One of the most important abilities developed by living organisms on Earth is adaptation to the light that comes from the nearest star in our galaxy – the Sun. This ability is called light perception. Interestingly, it defines not only the perception of a source of light, but the perception of all surrounding reality! It determines colors, shapes, orientation in space and in time.
Although our protective atmosphere shields the most energetic ranges of sunlight, we still have limited contact with the rest of the universe thanks to the fraction of light that reach us from space. The atmosphere is transparent to low-energy ultraviolet light, visible light, some radio waves and limited amounts of infrared radiation. Such low-energy spectrum still can be used in conversion of sunlight. Organisms, which developed photosynthesis, like bacteria, algae and plants, use light to produce organic compounds and molecular oxygen (O2).
Sunlight significantly contributed in development of existing life on our planet. Long time ago, all oxygen produced in photosynthesis, which was not combined with carbon to recreate CO2, accumulated in the atmosphere. After the impact of harsh cosmic radiation in the upper atmosphere, oxygen splitted for reactive single atoms, which bound to the oxygen molecules and created ozone (O3). An ozone layer appeared about 600 million years ago cutting off biologically lethal UV radiation (from 200 to 300nm). The presence of this protective ozone shield enabled organisms to evolve and allowed life to exist on Earth.
How does the light perception evolved?
We could account for three different directions:
Visual systems evolved to perceive and process diverse messages from the environment such as shape, color and motion. Vision is considered to be the most complex and the most developed sense. The basic role of the visual system is to perceive light stimuli by light-sensitive receptor proteins – rhodopsins – and transduce stimuli into neuronal action potentials. Electrochemical signals in photoreceptor cells are amplified, filtered and sorted via several interneuronal layers and get transmitted to higher visual centers in the brain, where complex signal processing occurs. The most primitive ever discovered form of eye is called stigma, which is a photoreceptive organellum inside unicellular green algae, Euglena. In flies, the proportion of the visual system to the rest of the body changes dramatically. The insect visual system occupies nearly 2/3rds of the brain’s volume. The most advanced and complex eyes in the animal kingdom belong to… a mantis shrimp! They contain 16 different photoreceptor pigments, 12 for colour sensitivity, others for colour filtering (for comparison, humans have only four visual pigments, three to see color, and one for the circadian clock setting). The mantis shrimp can perceive both polarised light and multispectral images. Additionally, each eye see objects with three parts of the same eye (so called trinocular vision), and depth perception.
In humans, the photoreceptive layer called retina consists of 6 to 7 million cones, which can be divided into “red” (64%), “green” (32%), and “blue” cones (2%) based on measured response curves. They provide the eye’s color sensitivity. One hundred million rods handle vision in low light, and cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The activated rhodopsin is formed, which is what creates electrical impulses in the optic nerve. Interestingly, we all see differently depending on how many rods and types of cones we have in our retina. Some people can observe differences in temperature and perceive different hue of colors (1).
Biological clock entrainment
The visual system continuously adjusts to exogenic light stimuli and the endogenous signals coming from the biological clock, but also to feedbacks due to behavioral situations and homeostatic (2) control. Most organisms on Earth display rhythmic activity patterns: they are active during the day and sleep at night or vice versa. Such activity rhythms are also visible at the cellular and molecular level, for example neurons swallow during the day, and shrink at night. Behavioral, metabolic and cellular processes are synchronized with daily environmental changes due to the robust modulation of the internal biological clock located in the brain. Clock cells contain the cell-autonomous photoreceptor cryptochrome (in most but not all clock cells), and several clock proteins expressed by clock genes (for example period, timeless, clock, cycle, doubletime). Cyclic expression of the clock genes induced by light generate oscillations that drive output pathways and generate rhythmic behavior such as locomotor activity, feeding, mating and so on. Desynchronisation of the biological clock may induce sleep disorders, jet lag or health problems.
Thermoreceptors are probably less understood elements of the light perception. All we know is that they are activated by changes in temperature and they are located in different areas of the body such as skin, cornea, urinary bladder and inside the brain in hypothalamic regions. This part of brain is involved in thermoregulation, a process, where thermoreceptors allow feed-forward responses to a predicted change in body temperature in response to changing environmental conditions.
Recent findings in optogenetics (neuromodulation technique), revealed deeper meaning of light for living organisms, especially in the light perceiving context. Light seems to be crucial factor for sustaining healthy condition of living systems. Such issues like light involvement in reprogramming DNA code by changing gene expression in cells, resetting memories from brain or light communication, still are waiting for profound investigation and understanding. On the other hand, lack of light also affects living organisms. In humans for example, we observe inhibited synthesis of vitamin D or seasonal depression. From the last decade of the 20th century until now we experience evolutionary changes in technology. Luminotherapy and laser techniques started to be commonly used in medicine. Definitely here comes the time for light and its enlightenment!
- There are many tests available on the internet, which can check your vision for example tests for color vision: www.color.method.ac, www.xrite.com/online-color-test-challenge; tests for far and near vision, myopia and astigmatism: www.essilor.com/en/EyeHealth/LensesForYourVision/TestyourEyes/Pages/home.aspx.
- Characterized by a state of equilibrium, as in an organism or cell, maintained by self-regulating processes.
Agata Kołodziejczyk is a neuroscientist at the Jagiellonian University in Kraków, Poland. Her passion is to decode neuronal networks, especially related with light perception, circadian clock and serotonin signaling. During PhD studies in Sweden she discovered ‘internal eyelid’ of insect eye and searched for neurotransmitters and neuropeptides involved in circadian communication. Now she continues neurobiological studies directed toward evolution of the nervous system and neuronal communication. She designed and organized several interdisciplinary international events in Poland related with space exploration. As an active member of Astronomia Nova Association and forScience Foundation, her aim is to network people to increase development of space science and space industry in the Middle and Eastern Europe.