Behavioral Apathy and its Relationship with Physical Activity, Affective Attitudes, and Motor Cortex Excitability

Abstract

Apathy is a multidimensional syndrome characterized by diminished motivation and reduced goal-directed behavior. Although apathy is commonly studied in neurological and psychiatric populations, it is also observed in otherwise healthy adults and may represent an important barrier to engagement in physical activity. Because physical activity requires effort, action initiation, and persistence, individuals with higher levels of behavioral apathy may be less likely to initiate and maintain active behavior. Despite growing interest in the relationship between apathy and physical activity, the behavioral and neurophysiological mechanisms underlying this association remain poorly understood. This thesis examined the relationship between behavioral apathy, physical activity, affective attitudes toward physical activity, and motor cortex neurophysiology. Across four complementary studies, behavioral and neurophysiological pathways that may contribute to reduced physical activity in individuals with higher apathy were investigated. Study 1 is a systematic review and meta-analysis examining the association between apathy and physical activity across clinical and non-clinical populations. Study 2 examined whether intentions, explicit affective attitudes, and approach-avoidance tendencies toward physical activity mediated the relationship between behavioral apathy and physical activity in healthy adults. Study 3 is a systematic review and meta-analysis investigating the association between habitual physical activity and measures of motor cortex neurophysiology, including corticospinal excitability, intracortical inhibition, and intracortical facilitation. Study 4 used transcranial magnetic stimulation (TMS) to examine whether behavioral apathy was associated with these neurophysiological measures in healthy adults. Across the four studies, results demonstrated that apathy is associated with physical activity, but that this association is more strongly explained by motivational and affective processes than by resting-state motor cortex neurophysiology. The systematic review and meta-analysis showed a significant negative association between apathy and physical activity, with stronger associations in older adults and individuals with Parkinson’s disease. Behavioral analyses further demonstrated that higher behavioral apathy was associated with lower physical activity, partly through weaker intentions to be physically active and less positive affective attitudes toward physical activity. Individuals with lower apathy also showed stronger tendencies to approach physical activity stimuli and avoid sedentary stimuli, although these tendencies did not mediate physical activity behavior. In contrast, the neurophysiological studies showed no reliable associations between habitual physical activity and measures of corticospinal excitability, intracortical inhibition, or facilitation. Similarly, behavioral apathy showed negligible associations with these measures, with equivalence testing indicating that any potential effects were too small to be practically meaningful. These results suggest that the relationship between apathy and physical activity is primarily linked to motivational and affective mechanisms rather than stable differences in primary motor cortex neurophysiology. Specifically, higher behavioral apathy was associated with weaker intentions to be physically active and less positive affective attitudes toward physical activity, whereas neither habitual physical activity nor behavioral apathy showed meaningful associations with resting TMS-derived measures of corticospinal excitability or intracortical circuitry. Overall, this thesis refines the understanding of how apathy contributes to physical inactivity and highlights the importance of targeting motivational processes when designing interventions aimed at increasing physical activity engagement.