Psychological heterogeneity in AD/HD--a dual pathway model of behaviour and cognition.

Psychological accounts have characterised attention-deficit/hyperactivity disorder (AD/HD) as either a neuro-cognitive disorder of regulation or a motivational style. Poor inhibitory control is thought to underpin AD/HD children's dysregulation while delay aversion is a dominant characteristic of their motivational style. A recent 'head to head' study of these two accounts suggest that delay aversion and poor inhibitory control are independent co-existing characteristics of AD/HD (combined type). In the present paper we build on these findings to propose a dual pathway model of AD/HD that recognises two quite distinct sub-types of the disorder. In one AD/HD is the result of the dysregulation of action and thought resulting from poor inhibitory control associated with the meso-cortical branch of the dopamine system projecting in the cortical control centres (e.g. pre-frontal cortex). In the other AD/HD is a motivational style characterised by an altered delay of reward gradient linked to the meso-limbic dopamine branch associated with the reward circuits (e.g. nucleus accumbens). The two pathways are further distinguished at the levels of symptoms, cognitive and motivation profiles and genetic and non-genetic origins.