Dr. Grossi's Blog
Memory is divided into two major aspects --declarative and procedural. The first is declarative memory which is divided into semantic memory and episodic memory, which is the memory for autobiographical events including times, places, events, associated emotions and contextual knowledge. The counterpart to declarative memory is procedural or implicit memory. Implicit memory is memory in which prior experience aids in the performance of task without consciousness awareness of previous experience. Procedural memory, a type of implicit memory, is the memory of how to do things such as tying our shoes or ties or driving a car. There is no conscious control and it is long-term memory. Evidence for implicit memory comes from priming experiments and amnestic patients who are unimpaired in performing such tasks. There are two forms of amnesia: anterograde amnesia in which a person is unable to memorize new events or data and retrograde in which an individual cannot consciously recall pre-existing memories.
The hippocampus is a paired structure, mirror image on left and right, embedded deep in the medial temporal lobes and a core part of the limbic system. The textbooks tell us that it encodes new memories as memory traces that are eventually consolidated in the cortex for long-term storage. Once the memories are in the cortex, the hippocampus is no longer needed to store or retrieve the memories. Yet a new study raises some questions about this standard position.
Eleanor A. Maguire and colleagues at the University College of London published in the Proceedings of the National Academy Science USA in 2007 a study of 5 amnestic patients with primary damage to the hippocampus bilaterally, compared to 10 controls who were matched for age, education, and IQ. They presented short verbal cues and asked the individuals to elaborate a future experience based on those cues. The cues were beach, pub, port, museum, forest, and castle. They found that those with amnestic damage were markedly impaired in producing new imagined experiences. The productions were characterized by fragmentation and lacking in a holistic representation of the setting. The examples they gave are striking in showing the lack of spatial coherence. It would thus appear that hippocampal lesions withdraw the spatial context that allows for disparate elements of a new experience to be ordered. Within the standard understanding, the hippocampus is a time-limited contributor in episodic memory as the neocortex assumes long-term storage including generalized representations for spatial contextual (e.g. port) and nonspatial (e.g. object)memories. In this model, hippocampal time-limited memories should not interfere with imaginative productions because the neocortical areas should actively support such imaginative productions yet it seems to interfere. Therefore there is question about the standard explanation.