Schizophrenia, Parkinson’s disease, and attention deficit hyperactivity disorder (ADHD) discusses how hyperactive dopaminergic neurotransmission appears to underlie schizophrenia’s positive symptoms, loss of dopaminergic neurons in adulthood leads to Parkinson’s disease, and dopamine neuron hypofunction in childhood and adolescence may underlie ADHD. Positive schizophrenia symptoms may arise from excessive incentive learning that is gradually lost with antipsychotic treatment. Declarative learning and memory may contribute to delusions based on excessive incentive learning. Loss of responsiveness to environmental stimuli in Parkinson’s may result from a decrease of their conditioned incentive value and inverse incentive learning. Conditioned incentive stimuli not encountered while in a state of decreased dopaminergic neurotransmission may retain their incentive value, producing apparent kinesia paradoxa. Dopamine hypofunction in juveniles does not lead to hypokinesia but may result in loss of incentive learning that focuses attention. Pro-dopaminergic drugs have a calming effect in ADHD, presumably because they reinstate normal incentive learning.
Incentive learning and the motivational control of instrumental performance by thirst
