areas receiving ascending nociceptive input. Ploghaus and
colleagues [11] found significantly increased activation
during certain expectation of pain in rACC, anterior insula
and posterior cerebellum, but not in adjacent areas that
responded to pain (mid-cingulate, mid-insula and cerebellar
vermis). Hence, the function of rACC, anterior insula,
or posterior cerebellum might be to mediate the influence
of certain expectation on the perception of pain and other
aversive events.
Studies examining brain activity during pain modulation
by hypnosis and placebo confirm this hypothesis,
but also increase its scope. Using hypnotic suggestions,
Rainville and colleagues [19] induced powerful expectations
of increased as well as decreased unpleasantness
of experimental painful stimulation. They found significantly
increased activity in rACC during both conditions
compared with a hypnosis control condition. Petrovic and
colleagues [20] used a placebo treatment to induce expectations
of decreased pain, and found an activation pattern
that closely resembles the activation pattern during
pain anticipation shown by Ploghaus et al. [11]: placeboinduced
expectation of decreased pain activated rACC and
posterior cerebellum, whereas pain itself activated the midcingulate
and the anterior cerebellum around the vermis.
Importantly, during hypnosis and placebo, expectation
continues throughout noxious stimulation and dominates
the perception of pain. Thus, these studies provide us with
two important additional insights. One, rACC and posterior
cerebellum are active when expectation governs behavior,
irrespective of whether this is before or during an actual
aversive event. Two, rACC and posterior cerebellum are
active irrespective of whether expectation increases or
decreases perception of the aversive event. Thus, we propose
that activity in these areas indicates a perceptual bias
in favor of the certain expectation and against potentially
conflicting nociceptive input.
By contrast, uncertain expectation has very different
cognitive and behavioral consequences (see Introduction)
and would not be expected to cause such a bias. In the
following section we show that uncertain expectation is
also associated with a different functional neuroanatomy.
Anxiety and uncertain expectation
Starting with pioneering work by Reiman et al. [21],
several studies on uncertain expectation of pain have been
performed. Two different forms of uncertainty have been
examined: uncertainty about outcome type (painful or
innocuous), and uncertainty about pain intensity. We
will start our discussion with two studies from the
former category.
Uncertainty about outcome type
Using fMRI, Porro and colleagues [22], studied brain
activation after a tactile warning signal to the foot that
might be followed either by a painful ascorbic acid injection
into the same foot or by innocuous touching of the same
skin area with a needle. The authors found that the warning
signal increased activity in the foot representation of
primary somatosensory cortex (SI) as well as in ventromedial
prefrontal cortex (vmPFC) and mid-cingulate
cortex. As these activation changes might be due to the
tactile nature of the warning signal rather than due to
uncertain expectation, a second experiment included a
control group where the tactile stimulation ‘warned’ of
no further stimulation. This experiment replicated the
198 Opinion TRENDS in Cognitive Sciences Vol.7 No.5 May 2003
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activation pattern in SI, but no activation changes in the
cingulate cortex were reported.
Hsieh and colleagues [23] obtained similar results using
PET. Subjects in this study received either a painful
ethanol or an innocuous saline injection, 10s (ethanol) or
20s (saline) after the start of each 100s PET scan. Subjects
were instructed that the first two injections (‘saline
control’) would not be painful, but some of the later
injections (either ethanol or saline) would be. The authors
found higher activation in mid-cingulate and vmPFC
during saline compared with saline control scans. This
could reflect up to 20s of uncertain expectation of pain, but
also up to 90s of relief of not having received the painful
ethanol injection. However, the studies by Porro et al. and
Hsieh et al. share similar activation patterns but not
similar design limitations, therefore we can conclude that
uncertain expectation of outcome type is associated with
activation in the vmPFC, mid-cingulate, and SI.
Using a gambling task that also provided uncertainty
about outcome type (reward or punishment), Bechara and
colleagues [24] demonstrated that patients with vmPFC
lesions failed to show autonomic arousal (as measured by
skin conductance) during high uncertainty or risk. In
healthy volunteers, autonomic arousal is the first indicator
of behavioral inhibition and improved task-related risk
management [25]. The vmPFC activation in the present
context could therefore also mediate an autonomic ‘alert’
signal triggering reassessment of behavioral strategy. As
will be discussed below, vmPFC might receive the signal
from the hippocampal formation [26].
Mid-cingulate cortex, the other activation site identified
by Hsieh and Porro, is the second most frequently reported
activation site in pain neuroimaging after the insular
cortex [27]. These two brain areas are also implicated
in increased somatosensitivity resulting from uncertain
expectation of outcome type [28], a process that probably
also accounts for the mid-cingulate activity found by Hsieh
and Porro. The neural basis of anxiety-induced hyperalgesia
will be discussed in more detail below.
Uncertainty about pain intensity
A different activation pattern emerges when subjects are
certain that impending stimulation will be painful, but
uncertain as to how painful. The experimental paradigm
by Reiman et al. [21] used three PET scans per subject.
Subjects were informed that they would receive no pain
during the first and last scan, but would receive a painful
electric shock sometime during the second scan, and that
this shock would be more painful the longer the delay
between the start of the scan and delivery of the shock. In
fact, the shock was only delivered after the scan. Using
this paradigm, Simpson et al. [29] showed that uncertain
expectation of pain intensity was associated with decreased
activation in vmPFC, and that the extent of this deactivation
was inversely related to the anxiety about the
likelihood of impending pain: the more anxious the subject,
the less deactivation in vmPFC.
Drevets et al. [30] and Hsieh et al. [23] used the same
paradigm except that they repeated the scan involving
threat of shock multiple times within each subject, which
could have turned scanning into a safety signal by allowing
subjects to learn that no shocks are delivered during scans.
Drevets et al. found that uncertain expectation of pain
intensity was associated with decreased activation in SI,
which was positively related to anxiety about the likelihood
of impending pain: the more anxious the subject, the
more deactivation in SI. Hsieh et al. found decreased
activation in the vmPFC, consistent with Simpson et al.
[29], as well as in mid-cingulate cortex.
It seems plausible that subjects devote more processing
capacity to analyzing the likelihood of strong than mild
pain. In the Reiman task, the subjective certainty of strong
pain increases throughout the PET scan. One might
expect, therefore, both the neural substrates of certain
and of uncertain expectation of stimulus type to be
engaged, the latter decreasingly, the former increasingly,
in the course of the scan. Anxiety, as measured by Simpson
and colleagues, can be thought of as a marker of the
contribution of uncertain expectation of stimulus type
(painful/innocuous) to the activation image – that is, more
or less activation in the case of vmPFC. Under the
assumption that the deactivating influence of certain
expectation on vmPFC exceeds the activating influence of
uncertain expectation of stimulus type, the activation
pattern observed by Simpson et al. [29] would be expected:
the more anxious the subject, the less deactivation in
vmPFC. However, data pertinent to the assumption have
not yet been reported.
Anxiety-induced hyperalgesia
Uncertain expectation and anxiety have been shown to
increase pain sensitivity. Ploghaus et al. [8] performed
an fMRI study to examine the functional neuroanatomy
underlying this process. In the study, pain stimuli of
different intensities were preceded by visual signals. One
visual signal, which reliably predicted pain of moderate
intensity, came to evoke low anxiety about the impending
pain. Another visual signal was followed by the same,
moderate-intensity stimulation on most of the trials, but
occasionally by discriminably stronger noxious stimuli,
and came to evoke higher anxiety.
We found that physically identical noxious stimulation
was perceived as more painful in the context of higher
anxiety. This anxiety-induced hyperalgesia was associated
with activation in the entorhinal area of the hippocampal
formation, and correlated activity in the ACC and insula.
This is consistent with the Gray–McNaughton theory [31],
which proposes that the hippocampal formation responds
to aversive events whenever they form part of a behavioral
conflict (e.g. during uncertain expectation). It resolves the
conflict by sending amplification signals to the neural
representation of the aversive event, thereby biasing the
organism toward a behavior that is adaptive to the worst
possible outcome. According to the theory, this process is
accompanied by anxiety.
Conclusion
Cognitive and emotional processes interact with pain
perception, and the neural circuitry underlying this
interaction provides an untapped opportunity for targeting
acute and chronic pain states. In the last decade,
functional neuroimaging of pain has provided us with a
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sound understanding as to which brain areas are activated
by acute painful stimulation. However, research on the
neural pathways underlying pain modulation by cognitive,
emotional, pharmaceutical and other factors, is only
just beginning.
One important cognitive factor interacting with pain is
expectation. The present article argues that the behavioral
effects and the neural basis of expectation depend on the
level of certainty. Certain expectation is associated with
activity in the rostral anterior cingulate cortex and in the
posterior cerebellum, whereas uncertain expectation activates
the ventromedial prefrontal cortex, mid-cingulate
cortex and hippocampus. Our analysis illustrates that
apparent variability between results in functional neuroimaging
is often caused by seemingly subtle differences in
experimental design, in the present case different levels of
certainty. These differences can cause different behavioral
consequences and therefore different activation patterns.
Expectation is a multidimensional construct and is
associated with multiple brain activation sites. An important
future direction is to achieve a closer mapping between
the psychological components (cognition, emotion, autonomic
responses, motor responses and others) of expectation
on the one hand, and brain activation sites or
functional connectivity on the other hand. Recent studies
on the expectation of appetitive events [32], and of monetary
gain and loss [33], provide exemplary examples of this
research direction.
The ability to predict biologically relevant events is
essential for survival, but is also a critical factor in
common disease states (e.g. chronic pain and anxiety
disorders). A better understanding of the neural processes
underlying different forms of expectation is of great
interest from a basic science perspective, but will also
help to develop novel therapeutic strategies.
Acknowledgements
This work was supported by a grant from NIH to DB (NIDA DA 13650).
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