Imagine seeing a former high school classmate you always wanted to know better. Then imagine seeing that kid who used to push you in the hallways. Do you react differently? What happens in your brain during these encounters?
In fact, different areas of the brain react differently when recognizing others, depending on the emotions attached to the memory, a team of Cornell University research psychologists has found. The team, led by professor of psychology Robert Johnston, has been conducting experiments to study individual recognition.
But rather than crash high school reunions with an MRI machine in tow, the researchers stayed in their laboratory and created social encounters between golden hamsters. Then they examined the animals' brains for evidence of those encounters.
Last year Johnston's team conducted the first experiment to demonstrate the neural basis of individual recognition in hamsters and identify which areas of the brain play a role. The results were published in the Dec. 7, 2005, issue of the Journal of Neuroscience.
Better understanding these mechanisms, Johnston said, may be of central importance in treating certain forms of autism, Asperger syndrome, psychopathy and social anxiety disorders.
"This ability to recognize individuals underlies social behavior in virtually all vertebrates and some invertebrates as well," explained Johnston. "Humans clearly have an incredible ability to recognize, remember and store huge amounts of information about individuals - even individuals we have never actually met. This ability is the core of circuits that one might call the social brain."
Johnston's team uses hamsters to study recognition because their brains are strikingly similar to ours. "They are more sophisticated than you might think," he noted.
In the latest experiment, a male hamster encountered two individuals that he knew equally well but had different interactions with the previous day: a male that defeated him in a fight and a male that he had never fought. The encounters mimicked those that occur in the wild.
The hamster fled from the winning male but was attracted to the neutral male - suggesting that he both recognized the individuals and remembered his experiences with them.
An hour later, the researchers removed the hamster's breath-mint-sized brain and injected it with antibodies and enzymes. The antibodies bond to specific proteins produced by recently active brain cells, and the enzymes convert chemicals in the cells into colored dyes, leaving behind a map of where the action was. This technique, called immunohistochemistry, is also used to diagnose cancerous cells in humans.
Next the brain was frozen with dry ice, shaved into very thin slices using a miniature slow-moving guillotine and then studied under a microscope to determine where the dyes were activated.
"Functional MRIs provide similar information from human brains, but those images are relatively fuzzy and lack the spatial resolution necessary for small animals," explained Johnston. "With immunohistochemistry, on the other hand, we can see each individual cell that was activated."
The researchers found activity in the brain's anterior dorsal hippocampus and amygdala, among other areas. They then repeated the experiment with another hamster whose anterior dorsal hippocampus was numbed with lidocaine, a local anesthetic, and found that the animal did not avoid the individual who had defeated him.
"It showed us that this region is necessary for recognition memory," said Johnston. "The hippocampus has also been implicated for recognition memory in humans."
Although hamsters recognize individuals by smell, whereas humans use largely sight and sound, Johnston said that the underlying mechanism is the same.