Mind's Eye Re-Creates Visual Memories
By William J. Cromie
Using magnetic fields that disrupt brain activity, researchers have proved that visual memories are re-created in the brain as mental pictures. Stored images are played, like videotapes, on a screenlike sheet of tissue at the back of the head.
Brain scans previously revealed intense activity in this area of the brain when people recall a familiar object, scene, or letter. However, scientists debated whether the area actually plays a role in handling information. It's possible, some argued, that such activity is only a by-product of processing that occurs elsewhere in the brain. Now, by using magnetic currents to blur the "mind's-eye" screen, Harvard researchers have found they can delay visual imagination, strong evidence that memories of what the eye once saw are replayed in this little theater of the brain.
"Scenes that register on the retina of the eye are faithfully projected
by patterns of nerve signals activated in the visual area of the brain," explains Stephen Kosslyn, professor of psychology. "The images are then stored in the temporal lobes (under the temples) in a compressed form not unlike magnetic pulses on a videotape.
"To recall things like the number of windows in a house, information
about the geometry of the house is unpacked from memory and sent back to the visual area," continues Kosslyn. "Unpacking memories is what visualization is all about."
The area where visual images are recalled or first imagined can be pinpointed by a brain scanning technique known as positron emission tomography, or PET. If experimenters send pulses of magnetic energy to this same area, the visualization is delayed, which would not happen if activity in this area played no role in recall or imagination.
"This is a new use of a technique called repetitive transcranial magnetic
stimulation (rTMS) whereby magnetic fields are used to both map and change brain activity," notes Alvaro Pascual-Leone, a Harvard Medical School researcher who worked with Kosslyn on these experiments. He has used rTMS to treat depression, Parkinson's disease, schizophrenia, obsessive- compulsive behavior, and to help blind people learn Braille.
In 1993, Kosslyn and his colleagues showed that two small areas at the middle of the back of the brain become active when you close your eyes and visualize letters of the alphabet. A few years later, he found that smaller objects activate areas farther back in the visual cortex than larger ones. Much the same thing happens when we first view objects of different sizes.
"It's a peculiar feature of how this part of the cortex is folded,"
PET scans measure brain activity by how much blood flows to different sites. Kosslyn and his colleagues found that people with the weakest flows took the longest to use mental pictures. Once flow exceeds a certain level, however, subjects respond relatively rapidly. "It's not clear whether the latter group sees more vivid images, but they do classify parts of visualized objects quicker."
In their latest experiments, Kosslyn and colleagues at Harvard- affiliated Massachusetts General Hospital in Boston had eight people remember patterns of stripes located in four different boxes presented together on a single screen. Afterward, the researchers asked them questions such as whether the stripes in box number one were shorter or longer than those in box number four. Other questions dealt with whether stripes were vertical or horizontal, thicker or thinner, tilted or straight.
Later, people did the same test after Pascual-Leone waved a magnetic wand over their mind's eye. An electric current running through the wand rapidly turns on and off, generating a magnetic field that penetrates the scalp. In some sessions, the coil was positioned so the magnetism missed the key areas where visual images are replayed. Investigators then compared results with and without magnetic stimulation of the key visual area.
"Every one of the people who received magnetic stimulation of that area
took more time to visualize and perceive the patterns of stripes," notes Kosslyn. "The magnetic fields were directed at the small area where the PET scans had registered activity during visualization."
The memory delay, no doubt, stems from disruption of the brain cells. Other studies have shown that intense magnetic fields can impair short-term memory for words and for previously learned movement responses.
Would it be possible to wipe out memories - say, recurring traumatic images of abuse or assault - with magnetism?
"Possibly," Kosslyn answers. "But I think you'd need to do it within a
year, before the memory becomes consolidated."
Electroshock therapy is used for both memory obliteration and treating major depression, and physicians are starting to use magnetic stimulation for the same purposes. Magnetism has the advantage of not shocking people into convulsions.
In experiments at Beth Israel Deaconess Medical Center in Boston, Pascual-Leone improved the moods of 11 of 17 badly depressed people who had not been helped by electroshock treatment or drugs.
"If we prolong stimulation for five to ten days, we can sometimes extend
the benefit up to several months," Pascual- Leone notes. "I hope that it will eventually work well enough so that individual treatments can be custom-designed for patients."
Kosslyn notes that images created by pure imagination share the same projection equipment as actual memories. That makes it easy to confuse real and implanted memories of abuse, or of witnessing a crime.
There is, however, evidence that false and true memories can be separated by the amount and pattern of brain activity they produce. "Real memories contain more information, more detail, and thus activate the brain differently," Kosslyn points out. "It's only speculation at this point, but we may be able to exploit these differences to verify the truth of a memory."
Copyright 1999 President and Fellows of Harvard College
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