Deja vu part 2

Everyone’s felt the embarrassment of not quite recognizing someone.

She walks toward you across the parking lot, her arms full of groceries. There’s something about the way her blond curls frame her face, the brown paper bag accentuating the pale pink of exertion in her cheeks. You slow your steps, squinting in the evening light, your keys dangling from your hand. Something about the way she brushes the hair back from her eyes, unlocking her car, stirs your memory, but was it in front of a high school locker? The reception desk at the gym? The back hall of a museum? You swear you’ve seen her somewhere before, but it’s hiding in the corner of your eye, refusing to come into full view.

The embarrassment worsens when she turns and smiles at you, waving. You must know her! Otherwise, why would she acknowledge you? You desperately scrape around inside your marijuana-caked college memories for the face of a classmate, a friend’s girlfriend, something. But before you can think of her name, her smile has staled and she slips inside her car, gone. It’s only when you are in the produce aisle that you realize she was waving at the man behind you, who had honked in reply. You feel like a fool, rooting around for rutabagas.

What is that feeling, that nagging feeling of recognition, especially when it is not true? Similar to deja vu, it corresponds with decisionmaking in the memory center of our brains. A team of researchers at Johns Hopkins University recently explored the mechanism that produces this effect.

A cross-section of the mid-brain showing the hippocampus with the dentate gyrus labeled. Original image from Wikimedia Commons.
A cross-section of the mid-brain showing the hippocampus with the dentate gyrus labeled. Original image from Wikimedia Commons.

The research, published in the journal Neuron, linked the effect to two areas of the hippocampus— the dentate gyrus and CA3. Previous research has theorized roles for the two areas: The dentate gyrus has been thought to take in stimuli and classify them as new and disaparate (hence, the total strange aspect when you see someone with glasses rather than recognizing your grandpa). This process is called pattern separation.

The CA3 region, which is located at the back of the curved hippocampus, is associated with pattern completion. When the brain records stimuli such as glasses, the CA3 region connects it with long-term memory of your college boyfriend whose glasses looked great on him and insists that the person in front of you must be him, ignoring the differences.

The new research from Johns Hopkins, however, has provided evidence that the CA3 region may be more complicated than that. The region seems to make decisions on whether a stimulus is new or recognized. Different parts of the CA3 come to different conclusions and pass those conclusions along to the other brain regions for a full decision.

Using rats, the researchers wanted to test how new stimuli in familiar environments affects long-term memory retrieval. Placing the rats on a wheel surrounded by a black curtain, the researchers allowed the rats to form a mental map of where the various textures on the wheel were and the items hanging on the curtain. Then, they rotated the curtain slightly, changing the layout of the “room.”

The dentate gyrus area of the rats’ hippocampus took the new environment and began making new memories, like a slight change in file names on a computer creates an entirely new file. However, the pattern completion area of the CA3 reacted to the similarities, retrieving memories for review. The mismatch likely created an effect like the one we think of as “half-recognition.”

A better understanding of the hippocampus could help us treat and preserve memory as we live longer. The brain is an immensely complicated and interdependent organ of the body, and studying its separate regions without observing how they affect one another is like studying plants in a vaccuum– removed from their biome, many of the mysteries of how they formed and what purpose they serve is lost.

Observing the brain definitively in context provides us with a better understanding of why we react to the world around us the way we do.

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