In a case study of a girl who developed anterograde amnesia during childhood, it was determined that the patient ("C.L.") retained semantic memory while having an extreme impairment of episodic memory.One patient, known by the codename "Gene", was involved in a motorcycle accident that damaged significant portions of his frontal and temporal lobes, including his left hippocampus. As a result, he cannot remember any specific episode in his life, such as a train derailment near his house. However, his semantic memory is intact; he remembers that he owns a car and two motorcycles, and he can even remember the names of his classmates in a school photograph. In stark contrast, a woman whose temporal lobes were damaged in the front due to encephalitis lost her semantic memory; she lost her memory of many simple words, historical events, and other trivial information categorized under semantic memory. However, her episodic memory was left intact; she can recall episodes such as her wedding and her fathers death with great detail. Vicari et al. describe that it remains unclear whether neural circuits involved in semantic and episodic memory overlap partially or completely, and this case seems to suggest that the two systems are independent. Both of the patients hippocampal and diencephalic structures on the right and left sides were disconnected. When C.L. came to Vicari et al.s office, her chief complaint was forgetfulness involving both semantic and episodic memory. After administering a battery of neuropsychological tests, Vicari determined that C.L.
Another case described the onset of anterograde amnesia as a result of cell death in the fornix, another structure that carries information from the hippocampus to the structures of the limbic system and the diencephalon. The patient in this case did not show any disconnection syndrome, which is unexpected since the structures involved divide the brain hemispheres (both sides of her brain were able to communicate). Instead, she showed signs of amnesia. The final diagnosis was made by MRI. This particular amnesic syndrome is difficult to diagnose and often is misdiagnosed by physicians as an acute psychiatric disorder. Reorganization of memory
When there is damage to just one side of the MTL, there is opportunity for normal functioning or near-normal function for memories. Neuroplasticity describes the ability of the cortex to remap when necessary. Remapping can occur in cases like the one above and with time the patient can recover and become more skilled at memory retention. A case report describing a patient who had two lobectomies – in the first, doctors removed part of her right MTL first because of seizures originating from the region, and later her left because she developed a tumor – demonstrates this. This case is unique because it is the only one in which both sides of the MTL were removed at different times. The authors observed that the patient was able to recover some ability to learn when she had only one MTL, but observed the deterioration of function when both sides of the MTL were affected.