(Pioneer Press file: Richard Marshall)



Their research, published Wednesday in the journal Nature, was conducted on mice, not humans, so the findings cannot immediately be translated to the treatment of patients. But experts said the experiments may eventually lead to more effective therapies for people with psychological problems like depression, anxiety or post-traumatic stress disorder.




Later, mice that had been shocked were put in the company of females — a positive experience — while scientists used the light to activate the memory of the shock. As a result, the negative memory became less negative; the mice became less fearful in the place where they had been conditioned to remember the shock.

The mice that originally spent time with the females later received mild electric shocks while scientists activated the neurons associated with this positive memory. The memory became less enticing, the scientists concluded: the mice froze more and sniffed less, standard measures of fear and reward in rodents.

The study’s senior author, Susumu Tonegawa, a professor of biology and neuroscience at MIT, said the findings provided a neurological basis for psychotherapy in which patients are encouraged to unearth a good memory to “reduce the feelings of a bad memory they have or stress they have had.”

Roger Redondo, a postdoctoral student and one of the study’s lead authors, said the experiments showed it was possible to alter the emotional perception of a memory “without any drugs and without the mice being brought back to the area where the memory occurred.”

The scientists also discovered important differences in the way neurons work in two key brain areas, the hippocampus and the amygdala. The hippocampus is involved in forming new memories and encoding factual details like where, what and when. The amygdala helps link emotions to memory.

The experiments found that neurons in the hippocampus can be changed to make a bad memory less negative, and vice versa. But neurons in the amygdala did not change, leading scientists to conclude that these neurons are prewired to reflect positive or negative emotions.

“That’s absolutely new,” said Robert Morris, a neuroscientist at the University of Edinburgh who was not involved in the research. He said it made sense that the amygdala would have specialized neurons, because it connects to several brain areas involved in taking action.

The brain must understand quickly if an experience is, say, fearful or rewarding, so it can coordinate its response, he said.

Susan Sangha, an assistant professor of psychological sciences at Purdue University who has studied neurons in the amygdala, said a key question is “what is special about these neurons.” Her research has suggested that besides reward and fear, some neurons transmit feelings of safety, and some seem to fire with any emotion.

Joshua Johansen, a neuroscientist at Japan’s Riken Institute who was not involved in the research, said the findings “provide clues as to how we go about tackling things like anxiety disorder.”

He cautioned that this research is in its infancy and that it involved normal behaviors, not disorders. But, Johansen said, “if we can figure out how to associate bad experiences with neutral or pleasurable experiences,” it could guide development of drugs, gene therapy or other treatments for mental illness.

Current optogenetics techniques are not appropriate for people because they involve inserting fiber optic wires and injecting a virus containing a protein into the brain.

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