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Study Shows How the Brain Recalls What Turns It On

Small groups of cells are found to use abstract memories to recognize specific objects.

June 23, 2005|Brad Wible | Times Staff Writer

Whether drawn as a cartoon or disguised as Catwoman, the striking features of Halle Berry are readily recognized by movie fans. That recognition is achieved by a surprisingly small group of brain cells, an international team of researchers reports today in the journal Nature.

Most researchers had thought that specific memories were spread out over large groups of brain cells, or neurons. However, the new study showed that small clusters of cells responded to specific objects, such as Jennifer Aniston or the Sydney Opera House, regardless of changes in their overall appearance, and sometimes just by seeing the name. The findings should provide insight into the still unknown mechanism by which new memories are stored and how those memories are recalled.

"When we see an object, the brain first has to process the geometric shape. But that's not enough. The brain also has to invoke all the memory associations that actually give it meaning," said Johns Hopkins University neuroscientist Charles E. Connor, who did not participate in the study. "We don't just take in information, we use it."

The researchers studied eight patients who had 100 tiny electrodes implanted in their brains before epilepsy surgery to identify the source of their seizures. Before surgery, however, researchers used the electrodes to test each individual's ability to recognize faces and other objects.

The electrodes were implanted in the region of the brain known as the hippocampus, which is important for forming and recalling memories. Nearby temporal lobe regions process signals from the eyes, combining basic colors and shades into recognizable objects. These regions are informally described as the "what" visual pathway because of their role in determining what we're seeing.

The electrodes recorded neuron activation while researchers showed patients images of different things (such as Berry, Aniston or the opera house) or different images of the same thing (such as a drawing of Berry and a photo of her in Catwoman's skintight black vinyl).

The neurons had clear favorites. A "Halle Berry" cell fired primarily for Berry pictures, a "Jennifer Aniston" cell primarily for Aniston. These neurons responded to different images of their favorite, even though the patterns of colors and shapes, hairstyles and wardrobes changed considerably from picture to picture. Specific features were ignored, but the general concept was recognized.

"The way we store memory is abstract concepts," said the study's lead author, bioengineer Rodrigo Quian Quiroga of the University of Leicester in England.

"I won't recall every physical detail of a conversation, but I'll remember what it was about."

The neurons fired even when presented with the written name of their favorite object, such as Kobe Bryant. This indicated that the neurons were providing memory recall rather than simply processing an image.

Insights into how particular patients remembered and associated things were hinted at by different responses to combinations of objects. A photo of Aniston with former "Friends" co-star Lisa Kudrow caused a neuron to fire, but not pictures of Aniston with Brad Pitt, suggesting strong associations between the former pair, and a lack of connections between the latter.

"These are cells that have the capacity to get the abstract concept of things," Quiroga said. "For whatever is linked to this person, the cell will fire. I think that if you show the ankle of Halle Berry, and you can tell that it's Halle Berry, the cell will fire."

Speculating on potential applications for this work, Quiroga suggested implantable prosthetic communication devices.

"We may be able to help patients communicate with the outside world: Think of Halle Berry and a computer can interpret it," he said.

But Connor cautioned that such technologies couldn't yet approach the performance of the brain. "The human brain is an object recognition system par excellence that far exceeds the best computer systems around," Connor said.

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