Essentials: How Your Brain Functions & Interprets the World | Dr. David Berson

Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, opens this Essentials conversation by describing Dr. David Berson as his go-to source on the nervous system for more than 20 years. His first question is basic: when a photon of light enters the eye, how does that become the experience of seeing a passing truck or a photo of a dog on the wall?

Berson's answer begins with the brain, not the eye. A visual experience, he says, comes from a pattern of brain activity that the brain associates with input from the periphery. But that input is not strictly required: in dreams, people see things that are not coming through their eyes. Asked whether dreams are memories, Berson is careful. He says they may reflect visual experience in a sense, and are not necessarily specific visual memories, though they can be.

But the point is that the experience of seeing is actually a brain phenomenon.

Under normal conditions, though, what matters when we look at the outside world is what the retina tells the brain. Berson calls the eye a camera. It detects the first image and does some early processing. Ganglion cells, the neurons that carry signals from eye to brain, then send that signal back to the brain. Conscious visual experience arises at the level of the cortex, though many other brain regions also receive visual input and use it for other purposes.

Turning to color vision, Berson describes light as a form of electromagnetic radiation that oscillates. Huberman asks whether that means photons are actually moving. Berson says photons certainly move through space, and that light can be treated either as particles or as a wave, like a radio wave. Either view is acceptable. Certain frequencies in the electromagnetic spectrum can be detected by retinal neurons, and those are the ones we see.

Within visible light there are different wavelengths, and the nervous system decodes them into the experience of color. In Berson's words, different wavelengths produce the sensation of different colors through different neurons tuned to different wavelengths. This starts in the first layer of the retina, where light becomes neural signals. Sensitive cells there make specific molecules whose job is to absorb photons, the first step in seeing.

Berson says the typical retina has about five such light-absorbing proteins, but color vision depends on three. Each absorbs light best at a different preferred frequency. The nervous system then compares and contrasts their signals to work out the wavelength composition of the light. His everyday example is a landscape that looks golden late in the day. We read the time of day from the changed composition of the light reaching our eyes.

Huberman asks whether his perception of red is the same as Berson's. Berson calls it a deep philosophical question and draws a clear line around what science can settle here.

It's a question that really, probably can't even ultimately be answered by the usual empirical scientific processes, because it's really about an individual's experience.