Cortical-subcortical re-entrant circuits and recurrent behaviour

Objective: To explore the application of Parallel Distributed Processing (PDP) models to biological cortical-subcortical re-entrant circuits, and their implications for recurrent psychiatric symptomatology. Method: The literature on cortical-cortical and cortical-subcortical re-entrant circuits is reviewed and possible neural network (PDP) analogies are described. Examples from the attention deficit hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD) literature are explored and compared. Results: A common circuit architecture for at least five circuits was found. Recurrent PDP models suggest that subcortical centres can exert significant influences on behaviour, but their effect will depend on gating processes at higher centres, governed by both feedback (phasic arousal/reinforcement) and synaptic activity at higher levels. The latter can vary around a 'Golden Mean' at cortical receptor levels, which degrades when it is either over-aroused or under-aroused. The Continuous Performance Task, which models both working memory and inhibitory processing is a useful task for functional imaging of an attentional network. Conclusion: Recurrent symptoms occur as a result of incoordination between phasic subcortical inputs and synaptic gating processes in cortical-subcortical circuits. Obsessive-compulsive disorder and ADHD represent opposite manifestations of recurrent cortical-subcortical processing deficits, with excess arousal in the former and insufficient maintenance of reinforcement and working memory in the latter. These processes may be subject to separate genetic influences at cortical versus subcortical levels.