Every memory that we acquire undergoes a "ripening" process (called consolidation) immediately after it is formed. In this process, it becomes impervious to outside stimulation or drugs that would obliterate it. Until recently, the accepted dogma was that for each separate item of memory, consolidation occurs just once, after which the time window that allows for "memory erasing" closes (usually about an hour or two after the memory is acquired).
However, evidence has lately come to light that a memory is open to disruption for a short period following each time this memory is recalled. If this is true, it means that it would be possible to recall a memory and, immediately after the act of remembering, to activate a "memory eraser" and wipe it out, even though years may have passed since the original memory was formed.
Research into the subject took place in leading labs around the world, but the results were indecisive, as in some cases it was found possible to erase old memories upon recall, while in others no evidence for this was found.
Prof. Dudai's group have now identified a new principle guiding the activity of the brain's memory systems, which sheds light on how memories are recalled and stabilized, and which can explain the puzzling discrepancies in the findings.
This principle delineates the conditions in which the recalled memory becomes re-sensitized to the activity of the "memory erasers." In order to understand the rule, think of the bits of information stored in our memories, each with many associations, some of which conflict with others. For instance, a certain food can bring up memories of taste - delicious or disagreeable; a person can be remembered in pleasant or unpleasant contexts, and so on.
When we next taste the food or see the person, all of the associated memories are called up in the blink of an eye, but in the end, only one of those memories will dictate our reaction (e.g. become dominant.) This memory will decide whether we will eat the food or reject it, or whether we will smile at our acquaintance or ignore him.
Prof. Dudai's team found that only that recalled memory that won the competition for dominance was re-exposed to the time window of sensitivity to memory erasers, and it is this memory that must be consolidated once again before being reinstalled in the long-term memory.
In other words, the winner, in the appropriate circumstances, may lose all. Put succinctly, one can say the stability of the recalled memory is inversely correlated with its dominance. This discovery is likely to assist in the future in developing new methods of wiping out unwanted memories, and thus of treating some kinds of psychological trauma.
Research that deals with the physical basis of the processes and mechanisms of memory, especially those that involve chemical or other intervention, relies on animal subjects. Prof. Dudai and his team carried out their research with rats and fish, which are especially suited for this type of research. The rats learned to remember flavors; the fish learned to remember flashes of light, and in both instances, the animals were trained to associate them with conflicting memories. That is, the tastes were sometimes good and sometimes bad, and the light sometimes signaled danger and sometimes didn't.
In both species, it was possible to show that the dominant memory - that which won out over other associated memories and determined subsequent behavior - was the only one that could be erased by giving the appropriate drug within a few minutes of the memory's recall. The fact that the closer we get to the "basic hardware" of memory, the more similarities exist between different animals, including humans, paves the way to the possibility that certain drugs found to be effective in eliminating memories in animals will also work on humans. Studies on humans, however, are yet to be conducted.
The results of the study were published today in the scientific journal Science. Other than Prof. Dudai, participating in the study were research students Mark Eisenberg, Tali Kobilo, and Diego Berman.
Prof. Yadin Dudai's research is supported by: Abe and Kathryn Selsky Foundation; Nella and Leon Benoziyo Center for Neurosciences; Lester Crown Brain Research Fund; Abramson Family Brain Research Program; Carl and Michaela Einhorn-Dominic Brain Research Institute; and Murray H. & Meyer Grodetsky Center for Research of Higher Brain Functions.
Prof. Dudai holds the Sara and Michael Sela Professorial Chair of Neurobiology.
The Weizmann Institute of Science, in Rehovot, Israel, is one of the world's foremost centers of scientific research and graduate study. Its 2,500 scientists, students, technicians, and engineers pursue basic research in the quest for knowledge and the enhancement of humanity. New ways of fighting disease and hunger, protecting the environment, and harnessing alternative sources of energy are high priorities at Weizmann.