Science Scientists Trace Memories

2013.07.25 19:12

이한중*65 Views:2057

Scientists Trace Memories of Things That Never Happened


    The vagaries of human memory are notorious. A friend insists you were at your 15th class reunion when you know it was your 10th. You distinctly remember that another friend was at your wedding, until she reminds you that you didn’t invite her. Or, more seriously, an eyewitness misidentifies the perpetrator of a terrible crime.     
        Not only are false, or mistaken, memories common in normal life, but researchers have found it relatively easy to generate false memories of words and images in human subjects. But exactly what goes on in the brain when mistaken memories are formed has remained mysterious.  
           Now scientists at the Riken-M.I.T. Center for Neural Circuit Genetics at the Massachusetts Institute of Technology say they have created a false memory in a mouse, providing detailed clues to how such memories may form in human brains.     
        Steve Ramirez, Xu Liu and other scientists, led by Susumu Tonegawa, reported Thursday in the journal Science that they caused mice to remember being shocked in one location, when in reality the electric shock was delivered in a completely different location.   
          The finding, said Dr. Tonegawa, a Nobel laureate for his work in immunology, and founder of the Picower Institute for Learning and Memory, of which the center is a part, is yet another cautionary reminder of how unreliable memory can be in mice and humans. It adds to evidence he and others first presented last year in the journal Nature that the physical trace of a specific memory can be identified in a group of brain cells as it forms, and activated later by stimulating those same cells.     
        Although mice are not people, the basic mechanisms of memory formation in mammals are evolutionarily ancient, said Edvard I. Moser, a neuroscientist at the Norwegian University of Science and Technology, who studies spatial memory and navigation and was not part of Dr. Tonegawa’s team.    
         At this level of brain activity, he said, “the difference between a mouse and a human is quite small.” In both, memories form in an area of the hippocampus called the dentate gyrus.     
        “What I find fascinating about this,” Dr. Moser said, “is that you actually can point to a physical substrate to memory,” what the researchers call an engram. Neuroscientists have long talked about engrams, but Dr. Moser said the recent research is the closest they have gotten to pointing to a spot in the brain and saying, “That is the memory.”    
         In the research reported Thursday, Dr. Tonegawa’s team first put mice in one environment and let them get used to it and remember it. They identified and chemically labeled the cells in the animals’ brains where that memory was being formed. The mice were not shocked in that environment.      
       A day later, in a completely different environment, the researchers delivered an electric shock to the mice at the same time that they stimulated the previously identified brain cells to trigger the earlier memory.   
          On the third day, the mice were reintroduced to the first environment. They froze in fear, a typical and well studied mouse behavior, indicating they remembered being shocked in the first environment, something that never happened. The researchers ran numerous variations of the experiment to confirm that they were in fact seeing the mice acting on a false memory.    
         The tools of optogenetics, which are transforming neuroscience, were used to locate and chemically label neurons, as well as make them susceptible to activation by blue light transmitted by a fiber optic cable. With these techniques the researchers were able to identify and label which neurons were involved in forming the initial memory of the first environment, and to reactivate the labeled cells a day later with light.    
         Dr. Tonegawa said that because the mechanisms of memory formation are almost certainly similar in mice and humans, part of the importance of the research is “to make people realize even more than before how unreliable human memory is,” particularly in criminal cases when so much is at stake.    
         That unreliability, he said, prompts a question about evolution: “Why is our brain made in such a way that we form false memories?”     
        No one knows, he said, but he wonders if it has to do with the creativity that allows humans to envision possible events and combinations of real and imagined events in great detail. That rich internal experience fuels work in the arts and sciences and other creative activities, he said. “Unless you have that kind of ability, there is no civilization,” he said.    
         But it could also provide raw material for false memories — a possible “tradeoff for this tremendous benefit.”
    ■ PUBLISHED JULY 25, 2013 http://www.nytimes.com/2013/07/26/science/false-memory-planted-in-a-mouse-brain-study-shows.html
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