Orbital And Medial Prefrontal Cortex And Cingulate Gyrus

Cortical areas on the ventromedial surface of the frontal lobe, in the cortex dorsal to the orbit and the region rostral and ventral to the genu of the corpus callosum, are substantially interconnected with all the limbic areas discussed previously and participate in many of the same functions. These areas vary from periallocortical agranular areas with a relatively simple cortical structure in the caudal part of the region to fully developed granular cortical areas more

Amygdala Prefrontal Connection

Figure 5 Schematic summary of major axonal connections of the hippocampal formation. Interactions with the polymodal sensory cortical areas occur primarily through the entorhinal cortex (EC). The amygdala, the orbital and medial prefrontal cortex, and the cingulate cortex interact with both the EC and the subiculum (Sub), whereas the Sub provides the primary output to the anterior thalamic nuclei, mammillary nuclei, and hypothalamus. Within the hippocampal formation, the principal flow of activity is from the EC to the dentate gyrus (DG), field CA3, field CA1, the Sub, and back to the EC.

Figure 5 Schematic summary of major axonal connections of the hippocampal formation. Interactions with the polymodal sensory cortical areas occur primarily through the entorhinal cortex (EC). The amygdala, the orbital and medial prefrontal cortex, and the cingulate cortex interact with both the EC and the subiculum (Sub), whereas the Sub provides the primary output to the anterior thalamic nuclei, mammillary nuclei, and hypothalamus. Within the hippocampal formation, the principal flow of activity is from the EC to the dentate gyrus (DG), field CA3, field CA1, the Sub, and back to the EC.

rostrally. In rodents, the agranular areas dominate and the granular areas are almost nonexistent, whereas in primates and especially humans the granular areas are much larger than the agranular areas.

As a whole, the cortical areas can be divided into two relatively interconnected networks that appear to have related but distinct functions (Fig. 6). Sensory inputs from other cortical regions or the thalamus enter many of the orbital cortical areas, and these are integrated by corticocortical connections between these areas. Many of these are related to food or eating (e.g., olfaction, taste, visceral afferents, somatic sensation from the hand and mouth, and vision), and neurons in the orbital cortex respond to multisensory stimuli involving the appearance, texture, or flavor of food. In contrast, many of the areas in the medial prefrontal cortex and a few related orbital areas provide output from the cortex to visceral control centers in the hypothalamus and brain stem. Therefore, the orbital and medial prefrontal cortex appears to be adapted to evaluate feeding-related sensory information and to evoke appropriate visceral reactions.

The function of the ventromedial frontal cortex is considerably wider, however. Food is a primary reward, and many of the orbital neurons respond to rewarding or aversive aspects of stimuli beyond their sensory characteristics. In this, the cortex is closely tied to the function of the related ventromedial striatum, in which reward-related neural activity has also been found.

In addition, lesions of the ventromedial frontal lobe produce dramatic behavioral deficits, which suggests that visceral reactions evoked through this cortical

Figure 6 Diagram illustrating connections of architectonic areas in the ventromedial prefrontal cortex in macaque monkeys, drawn onto the orbital (right) and medial (left) cortical surfaces. Sensory input is particularly directed to an ''orbital'' network of areas (in the orbital cortex), whereas output to autonomic control centers in the hypothalamus and brain stem (especially the periaqueductal gray) arises from a ''medial'' network of areas (in the medial and orbital cortex). Corticocortical axonal connections between areas, which largely define the two networks, are indicated by lines connecting the areas. In addition to this sensory/visceromotor transfer, interactions with the amygdala, hippocampus, entorhinal cortex, and other limbic structure involve this cortical region in reward appreciation and affective behavior. The numbers and letters are designations of architectonic areas, modified from Brodmann; cc, corpus callosum.

Figure 6 Diagram illustrating connections of architectonic areas in the ventromedial prefrontal cortex in macaque monkeys, drawn onto the orbital (right) and medial (left) cortical surfaces. Sensory input is particularly directed to an ''orbital'' network of areas (in the orbital cortex), whereas output to autonomic control centers in the hypothalamus and brain stem (especially the periaqueductal gray) arises from a ''medial'' network of areas (in the medial and orbital cortex). Corticocortical axonal connections between areas, which largely define the two networks, are indicated by lines connecting the areas. In addition to this sensory/visceromotor transfer, interactions with the amygdala, hippocampus, entorhinal cortex, and other limbic structure involve this cortical region in reward appreciation and affective behavior. The numbers and letters are designations of architectonic areas, modified from Brodmann; cc, corpus callosum.

area form a critical component in evaluating alternatives and making choices. As exemplified by the famous 19th-century case of Phineas Gage, individuals with damage to the ventromedial prefrontal cortex do not show deficits in motor or sensory function, or in intelligence or cognitive function, but have devastating changes in personality and choice behavior. Recent studies of such cases by Damasio and colleagues have shown that patients do not show usual visceral reactions to emotional stimuli, and this lack of response correlates closely with their difficulty in making appropriate choices. This has been explained by a somatic marker hypothesis, which postulates that visceral responses are monitored as a quick warning of choices to avoid.

In monkeys the cingulate gyrus is situated dorsal to the corpus callosum, but in humans it extends rostral and ventral to the genu of the corpus callosum and caudally around the splenium. The pre- and subgenual parts of the cingulate cortex are closely related to the medial prefrontal cortex in connections and presumably function, but more posterior cingulate regions appear to be distinct. Although the caudal pole of the cingulate gyrus is connected to the hippocampal formation, the central part of the cingulate gyrus has little relation to limbic structures.

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