What Is Vision?
Vision is the process through which light stimuli received through the eyes are transformed into a mental image by the brain. It is our sense of sight. This process is accomplished by the visual system, the sensory system that enables vision, which includes the eyes—the sensory organ for vision—and the neuronal visual pathways of the brain, from the retinas to the cerebral cortex.
The process of vision requires that light entering the eye be captured by sensors that convert it into neural signals that can be sent to the brain. There, these neuronal messages are processed to generate mental visual representations of the world.
Visible Light: Understanding the Visual Spectrum
What we see is light, or visible light. We can see what surrounds us because light is emitted or reflected by objects and captured by our eyes.
Light is electromagnetic radiation made of particles called photons that travel in waves of different wavelengths and energy. Visible light is a small fraction of the vast electromagnetic radiation spectrum, which ranges from high energy (high frequency) radiation such as gamma (γ) rays or X-rays, with very short wavelengths of less than 1 nanometer (nm), to low energy (low frequency) radiation such as microwaves and radio waves, with wavelengths greater than 1 millimeter (mm).
Visible light is the portion with wavelengths ranging from around 380 nm, corresponding to violet light, to 750 nm, corresponding to red light; it is bordered by ultraviolet (UV) radiation below 380 nm and infrared (IR) radiation above 750 nm. These are approximate numbers as there are no agreed limits to the visible spectrum; on average, light visibility falls within this interval, but it has been reported to extend as far as 310 nm at the short wavelength end and 1100 nm in the near-infrared .
This fraction of the electromagnetic spectrum is visible because it reaches the retina, is captured by neuronal receptors, and generates a neuronal signal that our brain can translate into a mental image. Only visible light and some wavelengths of near-IR light reach the retina; most radiation outside of the visible spectrum is filtered by the tissues and fluids of the eye that light has to cross before it reaches the retina. And that that does reach the retina, is outside the limits of detection of human vision .
Visible light is itself a spectrum, but this spectrum does not contain all the colors that the human visual system can distinguish. The mix of all wavelengths of light emitted by the sun appears to humans as white, whereas light of a single wavelength appears as one of the colors of the rainbow and is called a spectral color. In fact, that’s exactly what the rainbow is: the spectrum of visible light emitted by the sun, revealed by the reflection and refraction of light striking water droplets in the atmosphere.
Depending on the properties of an object, light is absorbed and reflected in different combinations of wavelengths that create different colors. We see in color because we see the combination of wavelengths that are reflected by objects. Many of the colors we perceive match those from the visible spectrum; some, such as pink, are produced when more than one wavelength are mixed together.
“What we see is the light that everything around us either emits or reflects and that is captured by our eyes."
Anatomy and Structure of the Eye
As the organs of vision, the structure of the eyes are designed for capturing light within the visible spectrum from the outside world and transforming it into signals to be sent to the human brain [3–5].
The eye is covered by a layer of connective tissue called sclera, which is what we see as the white part of the eye. In the front part of the eye, the sclera becomes a transparent disk called cornea, which allows the entry of light into the eye.
The interior of the eye is divided into two chambers by the lens: behind the cornea and in front of the lens, there is a small chamber filled with a low-protein fluid called aqueous humor. Behind the lens and the retina, there’s a much larger chamber filled mostly with a clear, gelatinous matrix called the vitreous humor, which helps to maintain the shape of the eyeball.
The aqueous humor is divided into two smaller parts (anterior and posterior parts) by the iris, the colored part of the eye. The iris has an opening through which light passes into the eye. That opening, which we see as the black spot in the middle of the iris, is the pupil, whose function is to regulate the amount of light that enters the eye (we see the pupil as a black spot because light within the eye is absorbed by the retina). Behind the pupil, there’s the lens, suspended within the eye by ligaments attached to the ciliary muscle, which forms a ring inside the eye. The lens is a transparent disk with two convex surfaces whose function is to focus light on the retina.
Lining the inner chamber of the eye is the retina, the actual sensory tissue of the eye. The retina contains light-sensitive neurons (photoreceptors) that capture light energy and convert it into neuronal impulses that are sent to other brain structures through the optic nerve. The retina and the optic nerve are actually part of the brain.
The optic nerve leaves the eye at an area called the optic disk, which is also where blood vessels enter the eye. Next to the optic disk there is a small dark spot called the fovea, which is surrounded by a ring of pigmented tissue called the macula, a region with a relative absence of large blood vessels. The fovea and the macula are the regions of the retina responsible for central vision—they form the center of the visual field, the zone with the sharpest vision. The macular pigments, lutein and zeaxanthin, absorb excess blue and ultraviolet light that can be damaging to the eye [3–5].
“The eyes are designed for capturing light energy within the visible spectrum and transforming its information into neural signals.”
The Structure of the Retina
The retina is a sensory neuronal tissue. The retina has several types of neurons organized into layers and supported by an epithelial cell layer called retinal pigment epithelium [3–5].
The neurons that convert the energy of photons into electrical signals are called photoreceptors. There are three main types of photoreceptors: rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). Rods and cones are visual photoreceptors, whereas ipRGCs function as sensors of variations in environmental light and have primary roles in the pupillary light reflex and in the regulation of circadian rhythms. (Whenever we refer to photoreceptors, we’ll be talking specifically about rods and cones.)
Rods are the most numerous photoreceptors, outnumbering cones by about 20 to 1 (around 92 million rods to 5 million cones). Rods are specialized for the low light levels we find from dusk to dawn and are thus used for night vision, creating images in black and white (as well as shades of gray). Cones are specialized for higher levels of light we find from sunrise to sunset and are responsible for color vision and for high-acuity vision (i.e., sharp vision).
Photoreceptors are located at the base of the retina (i.e., the outermost layer, facing away from the lens), meaning that light arriving at the retina must cross several layers of neurons before reaching the photoreceptors. This organization places photoreceptors next to the retinal pigment epithelium (RPE), whose cells have no photosensitivity or neurotransmission activity, but have a key role in vision by supporting photoreceptor function and the structural integrity of the retina. RPE cells form and maintain a blood-retinal barrier between the vascular layer of the eye (the choroid) and retinal cells. The RPE provides metabolic support to photoreceptors by transporting nutrients and water. The RPE, which contains melanin, also absorbs any light that passes through photoreceptors and provides antioxidant protection. Furthermore, RPE cells are also involved in the regeneration of visual pigments in a process known as the visual cycle.
Above the photoreceptor layer, the retina has several other layers containing other retinal neurons: bipolar cells, ganglion cells, and, on top, the axons of ganglion cells that will join to form the optic nerve. These three types of neurons form the pathway for visual information to exit the eye: from photoreceptors to bipolar cells to ganglion cells, whose axons form the optic nerve. In addition, there are two other neuronal cell types in the retina that influence retinal processing: horizontal cells and amacrine cells.
Only the retinal region containing the fovea and the macula has a different structure: it is thinner and organized in such a way that light reaches photoreceptors directly, without crossing several layers of cells. The fovea has only cones at a very high density. It is on the fovea that the lens focuses the light of an object to bring its image into sharp focus [3–5].
“The retina is the sensory tissue of the visual system.”
How Do We See?
Let’s go over the general steps of vision, from the capturing of light to the generation of a mental image [3–5].
1. Light enters the eye and creates an image on the retina
The visual process starts with the entry of light into the eye, where it can be captured by the retina. You’ll see that there is a lot in common between the eye and a camera: 1) both capture light through an adjustable opening whose size controls the amount of light (the pupil and the camera diaphragm); 2) and both create an image using a lens to focus light on a light-sensitive surface (the retina and the camera sensor).
Because the brightness of objects and of our environment varies widely, the amount of light that enters the eye needs to be constantly adjusted, so we can see both in bright and in low light environments. This is controlled by the size of the pupil, which, much like the camera diaphragm, widens or narrows down to adjust light entry. In bright sunlight, the pupils narrow to about 1.5 mm in diameter; in darkness, the pupils may dilate to as much as 8 mm.
When light rays pass from air into a medium of different density, such as the eye, they bend (the technical term is refraction). Light entering the eye is refracted twice: first when it passes through the cornea, and again when it passes through the lens. But whereas the cornea is a fixed structure, the lens is capable of changing its shape to focus light. The lens, which is a convex structure, bends parallel rays inwards, making them converge in a point called the focal point, and making that focal point fall in the retina. This is extremely important because light must fall precisely on the retina for the object to be seen in focus.
Objects that are far away are in focus when the lens is flatter. To bring closer objects into focus, the lens becomes more rounded to increase the angle of refraction and make the focal point fall on the retina. This process of adjusting the shape of the lens to keep objects in focus is known as accommodation and is controlled by the ciliary muscle.
The process of light bending causes light rays to cross over each other, turning the image falling on the retina upside down. It’s only later, through visual processing in the brain, that the image is mentally turned back to the original orientation.
2. Photoreceptors transduce light energy into neuronal impulses
We can only actually see if the information about our surroundings conveyed by light and captured by our eyes reaches the brain—if we can perceive it. To do so, light energy must be converted, or transduced, into a neuronal signal. Phototransduction is the process by which light energy is converted into neuronal electrical signals; it takes place in the retina and is carried out by the two types of visual photoreceptors: rods and cones.
Every photoreceptor has four regions: the outer segment, the inner segment, the cell body, and the synaptic terminal. The outer segment contains a stack of disks on whose membrane light-sensitive photopigments are found. Rods have a much larger number of disks, which makes them over 1000 times more sensitive to light than cones; that’s why they allow us to see in the dark.
The light-sensitive visual pigments of rods and cones function as the photoreceptor’s transducers by converting light energy (i.e., photons) into a change in membrane potential. All rods have one type of visual pigment called rhodopsin, made up of a protein called opsin and of retinal, a vitamin A derivative, which is the light-absorbing portion of the pigment. In the absence of light, retinal is bound to opsin and there is a continuous release of the neurotransmitter glutamate. When light activates rhodopsin, retinal is released from the pigment and a signaling cascade is initiated that increases membrane polarization, resulting in a decrease of glutamate release.
The process of phototransduction in cones is virtually the same as in rods. The major difference is in the type of opsins of the visual pigments. All cones contain the visual pigment photopsin, closely related to rhodopsin, but there are three variations in photopsin’s conformation that change its sensitivity to light. Consequently, there are three types of cones: S-cones, M-cones and L-cones, for short, medium, and long wavelength. All three types of cones are stimulated by a range of light wavelengths, but each is most sensitive to a particular wavelength that corresponds to a color: blue (S-cones), green (M-cones), and red (L-cones). Red, green, and blue are the three primary colors that, when combined, make all other colors of visible light (as in the RGB color model). This is what allows us to see in color.
Colored light is a combination of wavelengths that partially activates all three types of cones. The color we perceive depends on the relative contributions of each type of cone to the retinal signal, which in turn depends on the colors of the light that stimulated them. So, cones send to the brain a combination of signals that reproduce the RGB combinations we look at. The brain interprets those signals and recreates the colors we’re looking at.
The change in membrane potential created by light in photoreceptors creates a neurochemical signal that is passed, within the retina, to bipolar neurons and then to ganglion cells. Multiple photoreceptors converge on a single bipolar neuron and multiple bipolar neurons in turn converge on a single retinal ganglion cell. At each synaptic relay, neuronal responses are modulated horizontal cells and amacrine cells.
This means that the retina does not simply transmit passive input about the visual properties of what we look at, it also actively extracts and condenses information. There are roughly 100 million photoreceptors in the retina, but their information condenses down to about 1 million axons of ganglion cells that join to form the optic nerve.
Ganglion cells are the output cells of the retina and they are the only retinal neurons that can fire an action potential (the type of neuronal activity that allows information to be carried long distance through an axon). The visual information acquired by photoreceptors and codified in the retina will travel along the optic nerves to the brain for further processing.
3. The brain translates neuronal signals into mental images
The optic nerves that exit each eye first meet at the optic chiasm, which lies at the base of the brain. At the optic chiasm, a fraction of the retinal axons from each eye cross over to the other side. This selective crossing-over groups the information about each half of the visual field (i.e., each half relative to the midline of our visual field—left and right visual fields).
So, neurons from the section of the retina in each eye that sees the left side of the visual field join on one side, and neurons from the section of the retina in each eye that sees the right side of the visual field join on the other side. The information from each side is then carried onwards by two optic tracts. The visual information about the left visual field is sent to the right cerebral hemisphere and the visual information about the right visual field is sent to the left cerebral hemisphere, which means that the left visual field is “viewed” by the right hemisphere and the right visual hemifield is “viewed” by the left hemisphere.
Visual information is sent by the optic tract to several brain structures with different functions in visual processing. Most targets of retinal input are involved in conscious visual processing, but some are involved in other functions associated with vision, such as the pupillary light reflex (the midbrain in the brainstem) or regulating circadian rhythms (the hypothalamus), for example.
In the pathway of conscious visual processing, the first synaptic relay is located in a region of the thalamus called the lateral geniculate nucleus (LGN). At the LGN, input from retinal ganglion cells is segregated into different types of visual information, which means that different visual properties are processed in parallel by different LGN neurons. Visual information at the LGN is also integrated with inputs coming from the cerebral cortex associated with alertness or attention, for example. So, in the LGN, incoming visual information is already being influenced by what we’re doing or thinking.
From the LGN, visual information is sent to the primary visual cortex (PVC). The visual pathway is organized in such a way that neighboring cells in the retina send information to neighboring places in the LGN and the PVC. The result is that the retina is mapped on these structures. Visual information reaches the PVC as monocular—each neuron brings input from one side. Processing within the PVC merges corresponding inputs from the two eyes, i.e, input from the same point in the visual field, into “binocular neurons”. In the PVC, visual information about orientation, direction, analysis of color, form, contract, etc is processed.
The PVC is only the first site of cortical processing of visual information. Information is then sent to many other areas throughout the cerebral cortex for different types of specialized analysis, interpretation, and memorization.
With the concerted action of many neurons spread throughout the cortex, complex analysis and processing of visual information can take place and more intricate and specialized visual representations can be developed. At an intermediate level, the brain analyzes more elaborate properties of a scene, such as the layout, surfaces, contours, and the distinction between foreground and background. At the highest level, the most elaborate analysis takes place, including an integrated perception of the visual world, motion perception, the visual control of action, object recognition, and facial recognition.
Then, once a scene has been dissected and objects recognized, the brain matches what we’re seeing with memories, associated meanings, and concepts. This high-order processing is distributed throughout the cerebral cortex.
Visual Perception Is a Constructive Process
Perception is not a passive intake of stimuli from the outside world, it is an active and creative brain process that involves the integration of many types of information.
We’ve been comparing vision to the operation of a camera, but although there are many aspects of the functioning of the eye that are quite similar to a camera, the visual system is actually way more complex than that. Unlike a camera, the brain is able to create clear distinctions between many visual components of the world, such as separating figure from background, or discriminating contrast, orientation, position in space, depth, shape, and color, for example. It also perceives movement and uses vision to guide body movement, particularly hand movement. Furthermore, because we have two eyes, we may have two slightly different visual images that the brain is able to seamlessly merge.
The result of all this is that the many features of vision come together in a unified perception. But this can only be achieved through the collaboration of multiple areas and pathways in the brain. Vision is a very complex process, but we hope we’ve been able to paint the general picture .
To end, a fun fact: because the optic disk—the area at which the optic nerve leaves the eye—is made of axons, it has no photoreceptors, which means that images that are projected onto this region of the retina cannot be seen. This creates a blind spot in each eye but we don’t notice them in our field of vision because the brain fills in the gap. You can easily find your blind spot, just follow the instructions on the legend for the figure below.
Figure 7 - How to find your blind spot: Look at the image. Close your right eye. With your left eye, look at the plus sign. Slowly move your head back and forth while looking at the plus sign until the dot disappears from sight. When it does, you’ve found your left eye’s blind spot. Reverse the process to find your right eye’s blind spot: Close your left eye and look at the dot with your right eye. Move your head while looking at the dot until the plus sign disappears from sight.
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