Epilepsy and memory: Why some people have trouble distinguishing past from present

(Credit: peshkov via Adobe Stock)

Temporal lobe epilepsy seems to rewire a part of the brain that’s key to storing memories.

  • The dentate gyrus (DG) plays a role in recalling and creating new memories. 
  • The DG is rewired in patients diagnosed with temporal lobe epilepsy (TLE). 
  • Patients diagnosed with TLE have difficulty forming memories of new experiences when they are similar to past experiences.

by Peter Rogers

Most people don’t know they have epilepsy until their first seizure. This initial grand mal (literally, “big bad”) seizure is marked by violent muscle contractions and a loss of consciousness. As you can imagine, an unexpected collapse and a fit of convulsions is frightening and can have profound emotional, social, and physical consequences, especially if the person has never experienced one before. 

A study published recently in the Journal of Neuroscience provides a remarkable finding that may help physicians diagnose epilepsy before the first seizure: People with temporal lobe epilepsy (TLE) have difficulty distinguishing the present from the past.

The dentate gyrus, gatekeeper of memories

I have a stone on my desk that I picked up the first time I visited a beach with my fiancé. When I look at the stone, my brain conjures the memory of that visit. If I focus on that memory, my brain will recreate the sounds and smells of the ocean. Even the emotions of that day return to me. How is my brain recalling distinct, coherent memories just from seeing that rock? Why wouldn’t all this information suddenly explode into my consciousness in one blinding instant instead?

In patients with temporal lobe epilepsy (TLE), this is exactly what happens during a seizure. Instead of triggering memories in a controlled and deliberate manner, incoming information (like visual information about the stone on my desk) bursts into the memory center, activating thousands of memory pathways in a matter of moments. Like a fire in a fireworks factory, the result isn’t thousands of beautiful, distinct bursts. It is one massive explosion.

This is because the gatekeeper of memories — the dentate gyrus (DG) — is rewired when people develop TLE. This region of the brain is responsible for regulating what information is passed to the memory center. The rewiring can drastically change how effective the dentate gyrus is at silencing some information, while allowing other information to be passed to the rest of the brain.

The DG doesn’t just protect us from seizures, but also helps determine whether information should be stored as a new memory. Given that patients with TLE commonly exhibit memory problems, a team of University of Wisconsin-Madison researchers suspected that the DG rewiring was doing more than just causing seizures.

Distinguishing the present from the past

The present is never identical to the past; there is always some difference. It seems that one of the DG’s responsibilities is to find those differences so they can be properly stored as a memory. 

As information about present events — for instance, visual information about what I am currently seeing — enters the DG, the DG compares it with the information currently stored in memories. If the incoming information contains new details, they are passed along to be stored as a new memory. If no new details are found, the DG simply concludes the information must already be stored in a memory of the past. Thus, no new memory is necessary.

We take in so much information that it is rare for your brain to find absolutely no new details. But for most of us, this will happen at least once in our lifetime. It’s an eerie sensation, almost like being in a dream while still awake. For a moment, it feels like every detail of your current situation has happened before. This is the sensation of déjà vu. Scientists suspect that, during déjà vu, the DG “hiccups” and doesn’t find any new details for the entire present moment.

Déjà vu is an extreme example of not being able to identify new details. Normally, you are unaware of this process. For example, if you have had the same morning routine for the last five years, a lot of the information entering your DG is the same as information already stored in memories: the same blankets, the same bowl, the same towel, etc. Information about the blanket, bowl, and towel enters the DG, but the DG ignores most of this because it is already stored in a memory, and you are none the wiser (literally)…



F. Kaskais Web Guru

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