Qu'est ce qui nous pousse à agir? Les théories neuroéconomiques suggèrent que nos comportements sont déterminés par des mécanismes d'optimisation entre les coûts et les bénéfices attendus de nos actions. Dans ce travail, nous avons étudié les mécanismes de la motivation à l'effort grâce à des paradigmes de motivation incitative, et développé un modèle computationnel reposant sur une optimisation cout-bénéfice. Nous avons d'abord étudié les effets motivationnels d'interactions sociales, comme la coopération et la compétition, et montré que l'activité du complexe striatopallidal ventral corrélait avec les comportements utilitaires, tandis que l'activité et le volume de la jonction temporopariétale, prédisaient les biais altruistes, suggérant son rôle clé dans la détermination des efforts prosociaux. L'étude de patients parkinsoniens a ensuite permis, au niveau comportemental et computationnel, de dissocier les effets moteur et motivationnel de la dopamine. Ces effets prédisaient respectivement les variations des scores cliniques moteurs et d'apathie. Nous avons également montré que des traitements sérotoninergiques atténuaient les mécanismes de motivation incitative, pouvant ainsi rendre compte de l'existence de syndromes apathiques sous ces traitements. Enfin, nous avons pu identifier plusieurs profils d'atteinte motivationnelle au sein de pathologies neurologiques focales, dégénératives, ou psychiatriques. Ce travail propose ainsi un cadre normatif pour rendre compte des mécanismes déterminant la direction et l'intensité du comportement, et vise à progresser dans la compréhension des mécanismes de la motivation et de ses déficits dans les pathologies neuropsychiatriques. / Why do we do what we do? Recent neuroeconomic theories suggest that our behaviour is determined by optimizing the difference between the benefit and the cost of our actions. In this work, we studied the neural basis of motivation by using incentive motivation tasks, and we developed a computational model of goal-directed effort production that gathers theories of motor control with decision-making principles of cost/benefit optimization. We first studied the motivational effects of social interactions, such as cooperation and competition, and showed that the activity in the ventral striato-pallidal complex correlated with utilitarian behaviours, whereas the activity and the cortical volume in the temporoparietal junction predicted altruistic biases, suggesting a key role for this region in prosocial motivation.Next, we studied Parkinson’s disease patients, and showed a dissociation, at the behavioural and the computational level, between the motor and the motivational effects of dopamine. These effects predicted respectively the clinical change in motor dysfunction and apathy. We also showed that serotoninergic treatments in healthy subjects affected incentive motivation. This effect could account for the occurrence of apathetic syndromes with these treatments.Finally, we identified different profiles of motivational deficits in psychiatric, focal or degenerative neurological conditions.This work thus suggest a normative framework to account for the mechanisms that determine the direction and the intensity of the behaviour, and aims at better understanding the neural basis of motivation and its deficits among neuropsychiatric disorders.

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Psychological factors specific to overuse injury pain in physical activity contexts were explored within a social identity theoretical framework. Study 1 involved development of a method for designating overuse injury pain occurrence of hikers (N = 751), along with exploration of relationships between psychological measures, overuse injury pain occurrence, and effort levels. The findings of this cross-sectional, mixed-methods investigation revealed that social identification, social identity content, and mental toughness differentiated hikers who incurred overuse injury pain or selected a higher-effort behaviour from those who did not. From qualitative analysis, several social identity constructs (i.e., group member's presence, in-group status, social creativity, additional social identity content) emerged as contributors to overuse injury occurrence. The focus of Study 2 was a prospective examination of the aforementioned psychological factors in relation to overuse injury severity of hikers (N = 283). Additionally, the Test of Intentions to Reduce Effort (TIRE) was developed to identify individuals with susceptibility to higher overuse injury severity. Results provided evidence of factorial, construct, and predictive validity of TIRE factor scores. TIRE factors and social identity content significantly predicted higher severity of hikers' overuse injury pain. Mental toughness scores moderated the relationship between social identification and overuse injury severity. Study 3 consisted of a qualitative examination of social identity mechanisms of overuse injury pain in a physical activity context, CrossFit®, involving the presence of group leaders, and in which group members view each other. Findings revealed mechanisms pertaining to social identity content, in-group status, and social threats. Overall, the findings support a new means for assessing overuse injury occurrence and susceptibility to higher overuse injury severity, whilst demonstrating the potential applicability of social identity theory to the study of overuse injury. Knowledge gained may ultimately aid development of interventions to reduce overuse injury occurrence and severity of physical activity participants.