Neural circuit models of the serotonergic system

Serotonin is an important neuromodulator with wide range of functions that are broadly linked to decision making and reward/punishment processing. These functions include reward and punishment prediction, time scale of reward prediction, risk-seeking or impulsivity, risk-aversion among others. Dysfunction of the serotonergic system, therefore, is linked to disorders of decision making and reward/punishment processing like depression, addiction, anxiety, impulsivity and others. A major source of serotonin in the brain is a small cluster of cells known as the Dorsal Raphe Nucleus (DRN). Although the DRN neurons project to nearly every part of the brain, projections to two brain regions - the PreFrontal Cortex (PFC) and the Basal Ganglia (BG) - are important as substrates for decision making functions of serotonin. The first part of this chapter reviews systems-level computational models of the functions of the serotonergic system at microcircuit level. The second part presents models of the roles of sertonergic system in decision making. Particularly a line of modelling that reconciles the diverse roles of serotonin in reward/punishment sensitivity, risk sensitivity and time-scale of reward integration, is described in detail.