The historical interplay between akinesia, bradyphrenia and psychomotor retardation is reviewed. It is concluded that bradyphrenia is a common accompaniment to the motor disabilities of Parkinson's disease and consists of slowness of thought, impaired attention and motivation, lack of spontaneity, inflexibility and forgetfulness. Impaired attentional extra-dimensional set-shifting may be responsible for many of these problems. Some clinical overlap occurs between Parkinson's disease, severe depressive illness and obsessional-compulsive disorder and it is proposed that there may be shared biological disturbances involving basal-ganglia-limbic-frontal circuits in the three conditions.