Understanding Your Brain's Logic & Function | Dr. David Berson

The episode opens with the host, who introduces himself as a professor of neurobiology and ophthalmology at Stanford School of Medicine, presenting Dr. David Berson, Professor of Medical Science, Neurobiology and Ophthalmology at Brown University. Berson's laboratory is credited with discovering the cells in the eye that set circadian rhythms, the so-called intrinsically photosensitive melanopsin cells. For this introduction, the point is that these cells tell the brain and body what time of day it is. The lab is also described as having made other discoveries about how perceptions of the outside world are converted into motor action.

The introduction explains the personal connection: nearly two decades of conversation that began with a cold call and an invitation to Brown. It credits Berson with an unusually clear and organized view of a nervous system made up of many nuclei, connections, circuits and chemicals. The conversation is framed as a journey from the periphery of the nervous system inward, "layer by layer, structure by structure, circuit by circuit." The introduction also states that the podcast is separate from the host's teaching and research roles at Stanford and is meant to bring zero-cost-to-consumer science information to the general public.

The first question is a basic one: a photon of light enters the eye, so how does that become the experience of seeing a passing truck or a photo of a dog? The answer starts at the end of the process. A visual experience happens because the brain has a pattern of activity associated with input from the periphery. That input is not strictly required, though. Dreams show that people can see things that are not coming through their eyes.

Asked whether dreams are memories, the answer is that they may reflect visual experience in a sense. They are not necessarily specific visual memories, although they can be. The broader claim is that "the experience of seeing is actually a brain phenomenon." In ordinary waking vision, what matters is what the retina tells the brain. Retinal ganglion cells are described as the key neurons that carry signals from eye to brain. The eye works like a camera: it detects the initial image and does some initial processing. Conscious visual experience arises at the level of the cortex, and many other brain regions also receive visual input and use it for other purposes.

Color vision is explained from the physics up. Light is a form of electromagnetic radiation. It can be thought of as particles (photons) or as a wave like a radio wave, and "Either way is acceptable." Certain frequencies in the electromagnetic spectrum can be detected by neurons in the retina, and those are what we see. Within that visible range, the nervous system unpacks or decodes different wavelengths. Neurons tuned to different wavelengths are what give rise to the sensation of different colors.

The mechanism begins in the first layer of the retina, where light is turned into neural signals. Cells there make molecules whose purpose is to absorb photons, which is the first step in seeing. The discussion says a typical retina has about five such proteins, but only three are used for color. Each absorbs light best at a different frequency. The nervous system then compares and contrasts their signals to work out the wavelength composition of the light. The example given is looking at a landscape and inferring that it must be late in the day because things look golden. That inference comes from the brain interpreting the changed composition of the light reaching the eyes.

Whether two people perceive red the same way is treated as a deep philosophical question. The answer is that it probably cannot ultimately be settled by the usual empirical scientific methods, because it concerns an individual's experience. What science can say is narrower. The biological mechanisms thought to matter for color vision look very similar from one individual to the next, in humans and in other animals. Perception, understanding or experience is "a little bit tougher to nail down" with the scientific approaches used to study biological vision.

A question about "five different cone types" gets a correction: "it's not really five types of cones. There are really three types of cones." Color vision is thought to need three different signals. The other pigments serve other purposes. One is used mostly for dim-light vision, such as walking around on a moonless night; this is the rod cell, which has its own pigment. The other is melanopsin, which the conversation comes back to later. When ultraviolet and infrared are raised as possible examples, the discussion stays with the comparison of cone types.

Most humans have three cone types. Most mammals, dogs and cats included, have only two, and that limits how well they can tell wavelengths, or colors, apart. A dog is described as seeing the world somewhat like a particular kind of colorblind human, because with only two channels to compare, it is much harder to work out exactly which wavelength is being viewed. This answers the earlier guess that dogs see reds more as oranges.

On whether colorblind people suffer as a result, the answer compares colorblindness to other disabilities: "the world is built for people of the most common type." Problems come up when people are expected to see a contrast that their visual system cannot detect because a cone type is missing. Overall, though, colorblindness is called "a fairly modest visual limitation, as things go." Losing acuity, such as being unable to read fine print, can be much more damaging. The discussion adds that color is very meaningful to humans, while dogs, cats and many other mammals still do perfectly well in the world.

The conversation then turns to the unusual photopigment found in specialized ganglion cells, the intrinsically photosensitive cells, which handle biological functions rather than perception. The host refers to an earlier line, "We have a bit of fly eye in our eye," which had been illustrated with a slide of a giant fly from a horror movie. The pigment is described as the first sensitive element in a system that tells the brain how bright the world is. What makes it peculiar is that "it's in the wrong place, in a sense."

To explain the placement, the retina is described as a layer cake: a thin membrane at the back of the eye made of stacked thin layers. Conventional photoreceptors sit in the outermost layer. That layer is like the film of a camera or the CCD chip in a cell phone. The eye's optics form an image there, and an array of sensors turns it into something like a bitmap of neural signals across the retina. This arrangement, with rods and cones and, on closer inspection, three cone types, was known about 150 years ago, and it was thought to be the only place where light was converted into neural signals.

The surprise was that this last photopigment sits at the opposite, innermost side of the retina, where the ganglion cells are. Ganglion cells are the output neurons that talk to the brain. Some of them, which were not thought to have "any business being directly sensitive to light," turned out to make this photopigment, absorb light, turn it into neural signals and send those signals to the brain.

The fly comparison comes from what happens after light is absorbed. Getting from the absorption event to an electrical signal involves a complex chain of chemical steps. Insects, other invertebrates and "fuzzy, furry creatures like us" all have similar chains, but the specifics differ in characteristic ways. These extra photoreceptors use a cascade that "looks much more like what you would see in a fly photoreceptor" than what happens in a human rod or cone. When this is summarized as "a very primitive aspect of biology that we maintain," the answer is "Exactly right."

The function of these cells is described as carrying a single kind of signal, essentially a brightness signal, which the brain can use for many things. These cells track overall brightness, not details such as the edge of someone's ear. The difference is put as "the difference between knowing what the objects are on the table and knowing whether it's bright enough to be daylight right now."

The circadian clock is one system that needs that information. Jet lag is the example. After flying from California to Europe, the internal clock still runs on California time, so sunrise and sunset no longer match what the body expects. The brain holds an internal representation of the earth's rotation. The brightness system sends an "it's daylight now" signal, the brain compares it with its internal clock, and if the two disagree it adjusts the clock gradually until the jet lag passes.