The Influence of Catecholamines on Working Memory and the Activational Aspects of Motivation
Catecholamines are thought to have a major modulatory effect on cognition. However, there is only equivocal evidence that catecholamines have a direct impact on cognition. A body of research suggests that catecholamines influence motivation, through which they may have an indirect effect on cognition. Theories of motivation distinguish directional (behaviour toward or away from some stimuli) and activational aspects of motivation. The activational aspects of motivation (or behavioural activation) refer to the quantitative and qualitative features of motivated behaviour (speed, vigour and persistence) that enable organisms to overcome constraints that prevent them from obtaining a motivational stimulus. Here we investigated the contribution of catecholamines to cognition (using working memory as a cognitive model; working memory is a limited-capacity cognitive process that temporarily retains relevant information to guide thoughts and actions) and behavioural activation by augmenting catecholamine neurotransmission using the catecholamine reuptake inhibitors methylphenidate (MPH; 0.1-10 mg/kg) and atomoxetine (ATX; 0.01-1.0 mg/kg) in adult female Chinese rhesus macaques (Macaca mulatta). We also tested the effects of diminishing catecholamine neurotransmission on working memory and behavioural activation using an acute tyrosine phenylalanine depletion (ATPD) method, which we developed and demonstrated its effectiveness at impairing brain catecholamine synthesis and function. A visual sequential comparison (VSC) task, which allows the systematic manipulation of working memory load was used to assess working memory. Behavioural activation was assessed using several measures: the animals’ reaction time (speed), saccade peak velocity (vigour), successful trial completion (persistence), successful trial initiation (persistence), trial initiation time (speed), and anticipatory trial initiation (vigour) in the VSC task. A visual progressive ratio (PR) schedule of reinforcement task was also used to assess behavioural activation; the PR breakpoint reflects persistence. The results of the MPH, ATX and ATPD studies showed that catecholamines have little to no impact on working memory but a significant influence on behavioural activation.