Table 2.

Putative roles of neurotransmitters in active inference.

Neurotransmitter	Precision	Evidence
Acetylcholine	Likelihood	•Presence of presynaptic receptors on thalamocortical afferents (Sahin et al., 1992; Lavine et al., 1997) •Modulation of gain of visually evoked responses (Gil et al., 1997; Disney et al., 2007) •Changes in effective connectivity with pharmacological manipulations (Moran et al., 2013) •Modeling of behavioral responses under pharmacological manipulation (Vossel et al., 2014; Marshall et al., 2016)
Noradrenaline	Transitions	•Maintenance of persistent prefrontal (delay-period) activity (requiring precise transition probabilities) depends upon noradrenaline (Arnsten and Li, 2005; Zhang et al., 2013) •Pupillary responses to surprising (i.e., imprecise) sequences (Lavín et al., 2013; Liao et al., 2016; Vincent et al., under review) •Modeling of behavioral responses under pharmacological manipulation (Marshall et al., 2016)
Dopamine	Policies	•Expressed post-synaptically on striatal medium spiny neurons (Freund et al., 1984; Yager et al., 2015) •Computational fMRI reveals midbrain activity with changes in precision (Schwartenbeck et al., 2015) •Modeling of behavioral responses under pharmacological manipulation (Marshall et al., 2016)
Serotonin	Preferences or interoceptive likelihood	•Receptors expressed on layer V pyramidal cells (Aghajanian and Marek, 1999; Lambe et al., 2000; Elliott et al., 2018) in medial prefrontal cortex •Medial prefrontal cortical regions heavily implicated in interoceptive processing and autonomic regulation (Marek et al., 2013; Mukherjee et al., 2016)