Despite modern medicine's remarkable advances, psychiatric treatments remain stuck at 1960s technology because the brain was viewed as a simple 'soup bowl' of chemicals. Through this lecture, Professor Kim Sung-yeon explains the paradigm shift toward understanding the brain as a complex of precision 'neural circuits,' and how 'optogenetics' — the technology of controlling specific neurons with light — is leading a revolution in neuroscience. We are entering a new era of discovery that unlocks the secrets of mind and behavior through this technology.

1. The Last Frontier of 21st-Century Science: The Brain

Welcome everyone. I'm Professor Kim Sung-yeon from Seoul National University. Today we'll explore in depth what it means to study neural circuits of the brain. The brain, especially the human brain, is a core organ where 86 billion neurons exchange countless electrical and chemical signals through 100 trillion synaptic connections, creating our physiology and psychology.

The brain is called "the most complex machinery on Earth," and despite over a century of research, it's still considered the "last remaining frontier of 21st-century science." That's why countries worldwide — the US, Europe, Japan, Korea — are making massive national-level investments.

Neuroscientists study not from a single perspective but from multiple levels and scales in a three-dimensional way. They look at the entire forest, individual trees, and even the veins within leaves.

In the past, people studying molecules, circuits, and psychology worked as independently as if in different buildings. But since the 2000s, advances in genetic engineering and optical technology have torn down those barriers.

2. Why Have Psychiatric Drugs Stood Still for 60 Years?

Taking anxiety as an example — one of this lecture's core topics — anxiety disorders are so common that about 30% of adults experience them at least once in their lifetime, and they're closely linked to depression, dementia, and other conditions. Yet remarkably, the treatments we use still rely on benzodiazepine-class drugs like 'Diazepam (Valium)' developed in 1963.

Every year new revolutionary versions of iPhones and Galaxies come out, so why are brain-fixing drugs still stuck at 60-year-old technology? (...) Over the past decades, humanity eradicated smallpox and cancer and AIDS treatments have undergone revolutionary advances. So why are anxiety disorder treatments uniquely frozen?

The problem isn't that we don't know the brain's molecules — we know receptor structures down to the atomic level. The problem lies in 'how drugs act in the brain.'

3. The Brain Is Not a 'Soup Bowl' but a 'Precision Circuit'

Existing drugs (like diazepam) spread throughout the entire brain, forcibly shutting down (inhibiting) nearly all neurons. It's like treating the brain as a soup bowl where chemicals float around.

The brain is not only divided into functional zones, but within the same neighborhood, the same brain region, neurons with different jobs and personalities are mixed together. Just like students, doctors, police, and firefighters live mixed together in a city like Seoul.

Yet instead of fixing only the broken circuit, we're carpet-bombing the entire brain — no wonder side effects like drowsiness and memory loss occur. David Anderson aptly compared this to changing a car's engine oil by opening the hood and dumping oil all over the engine rather than pouring it into the correct port.

We now realize that "the brain is not a soup bowl but a complex network of networks" and are challenging ourselves to map every cell and connection in the brain through 'Brain Mapping.'

4. The Game-Changing Technology: Optogenetics

Having a map isn't the end — you need to actually travel the roads and manipulate the traffic lights to understand function. This is where optogenetics — the revolutionary technology of controlling neurons with light — enters the picture.

The answer was found, surprisingly, in a marine green alga called 'Chlamydomonas' that has light-sensing proteins (channelrhodopsin). In 2005, Karl Deisseroth's team at Stanford succeeded in implanting this algal gene into neurons, enabling millisecond-precise, cell-type-specific remote control using fiber optics instead of electrodes.

Using this technology, blue light activates neurons (channelrhodopsin) and yellow light deactivates them (halorhodopsin), enabling perfect 'gain of function' and 'loss of function' experiments at the neural circuit level.

5. Secrets of the Brain Revealed by Light

Thanks to optogenetics, neuroscience has achieved explosive discoveries over the past decade:

• Aggression control: Stimulating specific hypothalamic neurons with light in docile mice caused them to attack fiercely.
• Thirst and appetite: Stimulating the SFO region made mice drink water immediately; stimulating another region made already-full mice binge on high-fat food.
• Reward circuits: When a mouse's pleasure center was stimulated each time it performed a specific behavior, it became addicted and repeated the behavior endlessly.

At the center of all this innovation is Karl Deisseroth, a psychiatrist and engineer who channeled his despair from treating difficult patients into developing the technology.

6. Closing: New Discoveries Created by Tools

Modern neuroscience is more 'technology-driven' than perhaps any other field. Sydney Brenner, a giant of modern biology, said:

"Progress in science depends on new techniques, new discoveries, and new ideas, probably in that order."

When we can see what was invisible and manipulate what was untouchable, we can finally ask questions we could never have imagined before. Throughout this semester, we'll journey together through how these revolutionary tools have unlocked the brain's secrets. I hope you find questions that make your heart race. Thank you