The human brain has a right and left temporal lobe, wherein one mirrors the other. Mesial temporal lobe anatomy involves specific structures frequently implicated as a cause for seizures in patients with temporal lobe epilepsy. The structures include the hippocampus, the parahippocampus, and the amygdala. Many or all of these will be removed during surgery to stop seizures arising in this area. The most common reason for which seizure surgery is performed in the temporal lobe is for scarring in the hippocampus, or hippocampal sclerosis. Removal of the hippocampus in this situation will lead to stopping seizures in the majority of the patients.
The anatomy of this region is depicted in the following drawing.
Specific structures the surgeon looks for during surgery include arteries and veins supplying the region, the third and fourth cranial nerves (control eye movements), the brainstem, and the base of the skull. These structures are identified and protected during surgery to prevent post-operative neurological symptoms.
Blood supply to the hippocampus involves an arterial supply (shown in red) from the posterior cerebral artery and venous drainage (shown in blue) into the basal vein of Rosenthal as depicted in the cadaver specimen to the left. Other arteries sometimes seen during surgery include the middle cerebral artery system, the anterior choroidal artery, and the internal carotid artery.
Function of the Temporal Lobe
The hippocampus plays a role in memory processing; however, the exact memory circuit in the human brain is poorly understood. The hippocampus on the language-dominant side (usually the left side of the brain in most people) has a much more important role in memory than the non-dominant side. This hippocampus functions in verbal or language memory and short-term memory. Clinicians use several tests pre-operatively to determine the risk of impairing memory if the dominant hippocampus needs to be removed.
The dominant temporal lobe (left) often has a region specialized in language skills. This area of the brain is known classically as Wernicke's area and involves language comprehension. Its exact location is variable (see sketch) and, if surgery is mandated in the posterior lateral temporal lobe, this language site needs to be identified and protected.
Other functions in the temporal lobe include a portion of the visual fiber pathway known as Meyer's loop. These fibers project from the lateral geniculate nucleus of the thalamus to the visual cortex. They follow a variable course within the deep white matter of the temporal lobe and are sometimes injured unavoidably during temporal lobe surgery. This leads to a defect in peripheral vision with a black spot in the opposite upper peripheral field of vision ("pie in the sky" deficit).