For humans, seeing in the dark is a major challenge, yet other animals live their entire lives in darkness and seemingly have no trouble negotiating the world around them. So why is it that humans struggle in low light while many animals thrive? Exactly why is darkness a challenge for vision and how can these challenges be overcome?
In essence vision is all about the eye capturing photons, the fundamental particles of light, to form a picture of the world. Light-sensitive photoreceptor cells in the retina of the eye absorb photons, which trigger a neuronal response that, through various neurological processes, is interpreted by the brain as vision. However, photoreceptors are not perfect; they are inherently noisy due to spontaneous activation of the photopigment in the cells. This activation produces a signal that is indistinguishable from the signal that the photoreceptors produce when activated by photons.
Fortunately, light sources (such as the sun, or light reflected from objects) are made up of thousands or millions of photons, so the signal produced in the retina from actual photons being absorbed by the photoreceptors is far greater than the noisy signal from these spontaneous firing events. Difficulties start to arise when sunlight is in short supply, such as at night, underground or under water where sunlight diminishes by a factor of 10 for every 70m of depth. In these cases, the signal produced by the photoreceptors are not much above the level of the noise. The proportion of the true signal to the noisy signal, the ‘signal-to-noise’ ratio, is one of the limiting factors for how well we can see in the dark.
The other limit is having photoreceptors that are sufficiently sensitive to low photon numbers (dim light) such that the eye can achieve its full resolution, as shown in figure 1. Each of the four panels represent the same image, a dark circle on a lighter background, but at different light levels. Each circle represents a photoreceptor, where filled circles are photoreceptors that have ‘fired’; either through absorbing actual photons, or from spontaneous firing. In panel A, the lowest light level, there are few photons, so most of the photoreceptors that are firing are doing so spontaneously. It is very difficult, therefore to resolve the dark circle. As the light level increases in panels B and C, the signal from photon absorption becomes greater, until in panel D, the image of the dark circle is clear and unambiguous.
Figure 1: The effect of low light on the resolution of the eye. If there is not enough light (A-B), the photoreceptors (circles) cannot accurately detect (filled circles) an image of a dark region on a light background. As light levels increase, more of the photoreceptors are activated, allowing the eye to resolve the image fully. Adapted from MH Pirenne, Vision and the Eye, Chapman & Hall, 1967.
This simple example demonstrates that, while the eye has, in theory, sufficient resolution to distinguish the circle from the background, if there is insufficient light or the photoreceptors are insufficiently sensitive to low light levels, then the actual resolution of the eye is far below what it should be. There are two remedies for this resolution ‘underperformance’: increase the light level or increase the sensitivity of the photoreceptors. The eyes of nocturnal animals and those whose ecological niche involves the darker corners of the world, such as caves and the deep ocean, have evolved precisely to do this.
Pupil dilation
Clearly there are a limited number of ways an animal can increase the overall light levels in an environment. It is not as if they can simply flick a light switch or use a torch. However, they can increase the amount of light that enters the eye and that reaches their photoreceptors, which has the same effect. They can do this by increasing the size of the pupil, the aperture that lets light into the eye.
By increasing our pupil diameter from 2mm in daylight to 8mm at night, humans can increase the amount of light entering the eye by a factor of 16. But there is an obvious limitation to this strategy; a pupil can only expand so far before it is limited by the size of the eye. It is well known that nocturnal animals tend to have very large eyes. Additionally, their pupils tend to expand to a greater extent within the eye than diurnal animals, maximising the amount of light entering the eye.
However, there are drawbacks to having large pupils and large eyes. Firstly, the optics of a wide pupil can cause image degrading effects such as spherical or chromatic aberration, where light of different wavelengths (colours) is refracted (bent) through the lens by a different amount, causing a blurred image. As well as being energetically costly, having large eyes can also cause a reduction in image quality. If the photoreceptors in an eye were to scale up in direct proportion with the eye diameter, they would have a reduced acceptance angle due to the eye’s geometry. The acceptance angle refers to the cone of space from which a particular photoreceptor collects light. The smaller this cone, the less light it collects, which is clearly a disadvantage for a nocturnal animal since eyes operating at low light levels need to collect as much light as possible. So, as the eye diameter increases, the width of the photoreceptors must scale correspondingly wider.
The second way an animal can improve its low light vision is by increasing the sensitivity of the photoreceptors. Sensitivity can be improved by increasing the efficiency of a photoreceptor, that is the proportion of light it absorbs. Typically, a photoreceptor’s efficiency is proportional to its length. Therefore, a nocturnal animal could extend the length of its photoreceptors as well as increasing the width. Our rod cells, the photoreceptors responsible for our night vision, are slightly longer and wider than our cones. Probably the most extreme example of photoreceptor lengthening can be seen in the owl, which has tubular rather than spherical eyes. As a consequence of this elongated shape, they cannot move their eyes in their sockets, so have evolved an unusually mobile neck in order to view the whole world around them.
Another way of increasing the efficiency of a photoreceptor is to redirect the light that is not absorbed by the photon back to the photoreceptors using a highly reflective layer at the back of the eye called the tapetum. This is the structure responsible for the unsettling ‘glowing’ eyes of cats, racoons (figure 3) and many other animals. Incredibly, the optics of the tapetum act just to increase light capture, and do not cause any degrading effects. It reflects the light that has already passed, unabsorbed, through a photoreceptor directly back onto it in the same (but antiparallel) direction as the original direction as the incident beam of light. Therefore, the light reflected from the tapetum is indistinguishable from the light entering the eye, resulting in ‘normal’ vision.
Figure 3: The orange glow from the eyes of a raccoon caused by the light of the camera flash reflecting from the tapetum at the back of the animal’s eye.
Neurological adaptation
Alternatively, animals can avoid hypersensitivity by using neurological, rather than physical adaptations for seeing in the dark. Some animals pool the outputs of a group of small photoreceptors at the level of the ganglion cells. In other words, they make a single effective photoreceptor with an acceptance angle equal to the sum of the acceptance angles of all the photoreceptors in the group. Larger acceptance angles mean more light reaches the photoreceptors, but it does reduce the resolution of the eye, which reduces the image quality. But this grouping is dynamic, so the animal can have the best of both. In daylight conditions there is no pooling and the output of each photoreceptor is processed singly, resulting in full resolution vision. As night falls and light levels decrease, the number of photoreceptors in the ‘pool’ can gradually increase, as it is more important for the animal to be able to see something with poor resolution rather than seeing nothing at all in high resolution. This strategy is particularly useful for animals whose life histories involve them being active during both day and night, such as the locust. As well as this spatial summation, animals can make use of temporal summation by extending the integration time of the photoreceptor. In effect, this is the same as increasing the shutter time on a camera.
The longer the integration (shutter) time, the more light that is absorbed, so the better the signal. However, this strategy can lead to motion blur. Just as using a long shutter time to capture a moving scene can lead to a blurred image, a slow photoreceptor response can correspondingly lead to blurred vision.
Colour vision
So far, this discussion has centred on the adaptations that allow animals to see anything at all at night, but a few animals are able to see colours in the dark. Why is colour vision at night exceptional? Colour vision is based on a comparison between the signals produced by different types of cones (in vertebrates) that have different spectral sensitivities. Such signal comparison can be problematic when there is not much light to produce a signal in the first place. For this system to be reliable, the signal from the cones has to be large enough to be ‘heard’ above the background photoreceptor noise. At low light levels, most animals combine, or pool, the outputs of their photoreceptors as discussed above, to increase the signal-to-noise ratio. Since the signals are combined, there is little point in having photoreceptors with different spectral sensitivities. Taking ourselves as an example, we use our rods for night vision, which only have one photopigment, making our vision monochromatic. At night we are truly colourblind, seeing the world in shades of grey.
But not all animals are so limited, there are few who can see colour, even on the darkest of nights, like the helmeted gecko. The ancestors of the helmeted gecko were strictly diurnal; active only during the day-time when light is abundant. As a result, evolution did away with their rod receptors; after-all since they were not out at night, they did not need them. However, fast-forward a few million years (but not to present day) and, due to environmental changes, helmeted geckos found a new ecological niche for themselves as nocturnal agents. But they faced a problem: how to see at night without rods? Their solutions: make their eyes bigger, their pupils wider and their cones like rods, which comes with the added benefit of night-time colour vision. However, while we know that geckos can see colours at night, how well they can see them is still unclear. For most animals, the priority is to be able to see clearly with the maximum resolution, and colour vision, with its comparative system, does not lend itself well to this, which is probably why nocturnal colour vision is so rare.
In summary, seeing in the dark is tricky due to photons being few and far between. To overcome this challenge, animals make their eyes as large as is optically and physically feasible, and their photoreceptors as efficient as possible. Some use optical tricks to improve light capture, such as adding a reflective tapetum to the back of the eye, while others use neurological tricks to boost the signal from their photoreceptors. A few animals can even see the dark world in colour, though most animals, ourselves included, settle for being able to see anything at all.
Dr Ilse Daly is a research associate at the School of Biological Sciences at the University of Bristol.
