The hippocampal gyrus, uncus, isthmus, and gyrus cinguli are usually grouped together as the limbic lobe or limbic system. Closely related are the subcallosal and retrosplenial gyri, pyriform area, hippocampus, and various subcortical structures including the amygdala and septal nuclei. Certain of these are often included in the temporal lobe, but anatomically and physiologically they are placed in the limbic system, or "visceral brain." Irritation, or stimulation, of these produces either olfactory or gustatory hallucinations. These are often very disagreeable and are described with difficulty. For example, the patient may describe the taste of blood, the odor of burning rubber, or of decaying material. The hallucinations may be accompanied by smacking or licking of the lips, tasting movements of the tongue, swallowing, and salivation, and constitute an important part of the complex partial seizure. Destruction of these areas is not followed by loss of smell or taste because of their bilateral connections.
The limbic lobe and related structures have rich connections with the hypothalamus and thalamus, and play an important part in the central regulation of the autonomic nervous system. Abnormal affective and emotional responses, fear and aggressive behavior may be lessened by interruption of the connections between these cortical areas and the thalamus and hypothalamus. Bilateral or even unilateral lesions or resection of the hippocampus and hippocampal gyrus are followed by marked impairment of recent memory. Bilateral lesions of the cingulate gyrus cause apathy, akinesia, and mutism.