Limitations and perspectives

In this thesis, we presented theoretical and empirical works, in which we proposed using the appraisal theories of emotion to adapt predictions of the incentive salience hypothesis formulated on animals to humans. This work presented several limitations illustrating where new efforts need to be made and opening new perspectives; hereafter we described those that we feel are particularly interesting.

Relevance: From Physiological States to Current Concerns

The computational model of wanting define relevance based on the current physiological state of the organism, consisting of real internal state of the organism at a particular time in respect to ideal setpoint that regulates homeostasis, such as hunger, thirst, drugs effects or stress (also called k factor; Zhang et al., 2009). In the theoretical part of this thesis, we proposed to extend the concept of relevance to a larger variety of motivational states (i.e., current concerns) than simple physiological states. Nonetheless, in the empirical part we manipulated physiological states (i.e., hunger and stress) rather than other motivational states (e.g., concerns based on socialization, personal sensitivity or momentary goals). This was a specific choice driven by the necessity to proceed in small steps. Before extending the concept of relevance to other motivational states, we decided to verify whether we could find comparable results to the animal literature, by using manipulations that were as similar as possible to those used in studies conducted on animals.

Here, we brought evidence suggesting human and animal findings are similar when using comparable manipulations, this might provide a base on which future studies could build on, by investigating the role of the UCS relevance for other psychological concerns on the CS’s ability to bias attention and to trigger wanting peaks. This line of research is currently being conducted by Vanessa Sennwald (e.g., Sennwald et al., 2015) who is manipulating the UCS’s relevance to the current sexual concerns of the individual. In these studies, a group of homosexual men and a group of heterosexual men learn to associate a CSs to an erotic picture of man and a woman.

Erotic pictures of men are relevant for the current concerns of homosexual men, whereas erotic pictures of women are relevant for the current concerns of heterosexual men. The influence of the UCS’s relevance on the CS’s ability to bias attention and trigger wanting peak is then tested.

The meta-analysis we conducted (see chapter 2.1) provides encouraging findings for the conceptual extension of relevance from physiological states to current concerns: the attentional bias was larger for relevant compared to non-relevant rewards and reward associated stimuli. Some of the studies included in the meta-analysis used manipulation of relevance beyond physiological state, for instance the relevance of social motives (i.e., in group vs. out group; Hodsoll, Quinn, & Hodsoll, 2010).

Interactions Between Wanting and Other Systems Controlling Reward-Seeking Behaviors

Previously, we discussed how reward-seeking behaviors are controlled by three functionally interacting systems: Pavlovian, habitual and goal-directed. In the present thesis we focused exclusively on the functioning of the Pavlovian system on which wanting relies, without considering the interactions with the other systems.

An influential theoretical idea is that a number of irrational reward-seeking behaviors could be as emerging through interactions between two or more of these systems (Dayan, Niv, Seymour, & Daw, 2006). One of the studies in the present thesis, adapted from animal studies, has shown that stress amplifies CS-triggered wanting peaks, in this context the interactions with the habitual systems could be particularly interesting, since several other studies showed that stress also amplifies the control of the habitual over the goal-directed system (for review see Pool et al., 2015).

The notion that interactions between the Pavlovian system and the habitual system are especially important for irrational reward-seeking behaviors received some preliminary support from the work of Holland (2004). He induced habits vs. goal-directed actions by manipulating the amount of instrumental training the rodents received and then tested the amount of effort invested on habits and goal-directed actions leading to a reward during the presentation of a Pavlovian stimulus previously

associated with the same reward. He demonstrated that Pavlovian stimuli have a stronger motivational influence on habits than on goal-directed actions. Thus, if the reward engendered by the habitual action is no longer valued by the individual, the energizing influence of the Pavlovian system on the habitual system would be expected to result in the elicitation of irrational reward-seeking behavior.

Future studies are needed to further explore the extent to which in humans a privileged excitatory influence of Pavlovian stimuli on habits exists in humans, and to find out whether or not this interaction can promote the emergence of reward-seeking behaviors for a reward that is no longer liked.

The Incentive Salience Hypothesis in Humans: From Behavior to the Brain The incentive salience hypothesis has been formulated from behavioral observations triggered through brain manipulations (Berridge & Robinson, 1998). In animals, the neural correlates of wanting and liking have been investigated long before the precise definition of the underlying psychological computations (Berridge

& Robinson, 1998). This important aspect has not been investigated in the empirical part of this thesis and represents a particularly interesting perspective for futures studies. Here, we developed a human adapted version of the Pavlovian-Instrumental Transfer (a task used to measure wanting in animals) using a primary sensory reward (i.e., a rewarding chocolate odor). This paradigm enables the measurement of wanting but also the sensory liking experience for the same reward, in a similar way as what is commonly done on animals.

Such a paradigm combined with high-resolution functional magnetic resonance imaging (hr-fMRI) could be used to investigate some open questions in the human literature and notably the role of the nucleus accumbens in human wanting and liking. Animal studies have shown that different subnuclei of the nucleus accumbens contribute separately to the functions of wanting and liking. Critically, they demonstrated that manipulating the level of dopamine in the nucleus accumbens (principally the core part) increases the CS’s ability to trigger wanting peaks without simultaneously modifying the liking experience during the reward consumption (e.g., Mahler & Berridge, 2009; Pecina et al., 2003; Wyvell & Berridge, 2000). They also demonstrated that liking is based on a different neural network consisting of a series

of small hotspots located in the nucleus accumbens shell and other subcortical regions (e.g., ventral pallidum), which are modulated by opioids but not by dopamine (Berridge & Kringelbach, 2015; Pecina & Berridge, 2000, 2005; Smith & Berridge, 2007). Studies conducted on humans found the nucleus accumbens to be involved in CS-triggered wanting peaks (Talmi, Seymour, Dayan, & Dolan, 2008), however, they showed that others brain sites are involved in liking, most notably the orbitofrontal cortex (OFC; Kringelbach, 2005). The function of this cortical region appears to be more linked to the coding of the liking experience rather than causally generating the liking experience per se (Kringelbach, 2005; Kringelbach & Berridge, 2009;

Kringelbach, O’Doherty, Rolls, & Andrews, 2003). Indeed, several clinical observations reported that humans in which the prefrontal cortex was fully missing still showed liking hedonic reactions (Shewmon, Holmes, & Byrne, 1999), suggesting the existence of subcortical hedonic hotspot in humans similar to those showed on animals. In humans, the precise contribution of the accumbens’ subnuclei remains poorly understood because of the limitations in the spatial resolution of human brain imaging methods (Prevost, McCabe, Jessup, Bossaerts, & O'Doherty, 2011).

Recently, hr-fMRI protocols have been developed to overcome this obstacle (e.g., Prevost, Liljeholm, Tyszka, & O'Doherty, 2012). More particularly, Balik et al., (2013) developed an fMRI protocol through which they functionally parceled the human nucleus accumbens into putative core (i.e., the caudal and medial part) and shell (i.e., the rostral and lateral part).

The combination of hr-MRI protocols with animal paradigm adapted for humans could be promising to investigate the role of the nucleus accumbens in wanting and liking and it has already revealed to be effective to investigate the role of human amygdala subnuclei on the ability of the CS to influence reward-seeking behaviors (Prevost et al., 2012).