curiosity kills the kitten…
…or why too much memory is no good thing
Why people who are able to easily and accurately recall historical dates or long-ago events, have a harder time with word recall or remembering the day’s current events? They may have too much memory – making it harder to filter out information and increasing the time it takes for new short-term memories to be processed and stored.
Published in the Proceedings of the National Academy of Sciences (March 13, 2007 issue), a recent study of the Columbia University Medical Center reinforces the old adage that too much of anything – even something good for you – can actually be detrimental. In this case, the good thing is the growth of new neurons, a process called neurogenesis, in the hippocampus, the region of the brain responsible for learning and memory.
Long term memory (LTM) was once thought of as a huge database (Allard, 2001) where information was simply filed away in the same manner as a filing cabinet. LTM is often studied with normal people to assess its limits and characteristics. In order to determine the neural pathways to LTM, however, we must start with a brain that is damaged and then assess the deficiencies that individual faces with respect to LTM. This deficiency may arise from an accident, pathology, or in the case of animal models, intentionally inflicted. Today LTM in humans is believed to be partitioned into specialised modules:
1) Procedural memory – as memory storage of skills and procedures. This type of memory has also been referred to as “tacit knowledge” or “implicit knowledge”. Procedural memory is involved in tasks such as remembering how to play handball or how to ride a bike. This is “know how” memory, it often can only be expressed by performing the specific skill and people have problems verbalizing what they are doing and why. Procedural memory is therefore very important in human motor performance.
2) Declarative memory – as our memory for facts (Tulving & Schater, 1990). There is a common belief that declarative memory is further broken down into two components: Episodic memory (memory for past and personally experienced events), and Semantic memory (knowledge for the meaning of words and how to apply them).
Results of the study, conducted with mice, found that the absence of neurogenesis in the hippocampus improves working memory, a specific form of short-term memory that relates to the ability to store task-specific information for a limited timeframe, e.g., where your car is parked in a huge mall lot or remembering a phone number for few seconds before writing it down. Because working memory is highly sensitive to interference from information previously stored in memory, forgetting such information may therefore be necessary for performing everyday working memory tasks, such as balancing your check book or decision making.
“We were surprised to find that halting neurogenesis caused an improvement of working memory, which suggests that too much memory is not always a good thing, and that forgetting is important for normal cognition and behavior,” said Gaël Malleret, Ph.D., a research scientist at the Center for Neurobiology and Behavior at Columbia University Medical Center and the paper’s co-first author.
Many scientists had believed that neurogenesis in the hippocampus, and specifically, the dentate gyrus region, was wholly beneficial to memory. Previous research found that reducing neurogenesis causes long-term memory deficits.
“In our world, we are constantly bombarded by new information so we are constantly filtering –and if we did not do this, we would be overwhelmed,” said Dr. Malleret. “Our research indicates that those with better working memory may have fewer new neurons being developed in their hippocampus, which helps them forget old and useless information sooner and enable them to take in new information faster.”
Proc Natl Acad Sci U S A. 2007 Mar 13;104(11):4642-6. Epub 2007 Mar 5.
Paradoxical influence of hippocampal neurogenesis on working memory.Saxe MD, Malleret G, Vronskaya S, Mendez I, Garcia AD, Sofroniew MV, Kandel ER, Hen R.
*Center for Neurobiology and Behavior and Howard Hughes Medical Institute, Columbia University, 722 West 168th Street, New York, NY 10032.
To explore the function of adult hippocampal neurogenesis, we ablated cell proliferation by using two independent and complementary methods: (i) a focal hippocampal irradiation and (ii) an inducible and reversible genetic elimination of neural progenitor cells. Previous studies using these methods found a weakening of contextual fear conditioning but no change in spatial reference memory, suggesting a supportive role for neurogenesis in some, but not all, hippocampal-dependent memory tasks. In the present study, we examined hippocampal-dependent and -independent working memory using different radial maze tasks. Surprisingly, ablating neurogenesis caused an improvement of hippocampal-dependent working memory when repetitive information was presented in a single day. These findings suggest that adult-born cells in the dentate gyrus have different, and in some cases, opposite roles in distinct types of memory.
PMID: 17360577 [PubMed – in process]