Human beings are provided with a variety of senses that help them navigate the world around them, including five basic ones: sight, hearing, touch, smell, and taste. The organs associated with each of the latter report on certain sensations to the brain, which then translates them into understandable information, via a complex, yet fascinating process. Bright colours, a loud thud, an acute pain, a familiar flavour on the tongue or a sweet smell that tickles the nose – all these stimuli are put together into one big picture for us to identify our surroundings. However, while these systems are remarkably sophisticated ones in humans, some animals have super sensors. Felines are well-known for their nocturnal vision, elephants have the most powerful nose of the animal world, while bats rely on sound waves to hunt. Magnetoreception – the ability to detect the Earth’s magnetic field, is even considered a sixth sense that birds, along with certain mammals, reptiles, and fish, are gifted with.
The Physics of the Senses 101 series offers to explore the physical processes that make up each sense, including the so-called sixth sense, and to explain the extent to which they grant certain species 'super-capacities'.
1. Physics of the Senses 101: Visual Perception or Sight
2. Physics of the Senses 101: Auditory Perception or Hearing
3. Physics of the Senses 101: Haptic Perception or Touch
4. Physics of the Senses 101: Olfaction or Smell
5. Physics of the Senses 101: Gustatory Perception or Taste
6. Physics of the Senses 101: Magnetoreception or a Sense without a Receptor
Physics of the Senses 101: Visual Perception or Sight
Although recent research by the University of York demonstrated that there is no universal hierarchy of senses (Majid et al., 2018), vision is often considered as the most precious modality, at least in contemporary Western culture (Hutmacher, 2019). In an investigation on the dominance of vision in research by the University of Regensburg (Germany)'s Department of Psychology, 75% of participants to a survey indeed stated being most scared of losing sight over hearing, or even touch (Hutmacher, 2019, p.2). Bendong (2015) defines visual perception as ‘the ability to interpret the surrounding environment by processing information that is contained in visible light’ (p.1). In charge of this process are the highly specialised organ systems of the eye, while ‘the study for lights has been developed into an important branch of physics – optics’ (Bendong, 2015, p.2). Further, Hutmacher explains that, for debated reasons, 'the processing of visual information seems to dominate the processing of information from other modalities' (Hutmacher, 2019, p.3). This article will therefore start by describing the various physiological components involved in the important sense commonly referred to as sight, then explore the incredible abilities, such as near-nocturnal vision (Long et al., 2010) or even yaw gaze stabilisation (Daly et al., 2018), that cats, barn owls, and mantis shrimps are equipped with.
To start with, the very first element in the visual system is the eye – a teleceptor capable of receiving information from distant objects (Sybesma, 1989). It is a simple optical instrument of about 24 millimetres in diameter in adult humans, covered by a flexible tissue called the sclera, and ‘composed of only two positive lenses, the cornea and the crystalline lens, that project images into the retina’ (Artal, 2016, p.286). The cornea is transparent, allowing light to pass into the eye, and shaped like a meniscus lens, that is a lens with two spherical curved surfaces, convex on one side and concave on the other (Navarro, 2009). Its interior surface – the interface between the air and the eye – holds, by far, the highest refractive power in the human eye of ~ 48 dioptres (Navarro, 2009, p.2), while an ‘aqueous tear film on the cornea ensures that the first optical surface is smooth to provide the best quality’ (Artal, 2014, p.342).
Then, the light rays, or photons, pass through the dark circular opening at the centre of the iris – the pupil, whose ‘size changes with the ambient light, from less than 2mm in diameter in bright light to more than 8mm in the dark’ (Artal, 2014, p.342). The iris, with its two sets of muscles, therefore acts like a diaphragm, while the entrance pupil (the image of the iris through the cornea) and exit pupil (the image of the aperture through the lens) play a 'crucial role in image and vision quality' (Navarro, 2009, p.4). The light rays strike the inner lens of the eye, called the crystalline, which combines with the cornea, and focuses the rays on the retina. The crystalline lens is an active optical element, capable of changing the whole eye’s optical power by modifying its shape: ‘this is the basis of the mechanism of accommodation that allows the eye to focus on objects placed at different distances’ (Artal, 2014, p.343).
After going through the posterior chamber’s transparent and vitreous humour, the light rays finally reach the retina, which is the light-sensitive neural tissue on the inside back wall of the eye that acts as a screen where images are formed (Artal, 2014, p.343). ‘The retina has a two-fold role in the optical design of the eye’, Navarro explains (2009, p.6). Not only does its curvature seem to ‘match closely the image curvature, which has a major impact in maintaining a reasonable peripheral image quality’ (p.6), but ‘each retinal cone from the mosaic of photoreceptors [also] behaves as individual waveguide’ (p.6). As their name suggests, these ‘guide light from the inner to the outer segments where the visual pigments are contained’ (Rativa et al., 2011, p.1). Because they tend to be more efficient near their axes than at oblique angles, photoreceptor response to light entering the eye near the centre of the pupil is higher compared to that entering near the edge – a phenomenon known as the Stiles-Crawford effect of the first kind (Nilagiri et al., 2021). Such reduced sensitivity to peripheral rays attenuates their aberrations, i.e. ‘the difference[s] between the perfect (spherical) and the real wavefront’ (Artal, 2016, p.348) that cause the image formed by the lens to be blurred or distorted. The retina therefore plays a fundamental role in optimizing the performance of the eye’s optical system.
Consequently, whilst the peripheral parts of the retina detect movement and objects while rendering lower resolution (Artal, 2016, p.289), the centre of the visual field, called the fovea, is densely packed with photoreceptors to provide the highest possible resolution (Artal, 2016, p.289). Humans are provided with three types of photoreceptors: intrinsically photosensitive retinal ganglion cells (ipRGCs); rods – responsible for scotopic vision, i.e. at low light levels; and cones, which grant photopic vision, i.e. at higher light levels, as well as colour vision and high spatial acuity (Lamb, 2015).
Under twenty-first-century metropolitan conditions, almost all of our vision is mediated by the cone […] system, yet cones make up barely 5% of our retinal photoreceptors. Rods, on the other hand, contribute to human vision only under quite restricted conditions – namely, after an extended period (often tens of minutes) at very low light levels, of the order of twilight or lower (Lamb, 2015, p.1)
As few as ‘100,000 cones (0.1% of the total number of photoreceptors)’ (Lamb, 2015, p.1) packed in the rod-free foveola are responsible for the very high visual acuity of humans. Further, the cone system responds very fast to altered illumination, and functions over ‘a huge intensity range, from roughly twilight levels upwards’ (Lamb, 2015, p.2). The human eye possesses three cones: long-, medium-, and short-wavelengths, respectively peaking at red, green, and blue wavelengths. These underlie the colour sensation in medium and high brightness, while in very dim light, monochromatic rod cells take over (Davies et al., 2011). The reason why the latter are so numerically overwhelming in the retina despite their poorer performance ‘stems from a single property that the ancestral rods developed: the ability to respond reliably to individual photons of light’ (Lamb, 2015, p.3). This forced the retina to process these quantal signals as ‘discrete signals, rather than as analogue signals, and thereby achieved a huge advantage in extending the visual threshold down to exceedingly low levels’ (Lamb, 2015, p.6).
Lastly, the photons are converted via visual phototransduction into electrical impulses in the rod, cones, and ipRGCs, which can then be then analysed by the remaining parts of the visual system.
The retinal interneurons – the horizontal, bipolar, and amacrine cells – then pass that information to the retinal ganglion cells (RGCs), the output neurons of the eye. There are ~30 different types of RGCs, each firing action potentials depending on the quality and location of visual stimuli in the environment (Laha et al., 2017, p.1031)
Consequently, the action potentials are transmitted to the brain through the optic nerve, ‘where they are translated into perceptions and light-mediated behaviours’ (Laha et al., 2017, p.1031). The brain continues to filter the image in terms of colour, contrast, location, and direction of movement, before comparing them to the memory, giving an interpretation of what is being seen. While core object recognition, i.e. the ability to rapidly recognise objects despite substantial variation in their appearance, is yet to be fully understood (Rajaei et al., 2019), mounting evidence suggests that the brain solves such problems 'via a cascade of reflexive, largely feedforward computations that culminate in a powerful neuronal representation in the inferior temporal cortex' (DiCarlo et al., 2012, p.415).
Although the visual system is incredibly well designed in humans, some animals have a completely different perception. Felines, for example, are renowned for their improved nocturnal vision due to a few twists in key components, which a 2010 study (Long, et al.), aiming at creating Catalyst – an education game based on modelling human vision to cat vision within a puzzle game environment, described. Firstly, colour-wise, cats only have two cones of vision. Although a third one has been found, it is in too low quantity to have significant impact. This means that humans can better perceive colours and daylight. Vision acuity – the ability to tell objects apart, is also weaker in cats, but they have a wider field of vision for better peripheral motion perception. However, the presence of tapetum lucidum – a layer of tissue lying behind the retina that reflects the light and grants the cats their ‘phosphorescent eyes’, combined with a much higher concentration of rod sensors, allows for a minimum light threshold that is seven times lower than in humans (Long et al., 2010). Moreover, according to a study by the University of California (Banks et al., 2015), the vertical-slit pupils of domestic cats undergo area changes of 135- and 300-fold, whereas humans’ circular pupil changes by ~15-fold. 'Species that are active in night and day need to dilate sufficiently under dim conditions while constricting enough to prevent dazzle in daylight', Banks explains (Banks et al., 2015, p.2). Therefore, while cats can't see in complete darkness per se, their eyes are adapted to see in a much wider range of light levels.
Barn owls are also impressive nocturnal predators: as a 2012 study states, the ‘owl’s large, frontally oriented eyes […] are the basis of the barn owls’ stereovision’ (Orlowski et al., 2012, p.2). In addition, despite ‘large differences in neural organisation, higher levels of visual perception in barn owls generally show striking similarities to those of humans' (Orlowski et al., 2012, p.2). Barn owls therefore have ‘exceptionally good’ optical quality and a rod-dominated retina, but their photopic acuity is sacrificed to better exploit their dim surroundings and be scarily effective predators (Orlowski et al., 2012).
Nevertheless, the prize for the most complex visual system in the animal kingdom goes to the mantis shrimp. A recent, elaborate experimental study indeed showed that mantis shrimps have twelve channels of colour vision at their disposal, that is, three times more than humans and six times more than cats (Daly et al., 2018). Moreover, as an earlier paper discovered, they can sense polarised light – waves that only vibrate in one direction, which the human eye perceives as nuisance, but help the shrimps distinguish preys that otherwise blend into the background (Daly et al., 2016). Their rotational eye movements also actively enhance this information – an ability called dynamic polarisation vision, which has so far never been observed in other species (Daly et al., 2016).
To investigate further the visual mechanisms at play, researchers (Daly et al., 2018) installed a system of black and white stripes along a drum that rotated around an aquarium containing the stomatopod. The latter can perform pitch (up and down), torsional (around the fixed point the eyes are focused on), and yaw (horizontal) rotations with much higher angles and rotational freedom than most mammals and insects. The study based itself on the necessity of gaze stabilsation for all animals to perceive motion accurately as well as depth (using parallax, i.e. the difference in image between two eyes indicating close object versus far object). The collected date revealed that the mantis shrimp rotates its eye torsionally while also rotating for yaw, which is supported by the torsional motion system. This indicates that torsional rotations act as gaze stabilisators for yaw rotations, meaning the mantis shrimp consistently spins his eyeballs to adjust its sideway view. The researchers hence speculate upon the possibility that the nervous system of the mantis shrimp can negate torsional rotation effects to perceive a stable image, which humans and current machines struggle to accomplish, making this crustacean the only recorded case of torsional rotational independent vision within the animal world – a feature that lacks a clear reasoning of existence. Besides, the study raised the hypothesis that the stomatopod's visual system is equipped with a radial array of motion detectors capable of tracking stimuli from any direction beyond the yaw and pitch set, present in most animals (Daly et al., 2018) – an exciting capacity to investigate in future research.
To summarise, the biophysical system behind the sense commonly called sight is a very complex one, involving a broad range of processes starting with light refraction in the eye, image projection onto the retinal tissue, phototransduction in rods, cones, and ganglion cells, action potential transmission through the optic nerve, and finally complex feedforward computations in the brain to reconstruct an understandable visual information. Combined efforts of experimental methodologies and advanced modelling have enabled a deep understanding of optical mechanics along with the links between optical design, neurophysiology, and evolutionary needs. Indeed, while the human eye has evolved towards improved scotopic vision, certain species such as cats and barn owls have developed acute nocturnal vision and movement detection to match their predatory mode of life. Some of nature's mysteries remain unexplained to this day, though, as the mantis shrimp's uncanny visual system highlights. Future research could thus yield surprising discoveries, perhaps leading to breakthroughs in fields such as ophthalmic optics, or even neurosurgery.
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