Free will versus survival: brain systems that underlie intrinsic constraints on behavior.

This article discusses the neuroanatomical systems involved in the related functions of fear and discernment of the consequences of one's actions, two internal constraints on free will and action. Both mechanisms are related to a system for control and modulation of visceral function stretching from the spinal cord to the ventromedial prefrontal cortex, including the ventral striatum, ventral pallidum, and mediodorsal thalamus, the amygdala, the hypothalamus, the periaqueductal gray (PAG), and the brainstem reticular formation and autonomic nuclei. Reflexes at the lower levels provide rapid visceral and somatic reactions to threatening stimuli, while the PAG and hypothalamus coordinate these to produce more concerted responses. The amygdala interacts with the cortical sensory systems in the assessment of fear-related stimuli and modulates the reflex responses through projections to the hypothalamus and brainstem. The ventromedial prefrontal cortex, especially the "medial prefrontal network," is connected to the amygdala, hypothalamus, and PAG, and allows cortical control over the system in relation to a wider set of emotions. This cortical region is involved both in the assessment of reward and in mood disorders and it plays a central role in the ability to discern the consequences of one's actions and make appropriate behavioral choices. It also forms an interconnected circuit with specific cortical areas in the rostral superior temporal cortex, posterior parahippocampal cortex, and retrosplenial/posterior cingulate cortex. The overall function of this circuit is unclear, but may be involved in introspective monitoring of the individual.