The human eye is a wonder. It transducers light into patterns of neural activity.Then we use those neural signals to help us move around and understand our environment.
From light to retina
The sun radiates a photon (a packet of information), and 8 minutes later it reaches you. The first thing it hits is your eye, which suspended, supported and controlled by 6 muscles. There is one muscle each for up, down, left and right. The other two muscles twist and untwisted it at an angle.
Some radiation will fall on the sclera, the white of the eye. Sclera is Greek for hard, a good description of this 1 mm thick hollow ball. It is composed of fibrous strands running in parallel,, like strapping tape, and must 2x the pressure of the atmosphere.
Some radiation will land on the cornea, which bulges out from the sclera. The cornea is smooth, finely textured and very sensitive to touch. It is transparent and has no blood vessels it. It is nourished on the front by tears, and by the aqueous humor on the inside.
The cornea provides the primary focus of the eye. It is a fixed focus structure which bends the light and focuses it toward the center of the eye. The cornea very successfully narrows the light beam. It provides 2/3 of the eye’s focus.The only difficulty it has is inherited irregularities in its shape.
Light continues unabated because the aqueous humor is mostly water. It provides pressure to inflate the eye, is continuously refreshed, and contains an antioxidant to protect the eye from UV rays. The flow from top to bottom brings in oxygen and nourishment, and removes waste from back of lens. The sponge-like ciliary body starts the flow at the top which is balanced by the removal at the bottom through the Schlemm’s canal.
Light passes through the cornea and the aqueous humor, which are both transparent. The refracted light covers the eye but is filtered by the iris, eliminating extra light, and letting the most central rays pass through the pupil of the iris.
The iris has two layers. The outer layer is the colored part of the eye. The color blocks extraneous rays from going further into the eye. Albinos have an iris that is translucent, allowing unwanted light to hit the retina, and cause\ing a haze that lowers acuity. In everyone else, the color is the result of pigments which are light absorbing. The inner layer of the iris, behind the color, contains blood vessels
The pupil is the hole in the iris. It consists of two sets of muscles. One set pulls outwardly, opening the pupil to allow more light through. These radial muscles only pull outward or re lax. They\ don’t close the pupil. Open pupils give a shallower depth of field but allow more light in when lighting is dim. As you age, thes muscles get weak, which is why elderly people have trouble seeing in low light conditions.
The other set of muscles, the circular muscles, constricts the pupil, allowing less light through. Constricted pupils give a better depth of field. Tight pupils given sharp distance images in bright sunlight. As you age, these muscles lose some strength but not much. A 20-year old and an 80-year old don’t vary much in their ability to see in high light conditions.
The pupil can vary in size, up to a 4:1 ratio, allowing a 16:1 variation in light.
The narrow rays of light which go through the pupil are then focused by the lens, which is suspended by string-like ligaments called zonules. The lens looks like a bean or large aspirin, and is composed of three parts.
First, there is the lens capsule. It is a clear elastic membrane which surrounds the lens. It provides flexible stability. Second, there is the lens epithelial layer, which creates new lens fibers, and controls the aqueous humor. Third, the lens fibers. These are clear, long, thin fibers which are mostly protein.
The lens never stops growing. It adds fibers to the edge, so it becomes somewhat less bean shaped. The fibers you are born with are still in the center, so your sharpest vision is from your oldest fibers. These protein fibers are so clear they are sometimes called crystalline.
The lens refines the focus of light to go through the vitreous humor and hit the fovea of the macula. The vitreous humor is the region between the lens and the retina. It is the largest part of the eye. It is clear but has the texture of egg whites or jelly.
At the back of the vitreous humor there is a membrane to keep the jelly from covering the retina. This inner lining membrane (ILM) is composed of glial cells, in particularly Müller cells. The feet of these cells form a barrier which acts like a fiber optic array, gathering and filtering light before it reaches the retina.
Before reaching the photoreceptors, light must get past the central circulatory system (also called the retinal circulatory system) and several layers of support and processing cells. These structures are off to the side as much as possible but they do complicate things.
The eye is wired backwards, in the sense that photoreceptors should be first, followed by nerves and blood vessels. In the human eyes all the clutter comes first.
But the wiring plan in less silly when you realize how much blood the eye needs. There is a blood supply system just for the photoreceptors (the choroid). It is behind the receptors. It could not supply the support cells too. So it makes sense to halve a blood supply just for the support cells.
As to why the support cells aren’t behind the choroid, no one knows.I
Some think it is the Müller cells. We get too much blue light. We compensate for that by having less blue-sensitive receptors (S cones), and by passing light through the feet of Müller cells to enhance the greens and reds.
Regardless, we have arrived at the retina.
The retina, rete or ret (Latin for fisherman’s net) is a web of interconnected neural and glial cells. It is only the thickness of a postage stamp but has highest metabolic rate of any structure in the body. We think of the retina as being covered with light-sensitive receptors:, which is true, but that’s not the whole story.
The retina has 5 major layers of the retina (3 layers of nerve cells; 2 layers of connections), and another 5-6 other layers. The ganglion layer is is the region to be hit by light. You don’t notice these neurons carrying signals to the brain becomes they don’t move, so the brain simply ignores them.
Ganglion cells are the only nerve cells in eye with axons. They are unmyelinated until they reach the optic disk, where they form the optic nerve. There are 10-15 types of retinal ganglion cells, most coded to respond to a particular movement 0r stimulus pattern. Coding is important because input from 120 million rods and 6 million cones must be transmitted to the brain using only 1.2 million ganglion neurons.
Behind the ganglion cells are 30+ types of amocrine cells, which make lots of interconnections with other retinol cells.
On the same level as the amocrine cells, plus or minus, are the bipolar cells. They are bipolar because they have two arms, holding hands with a photoreceptor and an amocrine cell at the same time. Or a photoreceptor and a ganglion cell.
The last category of cells we will address is the horizontal cell. They have short dendrites and long horizontal processes. Horizontal cells inhibit neighboring cells from firing . When a photoreceptor fires, the non-firing of adjacent receptors makes edge detection easier. It is a clear cut distinction. It is an example of vision not being solely optical. A lot of extra-processing is involved.
Light from the sun finally reaches the photoreceptors. The rods are spread out across the retina. The cones are nestled in a small anti-dome called the macula. This dip is the reverse of the cornea and much smaller. It is a yellowish spot, about ¼ inch wide.
Inside the macula is a region called the fovea. It is about 1/16 of an inch wide, and is where the light rays have been focused. It contains no rods, and some of the cones are direct wired to the occipital lobe. A cone stimulated here shows a corresponding response in the brain.
Rods
Scotopic vision (rods) gives you black & white security footage. It works in low light, and is great for target detection. The 120 million rods are spread out across the eye, and wired together (summed) many to one. When you wake up at night and stumble to the kitchen, you’re using your rods. When you flip on the light and see in color, you’re using cones.
On the outside, rods are narrow and cylindrical in shape. There is the rod portion (closest to the choroid), the cell nucleus, and fibers terminate in a single end-bulb (a rod spherule). The overall structure extends into the outer plexiform layer, where it connects with the dendrites of bipolar cells. Hundreds, and sometimes thousands, of rods are summed together to trigger one bipolar cell.
The inside of the rod portion is filled with 900 free-floating rod ddisks (lamellae). The disks are separated from each other by cytoplasm, and contain visual purple (rhodopsin).
In terms of polarization, rods are different from most neurons. Most nerve cells have a resting potential of about -70mV which depolarises with stimulation to give an interior potential of +40mV.
Rods have an interior potential of -30mV in darkness. When stimulated, rods hyperpolarises in light to -60mV. Stimulating rods is easy. They are 500 times more sensitive than cones, and sensitive to a broader spectrum of light.
Rods sense intensity or shades of gray. They can’t discriminate between colors. Rods are perfect for night vision. They are great for detecting motion. Because of their location on the periphery of the retina, rods see off axis. There is nothing better at detecting potential threats in your field of view than rods.
Cones
Unlike rods, cones are color sensitive. They contain one of three different photopigments, referred to as long (L cones), medium (M cones) and short (S cones). They don’t actually cover three distinct regions of the spectrum of light. L and M cones overlap quite a bit/ S cones, the most sensitive to blue, are not in the fovea, and are less common than the other two types.
The 6 million cones in your eye are tightly packed in the fovea, not spread out like cones. On the outside, cones are shorter, broader, and more tapered than rods. The point of the cone is closest to the choroid. This outer segment resembles disks but is composed of one continuous folded surface. This is where the color sensitive proteins reside.
After the cone segment, the inner segment holds mitochondria and the cell’s nucleus. The fibers of the cone flatten out into a pedicule stalk, where it synapse with horizontal and bipolar cells.
The photopic system, composed primarily of cones, is great for day vision (high light) and target identification. Rods, particularly in low light, detect a target, while cones identify what it is. Rods act as motion detectors; cones take high quality, color photos.
8 differences between rods & cones
1. Light sensitivity
Rods are best for low light situations. They have an absolute threshold 1-10K lower than cones. This range of sensitivity is accomplished by dark and light adaptation. Photopigment “bleaches” (breaks down) in the light. It is regenerates in the dark. So in the dark, rods have more pigment available, making them more sensitive. Cones work best in high light.
2. Spectral sensitivity
In terms of which part of the light spectrum they respond to, rods win on responsiveness, and lose on color. Rods respond to a wide range of stimuli but only represent reality in shades of gray. They don’t indicate color, and don’t respond to deep red.
Cones deliver our sense of color by working together. The mixture of L, M and S cones determines the color we perceive. They differ in how much stimulation is needed for them to respond to a specific wavelength. All three cones can see green but it takes a lot of illumination for S cones to detect it. All can see blue but it takes very little light for S cones to see it.
3. Distribution on retina
Cones are mostly in the fovea.rods are everywhere else.
4. Interconnection
Rods are wired many to one. The scotopic system uses spatial summation to converge the signal. Cones use much less spatial summation. Some of the cones in the fovea are wired one to one to ganglion cells. The photopic system produces spatial discrimination (tell letters apart from each other).
5. Acuity
Cones give high quality, sharp images. You test their acuity with a Snellen eye charts. The test is set for a given distance (20 feet), and various sizes of letters are displayed. The patient is tested to see which is the smallest kine of text they can read. Scotopic acuity is tested by presenting a neutral field and presenting items (usually small dots of light) in different parts of the field and at different intensities. The patient is asked to indicate when they detect a target.
6. Response to movement
Rods are known for their ability to detect motion, and they do so very successfully. Four factors are involved.
First, rods are located on the sides of the eye, pointed to an opposing field of vision. The right sides of each eye have a good view of the left field of vision, even when the eye is facing forward. Cones look in the same direction as the eye. Rods always track what is going on on the edges.
Second, rods give relatively poor quality black and white images, which can be processed quickly. In photographic terms, rods have small file sizes. Cones have large files, and are much slower to process.
Third, rods are wired for spatial summation. Think of it as crowd sourcing. Lots of rods are used to trigger less and less neurons. This convergence means that the small stimulation of many receptors is the same as a large stimulation of one.
Fourth, there are a lot more rods than cones. Chances are that motion will be detected by even a portion of 120 million rods before it detected by a majority of 6 million cones.
7. Critical Flicker Frequency (CFF)
There is a point beyond which we can’t detect flicker in a light source. We can see a candle flicker because it does it so slowly. We can see a video chat with bad Wi-Fi connection flicker at 10 fps. Movies look pretty lifelike at w4 fps. We can’t consciously detect flicker above 60 fps, although some people say their eyes feel better viewing things at 120 fps.
There are several factors impacting CFF, including age, amplitude and wavelength. In general, rods fuse at lower frequencies than cones. It is easier to fool your rods than your cones. This may be because of temporal summation (add up light over time). Cones need more light to function but they have more temporal discrimination.
8. Signaling properties
Rods give transient signals. They fire at onset and offset. They operate like a convenience store threshold that beeps when you enter, and bee-s when you leave. Rods give brief signals, like snapshots. Cones are like video cameras.They provide sustained signals. They.continue to fire as long as a stimulus is present.
Behind the photoreceptors
Pigmented Epithelium
To reduce light scatter and ghostly images, there is a single layer of hexagonal cells behind and between the rods and cones. It keeps light from bouncing back, and nourishes the rods and cones.
Choroid Blood System
choroidal blood vessels = supply rods and cones. Receive about 75% of the retina’s blood flow. Macular degeneration occurs when abnormal blood vessels grow between retina and choroid. These blood vessels can also be damaged as a side effect of diabetes. This condition (diabetic retinopathy) includes leakage of blood and fluid into the eye (capillaries easily burst). The third major cause of blindness (retinitis pigmentosa) is a hereditary disease which causes rods (starting in the periphery) to degenerate. The result is the gradual onset of night blindness, followed by tunnel vision (only cones are working).
Choroid
This is the second and largest blood system of the eye. It accounts for 75-80% of blood flow to the eye, and primary nourishes the rods and cones.
From retina to brain
Rods have a direct but converging rout to the brain. In terns of wiring, the rod spherule connects to a bipolar cell which is spatially summed with amacrine and other bipolar cells. The resultant smaller bundle of bipolar cells connects to ganglion cells which go out of the optic disk (hole at the back of the eye), and become part of the optic nerve.
Cones are a bit different but not much. At the pedicule, a cone connects with a bipolar cell, which connects to a horizontal and a bipolar cell. The bipolar cell either connects directly with a bipolar cell, or is connects to an amacrine cell which connects to a ganglion cell. There is little summation.
Optic Disc
The round hole at the lower back of the sclera lets ganglion cells exit and blood vessels enter the eye. It is usually pink because there are small blood vessels present to nourish the optic nerve. There are no photoreceptors here, so it is a blind spot the brain ignores.
Optic Nerve
The ganglion neurons bundle together and head for the optic disk, where they exit the eye, become myelinated and form the optic nerve. Although there are 120 million rods and t million cones, there are only 1 million optic nerves.
Optic Chiasm
In order to consolidate the field of views, some information from each eye much be transferred to the opposite hemisphere. The goal is to have the eye portions facing left to be processed in the right hemisphere, and the right facing eye portions to be processed in the left hemisphere.
At the optic chasm, the crossover occurs. Fibers from the nasal hemiretinas (half retina) cross to the opposite hemisphere. Fibers from temporal hemiretinas continue to same hemisphere.
In the left eye, the outside (temporal) stream goes to the left hemisphere, but the stream close to the nose cross over to the right hemisphere. In the right eye, the temporal stream continues on to the right hemisphere, and the nasal stream crosses over to the left hemisphere. To remember this arrangement, think of it as crossing your nose.
Located just directly below the hypothalamus, the optic chasm is X shaped. X in Greek is chi (pronounced with a hard K), hence the name.
Two Paths
1. Superior Colliculus
Just below the thalamus and above the spine, there is one superior colliculus on each side of the head. A small portion of the feed from the optic chiasm comes here, and then upside the middle of the brain to the parietal lobe.
‘The parietal lobe is a busy place. It is the projection site for tough, temperature, and spatial location. It provides information from the tectopulvinar pathway to aim the eyes. It receives information from the occipital lobe to help give a 3-D view of the world which remaps every time you shift your gaze.
2. LGN
The Lateral Geniculate Nucleus (LGN) is located in, and is part of, the thalamus. It receives signals from the eye, the cortex, the brain stem, and other parts of the LGN. The information from the eye is crude, and is not processed in the LGN. It does add new information; it only regulates it.. It sends some information to the frontal lobe but receives more than it sends.
The LGN is a small oval with six layers. Layers I and II contain magnocellular cells. These are large cells so they can quickly handle the black and white images from the rods. Contralateral signals from rods go to layer I. Ipsilateral (same side) signals from cones go to layer II. M cells are sometimes called Y cells or Y ganglion cells.
The other four are composed of small parvocellular cells, which handle color images. Ipsilateral Data from cones go to layers III and V. Contralateral signals from cones go to layer IV and VI. P cells are sometimes called X cells or X ganglion cells. They handle reds and greens.
Koniocellular cells are extra small cells who specialize in blue light. These are interleaved between other cells.
The LGN outputs to the occipital lobe. All six layers of the LGN go the V1 of the occipital lobe but they maintain their structure by terminating in different sub-layers.
Color Vision
Want to jump ahead?
- What Is Perception? What
- Perceptual Efficiency
- Vision
- Taste
- Simple
- Tongue
- Throat
- Smell
- Basic
- Nose
- Olfactory bulb
- Flavor
- Touch
- Receptor
- Pressure
- Haptic Perception
- Temperature
- Pain
- Itch
- Hearing
- Ear
- Cochlea
- Pathway
- Temporal Lobe
- Vestibular
- Visceral
- Proprioception
- Time
Photo by Jake Nackos on Unsplash
