For a long time, experimental research on emotion has been mainly focused on negative emotions, particularly fear. Fear was one of the first emotions investigated in affective neuroscience, in particular through aversive conditioning in rodents (for review, see LeDoux, 1996). Findings from this animal literature had an enormous impact on researchers investigating human emotions. They tested whether these neuropsychological mechanisms could also apply to human fear (LaBar, Gatenby, Gore, LeDoux, & Phelps, 1998) and whether modeling these mechanisms in humans could improve the understanding of fear-related clinical disorders such as generalized anxiety, specific phobias, or panic attacks (see Bar-Haim, Lamy, Pergamin, Bakermans-Kranenburg, & van Ijzendoorn 2007).
The preferential interest for fear compared to other emotions as well as the key importance of fear for survivial raised the convinction that fear had a particulary privileged evolutionary status. Ohman & Mineka (2001) postulated that, through evolution, organisms developed a prepared and automatic fear module implemented in the amygdala, allowing the organisms to rapidly detect and react to stimuli representing a threat. However, this assumption has been contested by several theories of emotions (Moors et al., 2013; Scherer, 2001; Russell, 1980; Yik, Russell, &
Barrett, 1999), which have proposed that emotions are not modular but are rather underlain by common dimensions or mechanisms. For instance, appraisal theories propose that all emotions are determined by the appraised relevance of a stimulus event with respect to the individual’s current concerns (Frijda, 1988; Sander, Grandjean, & Scherer, 2005). Current concerns can be related to survival and threat avoidance, but also to another large variety of psychological motives (e.g., self-achievement), physiological states (e.g., hunger) values and needs (e.g., security) that are of major importance for the individual (Frijda, 1988). Therefore, according to these theories, the processing of rewarding stimuli inducing positive emotions should function in many similar ways than the processing of threatening stimuli inducing negative emotions.
Positive rewarding stimuli are particularly interesting to test such an assumption. They have been defined as positive, eliciting approach and inducing learning (Berridge & Kringelbach, 2008; Schulz, 2004). Therefore, similarly to negative threatening stimuli, they are also affectively relevant and they can be conceptualized as being symmetrically opposite in terms of valence and action
tendency: whereas the latter have a negative value and elicit avoidance, the former have a positive value and elicit approach.
Animal research has a long tradition of investigating appetitive conditioning (e.g., Hull, 1943; Spence, 1966), however, differently from aversive conditioning, it has not been related to emotions but rather to fundamental learning processes (e.g., associative processes, habits acquisition). The incentive salience hypothesis (Berridge
& Robinson, 1998) has been one of the first animal models highlighting the affective component of reward processing and clearly distinguishing it from the learning component in the context of appetitive conditioning. Indeed, apart from learning, the incentive salience hypothesis proposes two other distinct components: (a) wanting, consisting in a motivational state triggered by the perception of a reward or its associated Pavlovian conditioned stimulus (CS); and (b) liking, an affective component consisting of the hedonic pleasure experienced, triggered by the receipt of consumption of the rewarding unconditioned stimulus (UCS). Interestingly, these affective components can be dissociated, leading thereby to irrational reward-seeking behaviors where the amount of effort mobilized to obtain a reward is no longer justified by the reward ability to trigger hedonic pleasure. Animal studies showed that under some particular circumstances an organism can invest a considerable amount of effort pursuing a reward, even though once it obtains it, it is not experienced as pleasurable (e.g., Wyvell & Berridge, 2000). This kind of phenomenon occurs, because according to this hypothesis, wanting is driven by a mechanism that is different from the representation of the reward’s hedonic properties. This mechanism consists of the interaction between the current physiological state of the individual (e.g., level of mesolimbic dopamine, stress, hunger) and the perception of a CS (e.g., a photograph, a symbol) that is associated with a rewarding UCS (e.g., food).
The incentive salience hypothesis formulates two main postulates based on the aforementioned mechanism:
1. A relevant physiological state increases the CS perceptual salience that will then acquire the ability to rapidly and involuntary bias attention.
2. When the CS is perceived, it triggers wanting, the intensity of which is directly modulated by the relevant physiological state and can be
independent from the liking felt during the rewarding UCS receipt or consumption.
In the last decade, there has been a large increase of studies conducted to replicate findings from the animal literature on humans. A growing corpus of studies investigated the attentional bias for positive rewarding stimuli, as well as the possible dissociation between the influence of wanting and liking at both a behavioral and neural level. Several scholars have considered the independency of wanting and liking and the involuntary attentional orienting toward rewarding stimuli as potential mechanisms underlying a variety of human irrational reward-seeking behaviors such as over-eating, pathological gambling and addictive substance consumption (Finlayson, King, & Blundell, 2007; Tibboel et al., 2011; Wölfling et al., 2011).
However, experiments conducted on humans encountered several difficulties to coherently operationalize the constructs of wanting and liking formulated based on animal research (Havermans, 2011, 2012).
Appraisal theories of emotion might provide useful insights to adapt and translate the incentive salience hypothesis formulated on animals to a human population. In terms of psychological mechanisms, appraisal theories make a clear distinction between pleasantness or valence and affective relevance (Moors et al., 2013; Sander et al., 2005; Smith & Ellsworth, 1985). Pleasantness or valence is an evaluation of how good or how bad a stimulus is going to be, whereas affective relevance consists of the interaction between the stimulus and the current concerns of the individual perceiving it. Affective relevance represents a mechanism that is very similar to the one proposed to underlie wanting in the animal literature. Both are composed of the interaction between the stimulus’s attributes and the organism’s motivational state, but while animal experiments manipulated relevance by inducing physiological states such as hunger or thirst (Balleine, 1994; Dickinson & Dawson, 1987; Robinson & Berridge, 2013), experiments on humans highlighted a larger variety of motives (i.e., concerns based on socialization, personal sensitivity or momentary goals; Frijda, 2010; Sander, Grandjean, & Scherer, 2005).
An overview of the goal and the structure of this thesis
The main goal of the present thesis was to test whether the two main postulates of the incentive salience hypothesis formulated on animals can also be
shown on a human population. More precisely we aimed to empirically test (a) whether the CS’s ability to bias attention was directly modulated by its relevance to the current physiological state of the individual and (b) whether the CS’s ability to trigger peaks of wanting to obtain the associated UCS was directly modulated by the current physiological state of the individual, independently from the liking experienced during the rewarding UCS consumption.
To this end, we first reviewed the existing human literature on these topics (chapter 2) and then empirically tested the two postulates on a human population (chapter 3).
The theoretical part (chapter 2) is divided in two parts. First (chapter 2.1), we systematically reviewed the existing literature on the attentional bias for rewards and reward-associated stimuli. We conducted a meta-analysis on the studies found through the systematic search, investigating whether the attentional bias for rewarding stimuli was modulated by the stimulus relevance for the individual concerns rather than its valence. Second (chapter 2.2), we systematically reviewed the existing literature on studies measuring wanting and/or liking on a human population. We systematically described the different operationalizations in respect to the main tenets of animal research. This highlighted how operationalizations of wanting and liking in humans are sometimes far from the constructs measured in animals, therefore introducing confounds at level of the mechanisms reflected in their measures.
Based on these observations, we decided to empirically test the two postulates of the incentive salience hypothesis by using paradigms that were as similar as possible to those used in the animal literature (chapter 3). More precisely, we used Pavlovian and Pavlovian to instrumental transfer procedures in which the rewarding UCS consisted of a chocolate odor. Using an olfactory chocolate reward presented several advantages in the context of this thesis. First, being a primary reward, it can be consumed, triggering a liking experience that can easily be reported. Second, being a food odor, the relevant physiological state (i.e., hunger) could easily be manipulated. Finally, odors could be precisely administered through a computer-controlled olfactometer. Through these methods, we conducted two main studies. The first study (chapter 3.1) included two empirical experiments investigating the attentional bias for CS associated with odors, in respect to the relevant physiological state of hunger. The second study (chapter 3.2) investigated whether a physiological
state of stress selectively increased CS-triggered wanting without a parallel modification of liking.
To conclude, in chapter 4, we integrated the findings of our empirical studies with the framework presented in the theoretical section. We discussed the contribution of our findings and of the utilization of appraisal theories to test the incentive salience hypothesis on humans. We also discussed the limitations of our
research and potential future perspective.