The medial temporal lobes and human memory

Abstract

Spatial memory tasks known to be sensitive to hippocampal lesions in the rat were adapted to humans. These tasks and others known to be sensitive to medial temporal lesions in the human, were administered in order to investigate the effects of selective damage to medial temporal lobe structures of the human brain. The patients had undergone thermo-coagulation with a single electrode along the amygdalo-hippocampal axis in an attempt to alleviate their epilepsy. With this surgical technique, lesions to single medial temporal lobe structures can be carried out. The locations of the lesions were assessed by means of digital high-resolution magnetic resonance imaging and software allowing a 3-D reconstruction of the brain. A break in the collateral sulcus, dividing it into the anterior collateral sulcus and the posterior collateral sulcus is reported for the first time. This division corresponds to the posterior border of the entorhinal/perirhinal cortex and the anterior border of the parahippocampal cortex, and therefore helped in the identification of the areas. The results confirmed the role of the right hippocampus in visuo-spatial memory tasks (object location, Rey-Osterrieth Figure with and without delay), and the left for verbal memory tasks (Rey Auditory Verbal Learning Task with delay). Patients with lesions to the right parahippocampal cortex were also impaired on a spatial oddball task, but not on the object equivalent. Surprisingly, patients with lesions either to the right or to the left hippocampus were unimpaired on several memory tasks, including a spatial one, with a 30 minute delay, designed to be analogous to the Morris water maze. Patients with lesions to the right parahippocampal cortex were impaired on this task with a 30 minute delay, suggesting that the parahippocampal cortex itself may play an important role in spatial memory.