Dr. David Berson: Your Brain’s Logic & Function

High-Level Overview

Andrew Huberman sits down with Dr. David Berson of Brown University, whose lab discovered the intrinsically photosensitive melanopsin cells in the eye that set our circadian rhythms. Huberman has called Berson his go-to neuroscience mentor for about two decades. The episode is essentially a guided tour of the nervous system, moving from the outside in: how the eye turns light into signals, how color vision works, the "fly eye hidden inside our own retina that tracks brightness and sets the body clock, the balance system and why we get motion sick, the midbrain and its reflexes, the cerebellum, the basal ganglia's go" and "no-go" control, the cortex and its remarkable plasticity, and finally the new field of connectomics.

The tone is warm, curious and playful, with two old friends swapping analogies (train stations, air traffic control, cold spaghetti) and trading jokes along the way. The conversation moves step by step up the nervous system, stopping regularly for practical takeaways about light, sleep, eyesight, motion sickness and self-control.

The Discussion Explores:

• How the retina converts light into signals, and how three cone types create color vision
• Melanopsin ganglion cells, the "fly eye" in our eye, and how light sets the circadian clock
• Light, melatonin, mood, blue blockers and why time outdoors protects children's eyesight
• The vestibular system, image stabilization, head-bobbing pigeons and motion sickness
• Ear popping, the midbrain, reflexes and combining signals from different senses
• The cerebellum as air traffic control for movement, and the basal ganglia's go and no-go roles
• Cortical plasticity, illustrated by a blind woman who lost braille after a stroke in her visual cortex
• Connectomics, citizen science through EyeWire, and how to start learning neuroscience

Deep Dive into Main Ideas

How We See, and How We See Color

Berson begins by pointing out that seeing is ultimately a brain phenomenon. We can "see" in dreams with no input from the eyes. In waking life, though, what matters is what the retina tells the brain. The retina works like a camera's film or image sensor: a layered sheet at the back of the eye where photoreceptors convert light into neural signals. Retinal ganglion cells then carry that information to the brain, eventually reaching the cortex, where conscious vision happens. Many other brain areas receive visual input for other purposes.

Light is electromagnetic radiation, and different wavelengths within the visible range produce different color sensations. Humans typically have three cone types, each with a pigment tuned to a different preferred wavelength. The brain compares their signals, which is how we can tell, for instance, that late-day light looks golden. Rods handle dim-light vision. Most mammals, including dogs and cats, have only two cone types, so they see roughly the way some color-blind humans do. Whether your red is the same as someone else's is, Berson says, a deep philosophical question that science probably can't answer, though the biological machinery is very similar from person to person. Color blindness is a real but fairly modest limitation compared with losing sharp vision, although a world built for typical vision can make it a problem at times.

The "Fly Eye" in Our Eye and the Circadian Clock

Berson's own discovery concerns a fifth pigment, melanopsin, found in the "wrong place: in some ganglion cells at the innermost layer of the retina, cells nobody expected to sense light directly. These cells convert light into signals using a chemical cascade much more like an insect's photoreceptor than a human rod or cone, which is why Huberman jokes that we carry a bit of fly eye" in our eye. They ignore shapes and edges and instead report overall brightness, essentially whether it's daytime.

That brightness signal goes to the suprachiasmatic nucleus (SCN) in the hypothalamus, which Berson calls the coordinator of our drives, "the source of all our pleasures and all our problems." Almost every cell in the body has its own roughly 24-hour clock built from genes regulating other genes, and the SCN coordinates them through neural connections, substances released into the brain and bloodstream, and the autonomic nervous system. Biological clocks aren't perfect: someone living in a cave stays within a few minutes of 24 hours each day, but without correction they drift. That is why people with retinal blindness often have insomnia or wake in the middle of the night, since their clocks run at something like 24.2 or 23.8 hours. Daily light is the synchronizing signal, and it's also what gradually fixes jet lag.

Light, Melatonin, Mood and Eyesight

Through a roundabout sympathetic nervous system pathway, the SCN controls the pineal gland, the body's main source of melatonin. Melatonin is low during the day and high at night, but turning on a bright bathroom light in the middle of the night will "slam" it "to the floor. Berson confirms Huberman's point that blue light is somewhat more effective at suppressing melatonin, but any light can do it, and bright red light can beat dim blue light. So using red light doesn't let you off the hook. He also agrees that wearing blue blockers during the day makes little sense, because that's when you want bright light, including blue. Seasonal affective disorder in dark northern winters reflects this same system in reverse, and light therapy can help. It's not about it's like good or bad for you, it's about what kind of light and when."

Two more findings stand out. The more time children spend outdoors, the less nearsighted they tend to become. Berson says it's unresolved whether that's due to light intensity or simply focusing on distant things rather than on everything close up, like a phone. Separately, Samer Hattar's lab (with Diego Fernandez) found a side pathway from these light-sensing cells through the thalamus to the frontal cortex, the region involved in planning and self-image. In animals, activating it at the wrong time of day produced depression-like behavior, and silencing it prevented the effects of disruptive lighting schedules. Huberman floats the idea of a wearable device that counts photons during the day and warns you about getting too many at night.

Balance, Image Stabilization and Motion Sickness

The vestibular system in the inner ear shares a nerve with hearing and uses similar hair cells. Three fluid-filled canals, which Huberman pictures as three hula hoops at different angles, sense rotation of the head in three axes (the yes nod, the no shake, and the "puppy head tilt"). This is much like how three cone types encode color. Gravity acts like a constant acceleration, and the brain has to judge whether a movement was self-generated or imposed, such as a plane accelerating down the runway.

Rotate your head left, even in complete darkness, and your eyes reflexively rotate right to keep the image steady on the retina. The brain works hard to stabilize the visual world because vision is best when the image holds still. We move our eyes in quick jumps, then rest. Hummingbirds hold their heads still at feeders, chickens keep their heads fixed as their bodies move, and pigeons "bob" by holding their heads back as their bodies move forward, then snapping forward. Motion sickness comes from visual-vestibular conflict. In the back of a car looking at your phone, your inner ear senses motion while your eyes see a still screen, and your brain complains with nausea. The fix is to see as much of the moving world as possible, ideally as if you were the one driving: sit in front and look out.

The cerebellum, which Berson likens to air traffic control, combines these signals. It takes in information about everything happening everywhere and coordinates and refines movement. Without it you could still move and sense, but your timing and coordination would suffer (cerebellar ataxia), and you'd struggle to learn a skill like an overhead tennis serve. Its oldest part, the flocculus, combines visual and vestibular signals for image stabilization and adjusts itself when the system makes errors. On ear popping during flights, Berson thinks the direction of the pressure difference matters less than simply opening the tube to the back of the throat. Holding your nose and blowing or sucking should both work, which settles a bet with anatomist Harvey Karten in Huberman's favor.

The Midbrain, Reflexes and Combining the Senses

Traveling up from the spinal cord, the brainstem thickens as evolution adds more processing machinery. Its top section, the midbrain, contains the superior colliculus, which is the main visual center in frogs and lizards (where it's called the optic tectum). It reorients the eyes, head and attention toward sudden events, like something splatting onto the page of a novel you're reading. Related midbrain areas drive reflexes such as ducking at a looming object. It also takes in touch, sound and, in rattlesnakes, which Berson once studied, infrared input from facial heat-sensing pits. Combining a weak signal from one sense with a weak signal from another pointing to the same place gives the brain corroboration, while conflicting signals cause confusion.

Asked why tilting through space while skateboarding, surfing or taking a corner feels so good, Berson admits "I really have no clue." He speculates that dopamine may reach this system and that the pleasure may come from a sense of agency, of being "the master of your own movement."

Reflexes Versus Deliberate Action: Basal Ganglia and Cortex

Many brain levels work on the problem of staying alive at the same time. Lower centers provide fast, rigid reflexes, and higher centers add nuance and can override them. If the Queen hands you a thin Wedgwood teacup of scalding tea, you'll find a way to set it down rather than drop it. But you want the reflex to fire instantly when something is flying at your head. Overthinking can also sabotage trained reflexes, like a batter who stops trusting the swing built up over thousands of repetitions.

The basal ganglia, deep in the forebrain and closely linked to the cortex, handle "go" and "no-go: starting some actions and holding back others. The marshmallow test is a classic no-go. Children can wait to get two marshmallows instead of one, using cortical strategies like covering their eyes, singing or counting. Berson says why some people find it easy to start tasks and others don't comes down to genes and experience, and that restraint, like any skill, improves with practice. Huberman describes his habit of doing 21 deliberate no-goes" a day, such as not reaching for his phone. Berson calls this a legitimate mental discipline, noting that we learn to defer gratification or hold back a remark that would inflame a partner. Huberman suggests that many attention problems, outside of clinical ADHD, may come from never learning to interrupt a reflex.

The Cortex: Many Maps and Remarkable Plasticity

The visual cortex is a map, or rather dozens of maps, of the visual world. Different maps seem to specialize in different jobs, such as reaching for things versus recognizing what an object is and what it means. Neurons that respond to specific faces exist, but actually perceiving your grandmother involves patterns across huge numbers of neurons, not one magic cell. Berson says the nervous system is both highly specific, with genes that direct precise connections, and broadly networked. The truth lies in holding both ideas at once.

His most striking example is a woman who was blind from early life and became a top executive secretary, reading and typing in braille. She had a stroke, and the neurologist told her the good news was that it hit her visual cortex, an area "you're not even using." But she lost the ability to read braille. Imaging studies confirm that in people blind from early life, the visual cortex is repurposed to process touch. It works as a general-purpose spatial processor, and the fingertips are just another spatial sense.

Connectomics and How to Learn More

Berson's current passion is connectomics: using serial electron microscopy to image nervous tissue slice by slice at nanometer scale, then tracing every cell and synapse to build a complete wiring diagram. Huberman compares it to thinly slicing cold spaghetti and tracking which strand is which, with the sauce standing in for blood vessels and glial cells. Anatomy has long been "the weak sister" to studies of function, Berson says. Now wiring diagrams can generate hypotheses directly. In his own work on image-stabilizing cells that respond only to slow motion, the connectome pointed to a specific cell type that could tune them, and follow-up experiments confirmed it. Huberman adds that you can't form a hypothesis about a cell you don't know exists.

For people who want to learn more, Berson recommends Sebastian Seung's EyeWire project, where anyone can help trace neurons from their living room. He also suggests asking knowledgeable people for pointers, reading accessible books such as the late Richard (Dick) Masland's We Know It When We See It, and starting with Wikipedia and following its references. Whatever your passion, or a loved one's neurological or psychiatric condition, "there's a branch of neuroscience that is devoted to" it. Huberman describes finding Berson in his office, door closed, drawing neurons by hand. Berson, whose wife is an artist, says the work is "pure joy" because the beauty of the form is, in a sense, the function.

Conclusion

In under two hours, David Berson takes listeners through the nervous system: from photons striking the retina and the ancient "fly eye" cells that tell the body what time it is, through the balance system, midbrain reflexes, the cerebellum and the basal ganglia, up to the cortex's many maps and its ability to repurpose itself. The practical lessons are clear: get bright light during the day and avoid it at night (whatever its color), get kids outdoors, keep your eyes on the moving world when you're prone to motion sickness, and treat restraint as a skill you can train. The deeper lesson is Berson's view of the brain as both precisely wired and broadly networked, a system that tools like connectomics are only beginning to reveal.

This is a summary of "Dr. David Berson: Your Brain’s Logic & Function" from Huberman Lab, giving you the GIST so you can decide if you want the REST. Listen to the full episode.