Guest Writer & Computational Neuroscientist Michael Chary
Knowing the structure of connections between brain regions, for example whether the projections are topographic, will help provide an anatomic basis for psychiatric diseases and understanding how deep brain stimulation works.
Three prefrontal cortical (PFC) areas play separable roles in reward-guided decision making and may also be important in understanding psychopathology. The anterior cingulate cortex, whose activity is increased in OCD patients, chooses among behavioral plans by monitoring for errors in reward prediction and shifting attention. The ventromedial prefrontal cortex, whose activity is heightened in addicts, computes the chosen value of a behavioral plan. The orbitofrontal cortex, whose activity is increased in depressed patients links stimuli to their reward values. Furthermore, increased OFC activity is associated with depressed affect in normal patients. And, decreasing OFC activity back to normal suffices to remove some affective symptoms of depression.
These regions overlap in the prefrontal cortex and project to overlapping regions in the striatum. Their projections comprise 22% of the striatum’s volume and arborizations so widely that single cells can take up 14% of the striatum’s volume. Akin to dendritic arbors spanning multiple layers of V1, this allows for integration across basal ganglia-cortex circuits.
The dorsal striatum is associated with motor planning. The ventral striatum, also called the nucleus accumbens, projects to its dorsal counterpart through the substantia nigra to link the limbic and motor systems. The axon terminal fields of ventral striatal projections to the substantia nigra are quire broad, which suggests that accumbal projections are divergent. As drug habits are formed, activation measured by fMRI shifts from the ventral to dorsal striatum through this pathway.
Motor and limbic pathways converge again subthalamic nucleus. The globus pallidus, a motor and associative nucleus, and the ventral pallidum, a limbic area, make convergent synaptic inputs onto subthalamic neurons.
Deep brain stimulation may be effective in psychiatric diseases because of its effects on white matter bundles. The above anatomic knowledge helps relate the symptoms of psychiatric diseases with diffusion tensor imaging (DTI) results. DTI indicates less white matter integrity in patients with psychiatric diseases. Disrupting prefrontal cortical input to subcortical structures could explain the attenuation of executive function and misattribution of rewards present in affective disorders.