Cat Vision: Can Cats See In The Dark?
Yes, cats can see in the dark. However, it is important to point out that when we talk about “seeing in the dark”, we do not mean total darkness. If you are referring to “dark” as the total absence of light, then no cat or any other animal will be able to see in the dark. Hence, it is important to make that distinction.
How the Eyes “See”
The sense of sight is the result of the interplay between the eyes and the brain, as well as the different structures between these two organs. In very simple terms, the images that we see or that our feline pets “see” are the result of how the brain processes light information coming from the cells in the eyes. Hence, the eyes themselves do not see. They only gather information from the environment in the form of light waves. These light waves stimulate photoreceptor cells in the eyes. They then convert these light waves into electrical impulses that travel to the brain. Upon reaching the brain, the electrical impulses get processed, analyzed, and interpreted as images.
The very first structure that light waves get in contact with is the cornea of the eyes. The cornea is one of three eye structures that are important in the refraction and focusing of light. The other two are the aqueous humor and the lens. As the light travels through the cornea, it gets refracted through the anterior chamber filled with aqueous humor.
From here, light passes through a small opening called the pupil. The pupil is another important structure of the eye. It helps regulate the amount of light that enters through the inner structures of the eye. Smooth muscles from the iris attach to the pupil. When some of these muscles contract, they stimulate the pupil to increase its diameter. In turn, this allows more light coming from the cornea and the aqueous humor to pass through. There are also smooth muscles that do the opposite. When they get stimulated, they cause the pupil to reduce its diameter. This limits the amount of light entering.
It is easy to see why the pupil is so important in maintaining optimum vision. By regulating the diameter of the opening where light passes through, it can ensure clearer images. In dark environments where there is very little light, the pupil will try to open as wide as it can. This is to allow as much light as possible. In very bright environments, the pupil will reduce the diameter of its opening so that only the right amounts of light will pass through.
As light passes through the pupil, it hits the lens. Try to recall your elementary science activity using a magnifying glass to focus sunlight on a piece of paper. The lens provides the same exact function. It gathers light entering the eyes and focuses them straight to the retina at the back of the eye. The lens changes its shape to accommodate the amount of light entering. It needs to do this so that it can focus the light to the retina with greater precision and efficiency.
- Vitreous Humor
Along the way, light passes through the vitreous humor. Many disregard this part of the eye, but it plays a very important role in providing nutrition for the photoreceptor cells in the retina. Without the vitreous humor, the cells can die and they will no longer be able to receive the light signals coming from the front portion of the eye.
The retina contains specialized cells that gather light, known as rods and cones. Rods are special cells that are responsible for seeing in low-light conditions and peripheral vision. They are also more sensitive to brightness levels and shades of gray. On the other hand, cones are special cells that function in color vision as well as daytime vision.
- The Brain
The photoreceptor cells convert the light energy into electrical signals. They send these electrical signals to the brain through the optic nerve. The signals reach the thalamus first where they get integrated with other inputs from other sensory organs. The resulting nerve impulse gets propagated to the occipital lobe, turning the nerve impulses into visual information. The newly-processed image information gets delivered to the parietal lobe of the brain for interpretation.
Things that Make a Cat’s Eyes Better Suited for Low-Light Vision
If you look at the different eye structures, there is one thing that is very specific to the eye’s ability to see in the dark. This is the photoreceptor cell called “rods”. However, because rods are an inherent part of the eye of ANY organism, there should be no difference at all in the night vision capabilities of humans and cats. The truth of the matter is that there are more to a cat’s low-light vision abilities than just its rods. Let us look at some of the things that make a cat’s eyes better suited for night and low-light vision.
More Rods, Fewer Cones
In our discussion above, we know that the retina is an important structure because it contains the special cells that gather and convert light into nerve impulses. When it comes to seeing in the dark or low-light vision, this task is the responsibility of rods. And this is where feline eyes have a definite advantage over humans.
The feline retina contains more rods than cones. What this means is that it has more photoreceptors that can gather low-light information. They are also more proficient when it comes to differentiating images based on the varying shades of gray.
The rods of a cat’s retina are more numerous than that of the human eye. In humans, the ratio of rods to cones is 4:1. In cats, the ratio is at 25 rods to 1 cone. This means cats will only require about a sixth of the amount of light to see the things that the human eye can see. For example, if there is a candle in the middle of a darkened room emitting light at 1 lumen, cats will only need one-sixth of this light to see it.
There is another advantage to having more rods than cones. In low-light environments, it will be quite difficult to distinguish colors. What the human eye can see will be grayed-out hues of colors. Since the cat has more rods, it can differentiate its prey from the surroundings by deciphering the different shades of gray. Without the rods, it would be almost impossible to do this.
Because the cat’s ability to see in the dark hinges on the availability of light waves, it would be impossible for it to see in total darkness. Even if we are to fill the retina with rods, if there is no light entering the eyes and hitting the retina, then there will never be electrical impulses generated. In other words, total darkness means the absence of light. And where there is no light, then there is no light information for the rods to gather and convert into neural messages.
Hence, a cat can see in the dark for as long as there is a very faint light source. But in the absence of light waves, then it will be as blind as any other organism without eyes.
Greater Peripheral Vision
We mentioned that the retina is located at the back of the eye. What we failed to mention is that there are regions within the retina where one can find either rods or cones in greater concentration. Rods are often located in the peripheries of the retina. This allows them to provide a wider field of view or greater peripheral vision.
There are about 150,000 rods per square millimeter (mm2) in the peripheries of the human retina. By contrast, the feline retina contains about 250,000 rods per mm2 near the area centralis. This is in addition to the 463,000 rods per mm2 in the peripheries. The greater concentration of rods in the peripheries of the retina accounts for the wide field of vision of cats.
Each feline eye can provide a field of vision of between 155 and 208 degrees. When these two visual fields overlap, they create a field of vision that is slightly better than that of humans. On the average, we can only see about 180 degrees in front of us. A cat can see as much as 200 degrees from side to side.
While it does not have a direct impact on the cat’s ability to see in the dark, having greater peripheral vision can help the cat hunt for its prey. Wild cats have to scan their environments not only for the presence of prey, but also larger predators. They use their peripheral vision to help them gain a better understanding of both their prey and predators.
The Mysterious Tapetum Lucidum
While the cat’s eye has almost the same structure as that in humans, it contains something very unique. This is the “tapetum lucidum”, a membrane that Mother Nature herself created to help the animal survive in the wild. The location of the tapetum lucidum suggests that its main task is to reflect light back to the retina where the photoreceptors are.
Going back to our physiology of the sense of vision, light enters the front of the eyes and gets focused onto the retina by the lens. Unfortunately, some of the light can disperse in the vitreous humor located between the lens and the retina. Sometimes, minute changes in the shape of the lens can allow some of the light waves to escape towards the back of the retina. There are also instances when light waves can pass through the gaps between photoreceptor cells.
This “stray” light will be wasted if not for the presence of the tapetum lucidum. It serves as a safety net for the retina. It catches all escaping light waves from the structures in front of it and redirects the light back to the retina. In a way, the tapetum lucidum amplifies the light-absorbing abilities of the photoreceptors. This is crucial to the cat’s ability to hunt in the night. It depends on its tapetum lucidum to reflect all escaped light back into the retina so that it will be able to see in the dark.
Identifying the tapetum lucidum in cats is quite easy. It is that characteristic yellow-green to turquoise-blue eyeshine that we call the ‘cat’s eye’.
If you look at your pupils, you will see that they have a circular shape. But if you check the pupils of your cat, they follow a more elongated, elliptical form in a vertical orientation. This helps explain why the cat can see a lot better in the dark than humans. The vertical orientation and elongated shape of the feline pupil allow it to open a lot wider than the human pupil.
The question people have is how wide can a cat’s pupil go? Let us put it this way. The human pupil can dilate up to 15 times its closed diameter. By comparison, a cat’s pupil can dilate to as much as 300 times its closed size.
Because the pupil is able to open wider, it can allow more light to pass through. This helps ensure greater amount of light reaching the lens and getting focused onto the retina. It does not matter if the light is imperceptible to the human eye. The cat can see it because its pupils are able to increase their diameter by at least 130 times.
Again, nature intended the cat to have this kind of pupil so it can hunt prey in the dark. It also uses it to sense other animals that may be the cat’s predator.
Cats can see in the dark but not in total darkness. The unique shape of their pupils and the addition of a failsafe Tapetum Lucidum help to improve their night vision capabilities. Having more rods than cones is also an important feature in feline night vision.