The Science of Vision, Eye Health & Seeing Better

The episode opens with the host identifying himself as a professor of neurobiology and ophthalmology at Stanford School of Medicine. He says the podcast is separate from his teaching and research roles there and is part of an effort to bring "zero-cost-to-consumer" science information to the public. He then announces a new topic that will run for the next four to five episodes.

The new series covers the senses: sight, hearing, touch, taste, smell, and interoception, described as the sense of "our internal real estate." The stated reason is that understanding how the senses work, including the cells and connections behind them, sets up the following month's topic of mental health. The host argues the material still matters for people with no sensory problems, because it bears on how people move through the world, make sense of information and organize their thoughts and emotions. He promises practical tools along with mechanisms, and notes that vision has been the focus of his career for "well over 25 years."

He speaks directly to younger listeners who ask what they should do, given his frequent point that neuroplasticity tapers off with age. His answer is that young people with good vision are in the best position to reinforce their visual system so they do not lose vision as they age, because a 12- or 15-year-old's nervous system is "far more plastic" than his. He also suggests that specific light-viewing behaviors, at particular times and in particular ways and at any age, can build an emotional system reinforced by the visual system.

The host is explicit about the limits of what he offers. He says he cannot make clinical diagnoses or hold one-on-one conversations in this format, and that he is not a clinician. To vet the protocols against the clinical literature, he says he consulted Dr. Jeffrey Goldberg, chair of ophthalmology at Stanford University School of Medicine, and several others. People with a severe eye problem, he says, should work with a good ophthalmologist and/or optometrist, and in particular an ophthalmologist, who is a medical doctor.

I'm a scientist, not a physician.

Before the vision material, the host answers listener questions from the previous episode on endurance training. That episode described four kinds of endurance training, and the protocols are posted as a zero-cost PDF at hubermanlab.com. The most common question was about "concurrent training," meaning how to combine endurance work with strength and hypertrophy work.

His framework is to decide what to emphasize and then emphasize it for a 10-to-12-week cycle. If endurance is the priority, he suggests a three-to-two ratio: three, maybe four, endurance workouts per week and two strength and hypertrophy workouts. If strength and hypertrophy is the priority, the ratio flips: three or four of those workouts and two endurance sessions. People new to a type of training should start with the minimum number of sets in the protocol. For example, if the protocol says three to five sets, start with three, "maybe even just two," and add sets each week.

He recommends at least one complete rest day per week. He says he personally benefits from two, while acknowledging that many people dislike rest days and that rest can be active, such as hiking. After each 10-to-12-week cycle, he suggests five to seven days completely off, as a way to stay injury-free, keep joints and tissues healthy and keep progressing. He then explicitly softens all of this: anyone who doesn't want the week off doesn't have to take it.

None of this is holy, none of it is a strict prescriptive.

The host argues that "vision" means more than eyesight, the ability to perceive shapes, objects, faces and colors. The eyes also affect mood and alertness. He says there is no "extraocular light perception": light on the skin may feel good, but the eyes are the only route by which light information reaches the body's cells. He describes the neural retinas as part of the central nervous system. They are the only part of the brain outside the cranial vault, placed in the eye sockets during development by a genetic program that squeezes out "three little layers of neurons."

He walks through the eye's supporting structures. The lens focuses light precisely onto the retina, and people who need glasses or contacts likely don't do this correctly. Other parts of the eye keep it lubricated. Eyelashes, he says, exist to trigger the blink reflex when dust approaches the cornea, and he calls the blink reflex the fastest reflex a person has. He also flags the eyelids as a later topic, through neural circuits linking the brainstem to the eyelids that can be used to raise alertness.

The basic job of the eye, in this account, is to collect light and send it to the brain in a form the brain understands. Light itself goes no further than the neural retina. There, photoreceptors come in two types: cones, mainly for daytime vision, and rods, mainly for night or low-light vision, "generally speaking." Chemical reactions inside these cells, involving things like vitamin A, convert light into electricity. The host compares this to touch receptors in the skin converting physical pressure into electrical signals. After several processing stages within the retina, a specific class of neurons, retinal ganglion cells, sends the information to the rest of the brain.

The central conceptual claim of this section is that people never see objects directly. What reaches the brain is a pattern of electrical activity, and perception is the brain's interpretation of it.

What you're doing is you're making a best guess about what's there based on the pattern of electricity that arrives in your brain.

His main illustration is a green apple. Cones respond best to the wavelength of light reflected off the apple, but "no green light arrives in your brain." Instead, the brain compares the amount of green reflection to the amount of red and blue around it. On a brown table or a white surface, the apple looks very green because the green signal is high relative to red. He offers a second analogy from music, prefaced by "I'm not a musician, so hopefully I won't get this too incorrectly": the brain doesn't recognize a single piano key until another key is played next to it and the two are compared, which he describes as the brain doing "the subtraction."

He acknowledges that the idea may be hard to grasp and says it will be explained in more depth later. His summary is that eyesight is not information passing directly into the brain. Light information is transformed into electrical signals that the visual system interprets.

To show why the comparison model matters, the host turns to other species. Dogs and cats, he says, are not colorblind, but they lack the cones that respond to red, or long-wavelength, light. They therefore can't compare green against red the way a trichromatic human can, so a green lawn looks more brownish or orange to them, and a red stop sign looks orange-ish brown. The general point is that each animal's view of the world depends on whether it has one, two or three cone types.

He lists further examples. The mantis shrimp sees hundreds of colors humans cannot. The pit viper senses heat emissions, literally seeing the heat coming off a person or a prey animal. Ground squirrels can see ultraviolet light, and he says they signal one another at a distance by standing up and reflecting sunlight off their stomachs, comparable to signaling with a mirror. He also describes primate species that urinate on their hands, wipe it over their stomachs and use reflected sunlight to signal. He adds that he doesn't know what they are saying and that people tend to assume it is "something cute and nice." He links these examples to larger questions about consciousness and how much of the world is accessible to us, calling the topic an early obsession of his in visual neuroscience.

His final example is a diving bird that hunts fish from above the ocean. Water has a refractive index, which he calls a "refractory index," that shifts the apparent position of the fish like a prism, so the bird must adjust its dive because the fish is not where it appears. He compares this to diving for a coin at the bottom of a pool with eyes closed and missing it. According to his description, these birds have a streak-like arrangement of retinal cells for viewing the horizon and a pupil "on the bottom of their eye" for accurate dives, whereas humans have pupils in the middle of the eye.