Figure 5.

Alternative schemes for integrating the action-selection substrates. (a) A hierarchical architecture: lower levels represent increasingly simple actions, selected by the higher layers. This is consistent with the output of the basal ganglia reaching the mRF via the PPN, and with the results of our modelling work. (b) A layered architecture: the mRF and basal ganglia form separate layers in a control system dealing with increasingly complex stimuli, the higher layers being able to veto the output of the lower layers. This design is consistent with the separate sensory input to the basal ganglia and mRF, and with the basal ganglia's access to the spinal cord via the PPN. (c) A combined architecture: the competences of each layer contribute to the whole system. This is consistent with the evidence for feedback pathways within the neural systems, particularly between the PPN and the basal ganglia. Arrows, excitatory pathways; open circles, inhibitory pathways.