A Beam of Light Into the Depths of Memory

On October 5, 2026, the Nobel Prize in Physiology or Medicine was awarded to Karl Deisseroth, Peter Hegemann, and Georg Nagel for their discoveries concerning light-gated ion channels and optogenetics.

The story of this transformative technology did not begin with Alzheimer’s disease. It did not even begin with the human brain.

It began with a tiny green alga.

From an Alga That Follows the Light

A single-celled green alga called Chlamydomonas can sense light and swim toward it.

Peter Hegemann became fascinated by a deceptively simple question: How could such a simple organism detect light so quickly?

Years of research eventually led Hegemann, Georg Nagel, and their colleagues to a crucial discovery: channelrhodopsins.

These are remarkable proteins. When illuminated, they open like tiny gates in the cell membrane, allowing electrically charged ions to flow through.

In other words:

Light can be converted directly into an electrical signal.

Later, Karl Deisseroth and his colleagues pursued an idea that would transform neuroscience:

What would happen if this microbial “light switch” could be installed in neurons?

The result was extraordinary.

When blue light was delivered to neurons engineered to express channelrhodopsin, the channel opened, ions crossed the membrane, the membrane potential changed, and the neurons could fire action potentials.

For the first time, scientists had an extraordinarily precise way to turn selected neurons on—or off—with light.

This became known as optogenetics.

Its importance goes far beyond the invention of another laboratory technique. It changed the kinds of questions neuroscientists could ask.

Previously, researchers could observe:

“Which neurons become active when an animal performs a particular behavior?”

With optogenetics, they could reverse the question:

“If we activate only these neurons, will the behavior occur?”

That movement from correlation toward causation is one of optogenetics’ most profound contributions to neuroscience.

What Does This Have to Do With Alzheimer’s Disease?

At first glance, the connection may not seem obvious.

Alzheimer’s research has traditionally focused on amyloid-beta, tau, neuroinflammation, synaptic damage, and neuronal death.

Optogenetics, by contrast, is about controlling neurons with light.

But the two fields meet when we shift our attention from what we see in pathological tissue to one of the most fundamental questions about Alzheimer’s disease:

Where does memory go?

A person may gradually forget what happened yesterday or what was said only minutes ago. Later, they may forget a familiar route, a telephone number used for decades, or even the identity of the person standing in front of them.

This raises a profound scientific question:

Are those memories truly erased from the brain?

Or do some of them remain, while the brain has lost the ability to reach them?

A Lost Memory and an Inaccessible Memory Are Not the Same Thing

This distinction is one of the most intriguing perspectives optogenetics has brought to Alzheimer’s research.

Memory involves several different processes:

Encoding — putting an experience into memory.

Storage and consolidation — maintaining that memory over time.

Retrieval — finding and reactivating it when it is needed.

When someone cannot remember something, the outward result is the same: the memory appears to be gone.

But inside the brain, the underlying causes may be very different.

One possibility is:

The memory itself has been lost.

Another is:

The memory still exists, but normal cues can no longer retrieve it.

In 2016, Susumu Tonegawa and colleagues published a landmark study in Nature using a mouse model of early Alzheimer’s disease.

The researchers identified and labeled hippocampal neurons that were active while a particular memory was being formed. Populations of neurons associated with a specific memory are often called engram cells—the cellular traces of memory.

Later, the mice developed memory impairment.

When researchers used ordinary cues to trigger the memory, the animals appeared to have forgotten.

Then they did something that would once have seemed almost unimaginable.

Using optogenetics, they directly activated the engram cells that had participated in forming that memory.

The memory-related behavior returned.

In other words:

Behaviorally, the memory appeared to be gone.
At the level of the neural circuit, however, the memory trace had not necessarily disappeared.

The brain could no longer retrieve it normally. But when scientists bypassed the impaired retrieval pathway and directly activated the relevant neurons, the memory could again be expressed.

This does not mean that forgotten memories in people with Alzheimer’s disease are all intact somewhere inside the brain.

But it demonstrated something extremely important:

At least in certain early stages of disease in animal models, forgetting may involve not only memory loss, but also memory retrieval failure.

Those are very different concepts.

A Different Way to Think About Alzheimer’s Disease

We often imagine Alzheimer’s disease as a library that is slowly being demolished.

Neurons die. Shelves collapse. One by one, the books representing our memories disappear.

There is truth in that image.

As Alzheimer’s progresses, synapses and neurons are indeed increasingly damaged and lost, and some information may ultimately become irretrievable.

But optogenetic experiments suggest that, particularly in earlier stages, the situation may be more complicated.

Sometimes,

the book may still be there.

The catalog system may be broken.

Or the path leading to the shelf may have disappeared.

The brain knows that a book once existed, but can no longer find it.

If this idea is eventually supported more fully in human disease, it could influence how we think about treatment.

We should not only ask:

How can we prevent neurons from dying?

We should also ask:

How can we help surviving neural circuits work again?

40 Hz: Another Window Opened by Optogenetics

Alzheimer’s disease does not affect only molecules and individual cells. It also disrupts the rhythms of large-scale neural networks.

One rhythm that has attracted particular attention is the gamma oscillation.

Research in Alzheimer’s mouse models has shown that manipulating neural activity around 40 Hz can influence gamma activity and, in some experimental settings, improve memory-related performance.

This does not mean that 40-Hz stimulation has been established as a treatment for Alzheimer’s disease. Human studies remain an active area of investigation, and results from animal experiments cannot simply be translated into clinical claims.

The deeper question is more interesting:

How much cognitive impairment in Alzheimer’s disease comes from neurons that have died, and how much comes from surviving neurons that have lost their normal timing and coordination?

Imagine an orchestra.

Some musicians have left the stage. That is neuronal loss.

But other musicians are still sitting there. They can still play; they have simply lost the conductor’s timing and can no longer coordinate with one another.

If we could restore coordination among those surviving neurons, could we restore some function?

Optogenetics has helped scientists ask this question with unprecedented precision.

From Molecules to Cells, Circuits—and Memory

This may be the deepest connection between the 2026 Nobel Prize and Alzheimer’s disease.

For decades, much of Alzheimer’s research has focused on a cascade involving:

amyloid → tau → inflammation → neuronal dysfunction and death

These processes are enormously important.

But a person’s memory is not determined by a single amyloid plaque.

Memory emerges from a living, dynamic nervous system:

Molecule
↓
Synapse
↓
Neuron
↓
Neural circuit
↓
Network
↓
Memory
↓
Behavior
↓
A human life

One of the great contributions of optogenetics is that scientists can intervene at carefully selected points along this chain and then observe what happens to the larger system.

That adds another question to Alzheimer’s research.

Not only:

“How do we reduce pathology?”

But also:

“How do we preserve and restore function?”

Can We Treat Alzheimer’s Disease With Light?

Not yet.

This distinction is essential.

Most research involving optogenetics and Alzheimer’s disease remains in cells and animal models.

Traditional optogenetics generally requires introducing genes encoding light-sensitive proteins into selected neurons and then delivering light to those cells. Doing this safely and effectively in the deep structures of the human brain is far more complicated than performing the same experiment in a laboratory mouse.

There is another fundamental limitation:

Optogenetics cannot bring a dead neuron back to life.

It does not simply remove amyloid. It does not automatically eliminate tau. And a beam of blue light cannot by itself stop the entire neurodegenerative process.

So its greatest significance for Alzheimer’s disease today is not that we can “treat AD with light.”

Its value is that it helps researchers determine:

Which cells need to be protected?
Which synapses need to be restored?
Which neural circuits are malfunctioning?
Which network rhythms might be worth correcting?

Once those answers are found, the eventual treatment may not involve light at all.

It could involve a drug.

It could involve gene therapy.

It could involve electrical stimulation, magnetic stimulation, or another form of neuromodulation.

Or it could involve an approach we have not yet imagined.

Optogenetics is, first of all, a key that helps us search for the right doors.

Perhaps the Most Human Question Remains: Is the Memory Still There?

For scientists, engrams, synapses, gamma oscillations, amyloid, and tau are research questions.

For the family of someone living with Alzheimer’s disease, the question is often much simpler.

When a mother no longer recognizes her daughter,

when a husband looks at the woman who has shared his life for decades but can no longer say her name,

the family may really be asking only one thing:

“Does he still remember me?”

Science cannot yet give a simple answer.

As Alzheimer’s disease progresses, neurons and synapses can become severely damaged and lost. We should not romanticize animal experiments into the comforting claim that every forgotten memory remains permanently intact somewhere in the brain.

But optogenetic research has revealed something important:

A memory that cannot be expressed is not necessarily the same as a memory that no longer exists.

At least in some animal models and during some early stages of disease, traces of memory may remain even when the normal pathways needed to retrieve them have been disrupted.

And perhaps this is the most thought-provoking connection between this year’s Nobel Prize and Alzheimer’s disease.

Decades ago, a scientist wanted to understand a seemingly modest question:

Why does a tiny green alga swim toward light?

That question led to channelrhodopsin.

Scientists then placed that light-sensitive protein into neurons.

A beam of blue light entered the brain, and neurons responded.

Eventually, that light reached the neural circuits of memory.

And it allowed us to ask an ancient question in an entirely new way:

When a person forgets, where does the memory go?

We do not yet have the final answer.

But because of that beam of light,

we can now see a little deeper into the darkness.