The Brain's Logic & Function: From Vision to Circadian Rhythms with Dr. David Berson

This episode features Dr. David Berson, a Professor of Neurobiology and Ophthalmology at Brown University and a long-time mentor to Dr. Andrew Huberman. Dr. Berson, renowned for his clarity in explaining the nervous system, guides listeners through the intricate processes of vision and circadian rhythm regulation. The discussion begins with the fundamental question of how we see, detailing the journey of a photon from the eye's photoreceptors to the brain's cortex, where conscious visual experience is formed. A significant portion is dedicated to color vision, explaining how different wavelengths of light are decoded by three types of cone cells in the retina, leading to our perception of various colors. The philosophical question of whether individual color perception is identical is also touched upon, alongside comparisons of human vision with that of other animals like dogs, highlighting the biological similarities and perceptual differences.\n\nThe conversation then shifts to Dr. Berson's co-discovery of intrinsically photosensitive melanopsin cells (ipRGCs), a unique class of neurons in the retina. These cells are peculiar because they are located in the innermost part of the retina, unlike traditional photoreceptors, and utilize a primitive chemical cascade akin to that found in insect eyes, leading to the "fly eye in our eye" analogy. Unlike rods and cones, ipRGCs do not contribute to image-forming vision but instead detect overall environmental brightness. This brightness signal is crucial for non-visual biological functions, primarily synchronizing the body's internal clocks.\n\nDr. Berson elaborates on the critical role of these ipRGCs in regulating circadian rhythms. He explains that while nearly all cells in the body possess their own internal 24-hour-ish clocks, a central master pacemaker, the suprachiasmatic nucleus (SCN) located in the hypothalamus, coordinates them. The ipRGCs provide direct light input to the SCN, allowing it to synchronize with the external day-night cycle, thereby correcting any drift in the internal clock. This synchronization is vital for maintaining proper sleep-wake cycles and overall physiological function, preventing issues like jet lag or the insomnia experienced by some blind individuals whose SCN lacks environmental light cues.\n\nFinally, the discussion touches upon how the SCN, once synchronized, communicates with the rest of the body. It influences the autonomic nervous system, which controls states of alertness and calm, and also utilizes humeral signals (substances released into the blood or diffusing through the brain) to coordinate the myriad local clocks throughout the body. This intricate system ensures that the liver, stomach, and other organs operate in harmony with the external environment, impacting everything from metabolism to cognition, underscoring the profound, often unconscious, influence of light on our entire biological system.

"Today my guest is Dr. David Berson, Professor of Medical Science, Neurobiology and Ophthalmology at Brown University. Dr. Berson's laboratory is credited with discovering the cells in the eye that set your circadian rhythms."

"You're going to realize today why Dr. Berson is my go-to source. He has an exceptionally clear and organized view of how the nervous system works."

"By the end of this podcast, you will know far more about how your nervous system works than the vast majority of people out there including many expert biologists and neuroscientists."

"But the point is, that the experience of seeing is actually a brain phenomenon. But of course, under normal circumstances, we see the world because we're looking at it and we're using our eyes to look at it."

"Essentially, different wavelengths give us the sensation of different colors through the auspices of different neurons that are tuned to different wavelengths of light."

"What we can say is that the biological mechanisms that we think are important for seeing color for example, seem to be very highly similar from one individual to the next whether it be human beings, or other animals."

"So these funny extra photoreceptors that are in the wrong layer doing something completely different are actually using a chemical cascade that looks much more like what you would see in a fly photoreceptor, than what you would see in a human photoreceptor, a rod or a cone for example."

"So why does your nervous system need to know whether it's daylight right now? Well, one thing that needs to know that is your circadian clock."

"The role of the central pacemaker for the circadian system is to coordinate all of these. And there's a little nucleus, a little collection of nerve cells in your brain it's called the suprachiasmatic nucleus the SCN..."

"So this is a part of your visual system that doesn't really reach the level of consciousness, it's not something you think about, it's happening under the radar kind of all the time and the signal is working its way into this central clock coordinating center."