No single symptom is indispensable to diagnosis. But measures of the condition include motor activity, attention, motivation and psychostimulant responsiveness. In the study of the condition, measuring biological correlates of unusual function may prove useful for management of the condition, but could mislead in the search for causes. Investigation of attention-related function and associated regional cerebral activity would be improved by the careful application of the results of neuropsychological and psychophysiological study. A more extensive use of within subject protocols would greatly assist interpretation of the relevance of physiological states and the contribution of activity in different transmitter systems.
Attention is drawn to the trophic effects of the monoamines.
Symptom-models attempt to investigate the determinants a single feature of the illness. An interesting observation here is that where NA activity is low, learning in a variety of situations occurs at normal rates to a modest criterion, but then slows severely before eventually reaching good stringent criteria. The resolution oif the enigma may well lie in a better understanding of the interactions of the mesolimbic with the mesocortical DA system. Mesocortical activity can suppress mesolimbic activity, impairment of frontal function releases the mesolimbic system - a change that can be countered by psychostimulant treatment. Disorder-models are concerned with mimicking a whole cluster of symptoms if not the syndrome itself.