Anterograde amnesia · Grey Matter

Anterograde amnesia is the inability to form new lasting memories of events and facts after a brain injury, while what was learned before the injury, the capacity to hold something in mind for a few seconds and the ability to learn new skills largely remain.

Anterograde amnesia. Anterograde amnesia is the inability to form new lasting memories of events and facts after a brain injury, while what was learned before the injury, the capacity to hold something in mind for a few seconds and the ability to learn new skills largely remain.

It follows bilateral damage to the memory circuit: the hippocampus and the surrounding medial temporal cortex, or the structures they work with in the diencephalon (the mammillary bodies and the anterior and medial nuclei of the thalamus, damaged in Korsakoff syndrome). The most studied case is the patient H.M., whose medial temporal lobes, including most of both hippocampi and the amygdalae, were removed in 1953 to treat severe seizures. From then on he could not keep new episodes or facts for more than moments once his attention moved, while his intelligence, language and older knowledge were preserved.

It usually comes with some retrograde amnesia, graded in time. H.M. lost much of the decade before the operation and kept his childhood; later testing showed that even his older personal memories had lost their vivid detail, while their facts remained.

What survives defines the memory systems. working memory, procedural memory (H.M. learned mirror drawing without remembering the practice) and semantic knowledge acquired long before were spared, which showed that memory is several systems.

Alzheimer's disease often starts with a slowly progressive version of it, because the medial temporal lobe is among the first regions affected.

Questions: How can someone with amnesia get better at a skill without remembering practising it? Skills are stored by circuits that do not need the hippocampus: the basal ganglia, the cerebellum and the motor cortex adjust with every repetition, whether or not the repetition is remembered. In the early 1960s the patient H.M. traced a star seen only in a mirror over several days and improved steadily, while each day he said he had never done the task before. The dissociation showed that memory is several systems, one that knows that something happened and others that change how something is done. Why did the patient H.M. keep his old memories but form no new ones? The 1953 operation removed most of both hippocampi with the surrounding medial temporal cortex, the structures that bind a new experience into a lasting trace, so nothing new could be stored once his attention moved on. Knowledge consolidated years earlier had already been reorganised into the cortex, which was untouched, so his vocabulary, general knowledge and childhood memories remained. The sparing was partial: he lost much of the decade before the operation, and later testing found that even his older personal memories had lost their vivid detail while keeping their facts. That gradient is the evidence that the hippocampus is needed for storing and for a time afterwards, but not forever for every kind of memory.