Fig. 1.

Structures of the vertebrate midbrain (not to scale) supporting the behavioral “core control system.” The vertebrate midbrain supports an integrated multisensory model of the state of the animal in space, which supports effective decision making. (A) Decision making involves an assessment of what is needed and where and how the needed resources can be obtained. Decision making can therefore be considered to involve three domains: internal motivations, target selection, and action selection [adapted from the “selection triangle” proposed by Merker (8)]). These domains interact and can be resolved by referencing an integrated neural model that contains information on the state of self, self movement, environmental state and structure, and memory of prior experience. These elements of the neural model are supported by midbrain structures (B). As a simplification, regions are colored according to their primary function(s) in the neural model described in A. The superior colliculus [part of the tectum (TEC) forming the roof of the midbrain] receives topographically organized multisensory input and creates an integrated neural model of the organism moving in space (8). The floor of the midbrain is formed by the hypothalamic structures (Hyp) and associated nuclei [pituitary (pt) and mammillary bodies (M)] that collate information on the physiological status of the organism referenced with prior experience, to identify needs to maintain a homeostatic optimum (15, 56). Integrative structures within the midbrain and basal ganglia, including the periaqueductal gray (P), substantia nigra (N), thalamus (Tha), striatum (St), and midbrain reticular formation (MR), integrate these sources of information (8, 33) (A). In advanced vertebrates the cortex and hippocampal structures clearly have a very strong input in to this system, but this midbrain system is not dependent on cortical inputs to function (140) and is highly conserved across all vertebrate lineages (33).