Why we mistake a stranger for an old friend found

Have you ever found yourself mistaking a stranger for an old college friend? One tiny spot in the brain is to blame!
Scientists, including one of Indian-origin, have found that two parts of hippocampus work together to determine whether stimulus is completely new or related to something familiar.
The findings, which validate models about how memory works, could help explain what goes wrong with memory in diseases like Alzheimer's and could help to preserve people's memories as they age.
"You see a familiar face and say to yourself, 'I think I've seen that face.' But is this someone I met five years ago, maybe with thinner hair or different glasses - or is it someone else entirely," said James J Knierim, a professor of neuroscience at the Zanvyl Krieger Mind/Brain Institute at the Johns Hopkins University, who led the research.

Brain researchers theorised that two parts of the hippocampus (the dentate gyrus and CA3) competed to decide whether a stimulus was completely new or an altered version of something familiar.
The dentate gyrus was thought to automatically encode each stimulus as new, a process called pattern separation. In contrast, CA3 was thought to minimise any small changes from one experience to the next and classify the stimuli as being the same, a process called pattern completion.

So, the dentate gyrus would assume that the person with thinner hair and unfamiliar glasses was a complete stranger, while CA3 would ignore the altered details and retrieve the memory of a college friend.

However, the new research shows that parts of CA3 come to different decisions, and they pass these different decisions to other brain areas.
"The final job of the CA3 region is to make the decision - is it the same or is it different," Knierim said.
Knierim and Johns Hopkins postdoctoral fellows Heekyung Lee and Cheng Wang, along with Sachin S Deshmukh, a former assistant research scientist in Knierim's lab, implanted electrodes in the hippocampus of the rats.
They trained the rats to run around a track. The track floor had four different textures - sandpaper, carpet padding, duct tape and a rubber mat. The rat could see, feel and smell the differences in the textures.

A black curtain surrounding the track had various objects attached to it.
Over 10 days, the rats built mental maps of that environment. Then the experimenters rotated the track counter-clockwise, while rotating the curtain clockwise, creating a perceptual mismatch in the rats' minds.
The study was published in the journal Neuron.