Limbic System Pathologies
Amnesia: various memory disorders Anterograde amnesia is the inability to form memories after a brain injury, while retrograde amnesia is the inability to recall events prior to the injury. A person might have one of these conditions or both. Hippocampal insults are usually involved in both conditions.
The case of Henry Molaison: anterograde amnesia Henry Gustav Molaison (19262008) underwent an operation in 1953 to stop epileptic seizures (Scoville and Milner 1957; Milner et al. 1968). The surgeon removed most of his hippocampi, what was left atrophied. Molaison became unable to form new long-term declarative memories of daily events. Time stopped for him in 1953. His working memory remained intact, as did his other intellectual abilities. His motor memory was good enough to learn how to solve the Tower of Hanoi puzzle, and he performed many other motor-dependent problem-solving puzzles well.
The case of Clive Wearing: anterograde and retrograde amnesia The brain of musician Clive Wearing (1938-)—including both hippocampi areas—was attacked by herpes virus in 1985 (Wilson et al. 1995). He lost virtually all long-term declarative memories. The virus also rendered him incapable of forming new ones. He was left with a working memory span of about 30 s, effectively living in the moment or constantly waking up and becoming conscious of the world around him. He remembered how to play many piano pieces but with no recollection of having learned to play the piano. This indicates, as does the Molaison case, that motor learning (knowhow) is biologically separate from declarative learning (knowing that).
Posttraumatic Stress Disorder As described, the amygdala screens incoming sensory patterns, looking for dangers. It can also learn and develop heightened sensitivity to certain stimuli. This could explain why a war veteran jumps when a car backfires in the distance—the amygdala connects it to being in battle. The amygdala learns well under heavy and prolonged stress—conditions under which declarative learning of facts and bits of information shuts down. The amygdala evolved to operate in this way to guide us through stressful periods. However, in the case of posttraumatic stress disorder, it remembers all too well a haunting past of terror.
Limbic System Summary
The limbic system evolved early to help our ancestors survive through mechanisms of emotion and memory. The way the limbic system connects with the frontal cortex indicates a complex collaboration. Impairments of the limbic system can yield memory and emotional disorders.