Video: Vision

There's a saying that goes “The eyes are the windows to the soul.” But if you think about it, our eyes are also really complex windows for our brain to see the outside world. These cleverly designed intricate structures take visible light, focus it, and convert it into signals that the brain can interpret, enabling us to experience the joy of sight.

And in this tutorial, we're going to see how they do that, as we learn about the physiology of vision.

Vision, very simply put, is our ability to see the world around us. What we actually see is the light that gets reflected off objects, of which there's a specific range of wavelengths that our eyes can detect. This is called visible light and it ranges from about 380 to 750 nanometers on the electromagnetic spectrum.

The function of our eyes is to first bring those light rays to focus on the retina and then to convert the stimulus into electrical impulses by a process called phototransduction. These impulses get carried to the brain along the optic nerve and through the visual pathway to reach the visual cortex where they get processed. That's three key steps to understanding vision, so let's take a closer look at the first one -- how the eye focuses an image on the retina.

Light entering the eye is traveling from one medium to another of different density. This change in medium causes the light rays to bend, a process known as refraction. Multiple structures in the eye participate in this process, however, most of the refraction happens at the cornea, followed by a smaller amount by the lens.

These structures are so carefully designed that they bring those rays of light to focus exactly on the retina. This is the focal point of the eye. If those rays were to focus in front or behind the retina, the image would be blurred. For clear vision, they have to focus exactly on the retina.

Interestingly, the image formed on the retina is actually inverted, but visual processing in the brain ensures that it innately recognizes it as upright. The rays coming from distant objects over six meters away are nearly parallel by the time they reach the eye, thus they get refracted and converge to focus at the focal point on the retina. However, for closer objects under six meters away, the rays are more diverging. Without adjustment by the eye, the rays would focus behind the retina and the image would be blurred.

So how does the human eye make the necessary adjustments? It does this by changing the refractive power of the eye using the lens, through a process called accommodation.

Accommodation is the ability of the lens to change its curvature and thus its refractive index, helping those rays focus on the retina. In addition to accommodation, to be able to see a nearby object clearly, the eyes converge, the pupil constricts, and together the trio form what's called the near response.

So what happens to the lens during accommodation?

The lens is suspended by zonular fibers, also called suspensory ligaments, which attach it to the ciliary body. This body contains the ciliary muscle. The ciliary muscle functions like a sphincter. When it contracts, it moves closer to the lens. When it relaxes, it moves away from it.

To focus on nearby objects, the ciliary muscle contracts, making the zonular fibers lax, allowing the curvature of the lens to increase. This makes the lens rounder, enhancing its ability to bend diverging light rays from close objects.

To refocus on far objects, the ciliary muscle relaxes, the zonular fibers become taut, and the curvature of the lens decreases, making it flatter. This decrease in curvature lowers the lens's refractive power, ensuring parallel light rays from distant objects converge onto the retina's focal point for clear vision.

The lens can change its curvature thanks to the elasticity of its capsule, which typically diminishes with the aging process. Thus by the age of 40 to 45, it no longer accommodates very well, making objects appear blurry. This is called presbyopia and it's one of the errors of refraction.

Another refractive error occurs in people who have eyeballs that are too short. In such cases, close objects would focus behind the retina, again creating a blurred image. This is called hyperopia, or farsightedness, since distant objects would still form a clear image.

On the other hand, if the eyeball is too long, distant objects would focus in front of the retina, again creating a blurred image. This is called myopia, or nearsightedness.

These refractive errors can be corrected by placing a lens in front of the eye. For example, a concave lens in myopia would make the image clear again, while a convex lens would fix the error in hyperopia. But if we assume the eye is able to focus the image perfectly well, it is considered an emmetropic eye.

And now, let's see what happens to that light in the retina.

The retina has a lot of layers, broadly divided into the pigmented layer and a neural layer. The neural layer has five types of cells -- the photoreceptor cells; the bipolar neurons, also known as bipolar cells; the retinal ganglion cells; and in between these are the horizontal neurons and amacrine cells.

The sensory receptors of the eye are the photoreceptor cells. When considering their position in the retinal layers, you might imagine them to be the closest to where light would fall, but then you'd be surprised to learn that they're actually the furthest. It's upside down. Light has to travel through the other neural layers before it can reach the photoreceptor cells. Any extra rays are absorbed by the pigmented layer of the retina to prevent them from reflecting back and distorting the image.

In the photoreceptor cells, light gets converted into electrical signals through the process of phototransduction. These cells are then carried via the bipolar neurons to the retinal ganglion cells. The axons of the retinal ganglion cells form the optic nerve, which exits the eye of the optic disc carrying these impulses.

But how does phototransduction happen?

Phototransduction starts with light being absorbed by photoreceptor cells, of which there are two primary types -- the rod cells and the cone cells. Cone cells have a lower sensitivity to light and thus function well in bright light, making them essential for day vision, also known as photopic vision. They provide a high visual acuity and are also needed for color vision.

Though we saw earlier that light has to travel through neural layers to reach the photoreceptor cells, there's one particular region where it directly reaches them -- the fovea. All we have here is a cluster of cone cells, thus making the fovea the region of highest visual acuity.

The number of cone cells decrease as we move away from the fovea; however, the number of rod cells increases towards the periphery. Rod cells have a high sensitivity to light, making them essential for night vision, also known as scotopic vision.

Rod and cone cells have similar structures, each having an outer segment and an inner segment. They have cell bodies with nuclei and axon terminals. However, the terminals appear slightly different, with rod cells having rod spherules and cone cells having cone pedicles. They contain synaptic vesicles with neurotransmitters.

The outer segments, shaped like a rod in rod cells and a cone in cone cells, have membranous discs. These discs have the visual pigment, which in the case of rod cells is called rhodopsin. The cone cells have iodopsins, or cone pigments, which are structurally similar to rhodopsin.

There are three kinds of pigments since there are actually three types of cones -- red, green, and blue -- as they absorb three different wavelengths. They are also known as L, M, and S-type cone cells as they absorb long, medium, and short wavelengths, and are responsible for color vision.

The inner segments of the photoreceptor cells have mitochondria, which provide energy needed for synthesis of the pigment and phototransduction, which is how light becomes electrical signals. The mechanism of phototransduction in rod and cone cells is similar, however, the process has been studied more in rod cells, so we're going to use them to understand how light becomes electrical impulses.

The visual pigment in rod cells is rhodopsin. Rhodopsin is a protein consisting of opsin, also known as scotopsin, which is a G-protein coupled receptor bound to cis-retinal, a derivative of vitamin A. Very soon, you'll see why we were told as children that eating carrots was good for our eyes.

To understand what happens in the light, let's first see what happens to rod cells in the dark.

In the dark, cis-retinal is bound to opsin. They are coupled to a G-protein called transducin, which is inactive in the dark. Without light, the cyclic guanosine monophosphate or cGMP levels in the cell are high. This keeps an ion channel on the membrane open, allowing sodium to enter the cell, thereby depolarizing it. Thus in the dark, rods have a continuous tonic release of the neurotransmitter glutamate.

When a photon of light hits rhodopsin, one important thing happens that sets off a chain of events. The cis-retinal gets converted to trans-retinal, which changes the conformation of opsin. This is called bleaching and it activates the associated G-protein, transducin. This in turn activates an enzyme, phosphodiesterase, which lowers the levels of cGMP by converting it to guanosine monophosphate or GMP, thus closing the sodium channels.

The cell hyperpolarizes and the glutamate release reduces. Thus photoreceptor cells hyperpolarize in the light and depolarize in the dark. These receptors synapse with bipolar neurons. The rod cells synapse with rod bipolar cells while the cone cells synapse with cone bipolar cells.

Glutamate from the photoreceptor cell could either depolarize or hyperpolarize these bipolar cells, called ON or OFF cells, depending on the type of glutamate receptor they have. It's the receptor that determines whether the neurotransmitter will be excitatory or inhibitory to the cell. For the sake of simplicity, let's understand what happens with the ON bipolar cells.

Glutamate hyperpolarizes these bipolar cells, acting as an inhibitory neurotransmitter. Thus in the dark, when the photoreceptor cells release glutamate, the ON bipolar cells become hyperpolarized and won't release their neurotransmitters. But in the light, when glutamate release reduces, they are now released from that inhibition, free to depolarize and release their neurotransmitters. Therefore, light deactivates the photoreceptor cells and in turn excites the ON bipolar cells while inhibiting the OFF bipolar cells, the opposite of what would happen in the dark.

It's important to note that bipolar neurons themselves don't generate action potentials. They have local potentials which can result in the release of neurotransmitters. These neurotransmitters can in turn trigger action potentials in the retinal ganglion cells. The axons of the retinal ganglion cells then carry those impulses along the optic nerve and thus visual information is now being transported as electrical signals.

But what happens when we go from a light room to a dark one? It takes our eyes some time to adjust. This is called dark adaptation and it can take up to 40 minutes. Initially, our pupils dilate in response to the reduced light almost immediately. thanks to the pupillary light reflex, which involves relaxation of the sphincter pupillae and contraction of the dilator pupillae muscles.

However, the photoreceptor cells are the ones that take time to adjust. They get bleached in bright light and need time to recover their visual pigments. The cone cells recover faster, but in dim light we need the rod cells to function and they need much longer to regenerate rhodopsin. This slow process accounts for the time required for our eyes to adjust to the dark.

When you return to a brightly lit room, such as when you exit a movie theater, you'll notice it takes time to adjust again. Our pupils constrict immediately, thanks again to the pupillary light reflex. The sphincter pupillae muscle contracts and the dilator pupillae relaxes, letting in fewer light rays. The bright light bleaches the photoreceptor cells, but since the cone cells recover their visual pigments faster, adjusting to the light does not take as long as with the dark. This is light adaptation.

And now we've reached the last step in vision -- the visual pathway and the brain.

The visual fields of the eyes overlap. Rays from the left visual field fall onto the left nasal and right temporal retina, while those from the right visual field fall on the right nasal and left temporal retina. Impulses from both the nasal and temporal retinas get picked up by the optic nerve, which exits the eye.

At the optic chiasm, the nasal fibers cross over such that each optic tract has the temporal fibers of the ipsilateral eye and nasal fibers of the contralateral eye, thus impulses from both eyes reach the lateral geniculate nucleus of the thalamus.

If you'd like to learn about the visual pathway in more detail, check out our video on the optic nerve.

Signals from the thalamus then travel along the optic radiation to reach the primary visual cortex located in the occipital lobe -- Brodmann area 17. Information from the right visual field is processed in the left cerebral hemisphere and vice versa.

The visual cortex is where the initial processing of information related to form, movement, shape, and color takes place; however, it also has connections with other areas of the brain known as visual association areas like Brodmann areas 18 and 19. They're also known as the secondary visual cortex and are located around the primary visual cortex.

Connections with areas further away help in deeper processing of visual information. Broadly speaking, there are two important pathways of connections. The dorsal stream connects the visual cortex with the parietal lobe, needed for things like interpreting motion. The ventral stream connects it with the temporal lobe, needed for the assessment of fine details such as color, form, and recognizing letters and faces.

That isn't all, however, as the visual cortex has a plethora of connections like those that enrich our sensory experience of the world around us and enable us to perform all types of activities, even something as simple as reading a book.

So the next time your eyes are quickly zipping across the page of a book, think about how much work your visual system had to put in for you to be able to clearly see those letters. This is just a start to understanding vision.

To learn more about this and all the other special senses, check out our study units and the many articles we have at Kenhub.