Serotonin and Dopamine: Unifying Affective,
Activational, and Decision Functions
Roshan Cools*
,1
, Kae Nakamura
2,3
and Nathaniel D Daw
4
1
Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Centre for Cognitive Neuroimaging,
Nijmegen, The Netherlands;
2
Department of Physiology, School of Medicine, Kansai Medical University, Moriguchi City,
Japan;
3
PRESTO, Honcho Kawaguchi, Saitama, Japan;
4
Center for Neural Science & Department of Psychology, New York
University, New York, NY, USA
Serotonin, like dopamine (DA), has long been implicated in adaptive behavior, including decision making and reinforcement
learning. However, although the two neuromodulators are tightly related and have a similar degree of functional importance,
compared with DA, we have a much less specific understanding about the mechanisms by which serotonin affects behavior.
Here, we draw on recent work on computational models of dopaminergic function to suggest a framework by which many of
the seemingly diverse functions associated with both DA and serotonin
Fcomprising both affective and activational ones, as
well as a number of other functions not overtly related to either
Fcan be seen as consequences of a single root mechanism.
Neuropsychopharmacology Reviews (2011) 36, 98–113; doi:10.1038/npp.2010.121; published online 25 August 2010
Keywords: aversion; reward; inhibition; impulsivity; activation; punishment
INTRODUCTION
The ascending monoamine neuromodulatory systems are
implicated in healthy and disordered functions so wide
ranging and so apparently heterogeneous that characteriz-
ing their function more crisply is an important scientific
puzzle. In the case of dopamine (DA)
Fwhich is involved in
cognition, motivation, and movement
Fnotable progress
has been made in the last decade using an interdisciplinary
and interspecies approach. In particular, computational
models of reinforcement learning (RL: trial-and-error
learning to obtain rewards) have been used as a framework
formally to interpret and connect observations from
neurophysiological, brain imaging, and behavioral/pharma-
cological studies in humans and animals.
In contrast, although the neuromodulator serotonin
(5-HT) has functional and clinical importance at least equal
to that of DA (eg, it is implicated in impulsivity, depression,
and pain), there is no similarly formal and well-developed
framework for understanding any of its roles. Here, we take
early steps toward such a theoretical framework by
reviewing aspects of function that have been prominently
associated with 5-HT, namely, aversive processing and
behavioral inhibition, and leveraging the example of DA to
suggest how the data supporting these ideas might be
interpreted, together with other functions, as manifestations
of a common, underlying computational mechanism.
In particular, we consider the implications of a recent
computational theory of DA (Niv et al, 2007) for offering a
common explanation for a number of seemingly distinct
functional associations of both DA and 5-HT. We discuss
the theory informally (omitting equations) and use it as a
framework to discuss studies using psychopharmacological
manipulations of 5-HT in humans and experimental
rodents, as well as single-neuron recording studies in non-
human primates. In the first half of the review, we discuss
how Niv et al’s concept of an opportunity cost of time offers
a common explanation for both affective (reward and
punishment) and activational (behavioral vigor and with-
holding) aspects of the neuromodulators’ functions. After
this, we develop this framework to discuss how a number
of additional, seemingly disparate, aspects of decision
making that have been associated with these systems, such
as time discounting and risk sensitivity, can also be seen
as consequences of the same mechanism. Throughout, we
stress many caveats, interpretation difficulties, and experi-
mental concerns; our goal here is to articulate a set of
important behaviors, computations, and quantities that
might guide more definitive experiments. In addition,
similar to Boureau and Dayan (2010; this issue) (see also
Dayan and Huys, 2008 and Daw, Kakade and Dayan, 2002),
Received 12 April 2010; revised 16 July 2010; accepted 16 July 2010
*Correspondence: Dr R Cools, Centre for Cognitive Neuroimaging,
Donders Institute for Brain, Cognition and Behaviour, Radboud University
Nijmegen, Kapittelweg 29, Nijmegen 6500HB, The Netherlands, Tel: + 31
243 610 656, Fax: + 31 243 610 989, E-mail: roshan.cools@gmail.com
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(2011) 36, 98–113
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our overall strategy is to push outward from our relatively
secure understanding of DA, through what is known about
the similarity and differences in DA and 5-HT functions and
about how the two neuromodulators interact, to extrapolate
a tentative extended understanding encompassing DA
and 5-HT collectively in a common framework. Boureau
and Dayan take a complementary approach, offering,
in particular, a more detailed discussion of the nature of
interactions between DA and 5-HT, and between reward
and punishment in the context of different components of
conditioning.
DA, REINFORCEMENT, AND BEHAVIORAL
ACTIVATION
The puzzles and controversies of DA have long centered
around the question of how to understand its seemingly
dual function in both reward and movement (Ungerstedt,
1971; Lyon and Robbins, 1975; Milner, 1977; Evenden and
Robbins, 1984; Berridge and Robinson, 1998; Ikemoto
and Panksepp, 1999; Schultz, 2007). On the one hand, DA
is implicated in motivation and reinforcement, for instance,
it is a focus of drugs of abuse and self-stimulation. On the
other, it is a facilitator of vigorous action: consider the
poverty of movement that accompanies dopaminergic
degeneration in Parkinson’s disease (PD) or the hyperactivity
and stereotypy engendered by psychostimulant drugs that
enhance DA, such as methamphetamine (Lyon and Robbins,
1975; Robbins and Sahakian, 1979). In principle, these two
axes of behavior might be independent, but they appear
instead to be closely coupled through the action of DA.
Thus, one early hypothesis (Mogenson et al, 1980)
characterized the nucleus accumbens (a key dopaminergic
target) as the ‘limbic-motor gateway’ in which motivational
considerations gained access to the control of action.
Echoing this idea, more recent RL theories link these
aspects by claiming that DA is involved in learning which
behaviors are associated with reward. Variants of the
reward/action duality also underlie longstanding contro-
versies about what psychological aspects of reward DA
might subserve
Ffor instance, hedonics, reinforcement, or
motivational and activational (Ikemoto and Panksepp, 1999;
Berridge, 2007; Robbins and Everitt, 2007)
Fand the
question whether DA impacts behavior via learning versus
performance (Gallistel et al, 1974; Berridge, 2007; Niv et al,
2007). We focus on this last question here.
Appropriately, given DA’s dual nature, theories of its
function have grown largely separately on two tracks,
rooted in different experimental methodologies and theore-
tical approaches. The predominant view in computational
and systems neuroscience holds that DA serves to promote
RL, that is, trial-and-error instrumental learning, to choose
rewarding actions (Houk et al, 1995; Montague et al, 1996;
Schultz et al, 1997; Samejima et al, 2005; Morris et al, 2006).
This idea is derived from electrophysiological recordings
from neurons in the midbrain dopaminergic nuclei of
primates performing simple tasks for reward (Ljungberg
et al, 1991; Hollerman and Schultz, 1998; Waelti et al, 2001),
together with the insight that the phasic firing of these
neurons quantitatively resembles a ‘reward prediction error’
signal used in computational algorithms for RL to improve
action choice so as to obtain more rewards (Sutton and
Barto, 1990; Montague et al, 1996; Sutton and Barto, 1998;
Montague et al, 2004; Bayer and Glimcher, 2005;
Frank, 2005). More recently, studies employing temporally
precise methods in freely behaving animals, such as
electrochemical voltammetric approaches, which enable
the measurement of phasic DA release directly (Day et al,
2007; Roitman et al, 2008), as well as optogenetic
approaches, which enable the transient activation of specific
DA neurons (Tsai et al, 2009), have substantiated these
ideas. Furthermore, functional neuroimaging has revealed
that similar prediction error signals in humans (McClure
et al, 2003; O’Doherty et al, 2003) might be modulated by
DA (Pessiglione et al, 2006), whereas microelectrode
recordings during deep brain stimulation surgery have
demonstrated that such prediction error signals are also
encoded by the human midbrain (Zaghloul et al, 2009) (see
also D’Ardenne et al, 2008).
At the same time, more psychological approaches, largely
grounded in causal manipulations (eg, drug or lesion) of
dopaminergic function, tend to envision DA as being
involved less in acquisition and more in the performance
of motivated behavior. Indeed, the most pronounced effects
of causal DA manipulations tend to be on performance
rather than learning, with DA promoting behavioral vigor
or activation more generally (Lyon and Robbins, 1975;
Ikemoto and Panksepp, 1999; Berridge, 2007; Robbins and
Everitt, 2007; Salamone et al, 2007). Two current inter-
pretations characterize these effects as arising via dopami-
nergic mediaton of incentive motivation (Berridge, 2007) or
cost/benefit tradeoffs (Salamone et al, 2007). Other authors
writing from a similar tradition have provided a more
general activational account, with parallel roles for DA in
the dorsal and ventral striatum (Robbins and Everitt, 1982,
1992; Robbins and Everitt, 2007), stressing both a perfor-
mance-based energetic component to DA and reinforce-
ment-related functions more akin to those posited in the
computational RL models, for example, conditioned re-
inforcement and stamping-in of stimulus–response habits
(Wise, 2004). Indeed, early experimental work by Gallistel
et al (1974) argued for both reinforcing and activational
effects of (putatively dopaminergic) brain stimulation
reward, distinguished as progressive and immediate effects
of contingent versus noncontingent self-stimulation.
MODELING THE DUAL FUNCTION OF DA
One attempt to reconcile these two streams of thought
(Niv et al, 2007) extended RL accounts, which had
traditionally focused on learning which action is most
rewarding, into an additional formal analysis of how
Multiple functions of serotonin and dopamine
R Cools et al
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