The Ecology of Humans – Sight

Sight

From a purely physical perspective, human vision is a miraculous appreciation of photonically-inspired excitement, employing an array of tissues set in peculiar configurations. Before relating the process let’s examine the instruments.

The Human Eye

The human eye is an intricately layered organ, sensing more than images, though that is its most evolved function. ~70% of the body’s sensory receptors are in the eye.

The human eyeball resembles a 2.5 cm sphere, but its exterior is a fused 2-piece unit.

The transparent, strongly refractive curved cornea at the front is linked to the sclera: the white of the eye that covers over 80% of the eyeball’s exterior. The sclera sits in the eye socket, tugged by 6 sets of muscles to control eye movement.

The cornea of primates, humans included, has 5 layers, whereas cats, dogs, and other carnivores have a cornea with 4 layers. The missing layer in carnivores, called Bowman’s layer, is an 8–14 mm thick protective layer of irregularly arranged collagen that protects the corneal stroma.

The transparent stroma accounts for 90% of the cornea’s thickness, comprising water (78%) scaffolded by collagen (16%), which gives the cornea its shape, strength, and elasticity. The collagen’s unique form is an ingeniously intricate arrangement, essential in producing the cornea’s transparency.

Behind the cornea is the aqueous humour: a watery reservoir in both the anterior and posterior chambers, continuously filled and drained in a precise ballet of coordination to maintain intraocular pressure at 15 mm Hg above atmospheric pressure.

Behind the aqueous humour is the iris: the highly individualized colorful disc of the eye. People with blue or light-colored irises tend to be more light-sensitive, as there is less light-absorbing pigment.

Muscles connected to the iris expand or contract the pupil: the aperture at the center of the iris. This adjustment process, termed adaptation, is a high-speed autonomic feedback loop, responsive to the level of light entering the eye, fed back from the mind-brain to the eye to optimize image input.

The pupil appears black because light entering the eye is absorbed by the tissues within, with none reflected. The human pupil is typically 4 mm diameter, with a range from 2 mm (ƒ/8.3) to 8 mm (ƒ/2.1), bright to dark. Aging lessens dilation capacity, to an elderly limit of 5–6 mm, which limits contrast receptivity, and hence visual acuity, especially night vision.

Primate pupils are round. Some other species, such as cats, have slit pupils.

Besides acting as a light access portal, the pupil is an autonomic storyteller: dilating to fear or pain. There is also a task-evolved pupillary response: pupils tend to dilate slightly with a heavy cognitive load, such as concentration or intense sensory discrimination. Certain drugs affect pupil size: alcohol and opioids constrict, while hallucinogens and stimulants, such as cocaine, cause dilation.

Behind the pupil sits the lens. Though the cornea accounts for most of the visual power of the eye, its focus is fixed. The lens curvature adjusts to focus based upon viewed object distance, becoming flatter for distance viewing, and curving for close-ups. The ability of the eyes to alter focal plane via the ciliary muscle is termed accommodation.

The interior of the eyeball, the vitreous body, is filled with a clear, viscous fluid: the vitreous humour. The vitreous humour is like the aqueous humour of the cornea except for 2 things. 1st, the vitreous is 99% water, with no blood vessels, and scant cells. A few phagocytes remove debris. Yet the vitreous humour is gelatinous, with a viscosity 2–4 times that of pure water; its refractive index is 1.336. 2nd, while the aqueous is a never-ending stream, the vitreous is a stagnant reservoir.

Although the vitreous contacts the retina, it does not adhere except at the optic nerve disc, where the retina has 1.2 million nerve axons for electrical signal transport to the brain.

The choroid lines most of the eyeball. It is a dark layer that cuts reflection within the eye and provides the blood vessels that feed the organ.

 Retinal Reception

The thin, layered retina is a lining just inside the choroid which covers 2/3rds of the eye. The outer layer, like the choroid, prevents light scatter within the eye. The inner layer, which has several sublayers, houses 126 million photoreceptors in a tiered mesh, covering 72% of the back of the eye: about 22 mm diameter.

The retina nerve complex, no more than 0.5 mm thick, is itself multi-tiered: 3 layers of nerve cells and 2 synapses layers, including a unique ribbon synapse. A fine resolution count of all cell layers in the retina renders 10 layers total.

The human eye has an approximate field of view 95° out, 60° in, 75° down, and 60° up. 12–15° temporal and 1.5° below the horizontal lies the optic nerve: a blind spot roughly 7.5° high and 5.5° wide.

Though the eye’s photoreceptor mesh is a field of about 200°, the acuity over most of that range is poor. To form high-resolution images, light must fall on the macula. That limits the acute vision angle to about 15°.

The macula is a 5-mm oval-shaped spot near the center of the retina, where visual acuity, particularly color vision, is best. The macula is a highly pigmented yellow, thus absorbing excess blue and ultraviolet light entering the eye, and so acting as a sunblock for this focal eye area.

At the center of the macula is the fovea, sight’s “sweet spot”: a 1 mm dimple of maximum acuity and color sensitivity, directly opposite the pupil at the back of the eyeball. The fovea is packed exclusively with green and blue cones (no rods).

The fovea covers only about 2° of visual angle. When looking at a scene at arm’s length, the fovea subtends a field about the size of the thumbnail.

The fovea’s small patch of clarity requires autonomically moving the eyes (microsaccade) so that an object of regard falls on the fovea with some degree of clarity. The frontal brain lobe is especially active during this autonomic task.

Fovea cones are thinner and more densely packed than anywhere else in the retina. Rods are connected to nerve fibers such that a single fiber can be activated by 1 in 100 rods. By contrast, cones in the fovea have a 1-to-1 wiring of cone to neuron.

While well-suited for fine tasks like reading, the fovea is quite slow in processing visual signals compared to the rest of the retina. This slow sensitivity is why motion is seen in flipbooks and movies. It also prevents seeing flicker unless we glance from the corner of our eye, where visual acuity is low, but processing is quicker.

The foveola sits in the center of the fovea. This is the retina’s little mind-brain: a 0.35 mm spot in the center of the fovea, with cone cells and a cone-shaped zone of Müller glia cells. Müller are the retina’s resident vision signal processors.

Most mammals lack a fovea. How they attain sharp vision is not understood.

Lacking photoreceptors, the retinal optic disc is the eye’s blind spot. Any light focused there is not received. But the eyes constantly shift so rapidly that this blank spot is seldom noticed.

The ribbon synapse is a linked neural mesh which functions like a hybrid of nerve and glial cell. The result is fast, precise, and sustained transmission.

A layer containing retinal ganglion cells (RGCs) is sensitive to the overall brightness of light, but not to specific wavelengths.

The retina has 2 primary neural photoreceptors: rods and cones. Rods are more numerous: in a human eye, 120 million rods to 6 million cones.

From an evolutionary standpoint, rods are a more primitive photoreceptor than cones. Individual rods receive patterns of light in black and white, which are gradated into shades of gray by the mind-brain by clustering adjacent receptors.

Cones absorb light indicative of color components. Cone cells do not themselves detect color.

Cone data is passed to a layer of bipolar cells before being passed to the retinal ganglion cells (RGCs). RGC axons make a sharp turn near the back of the eyeball to form the massive bundle called the optic nerve.

The rods that carpet the retina can capture a single photon, though it takes 3 for the eye to sense a flash of light. Photoreceptor proteins in a cell absorb photons, the energy of which causes a phototransduction that triggers a change in the cell’s membrane potential.

The photonic reaction causes a cascade function which leads to chemical deactivation, like erasing a chalk mark on a blackboard. Strangely, it is not the photonic chalk mark that sends an excitatory signal down the optic nerve to the brain for processing. Instead, depolarization, the chemical erasure reset, switching off the cell, does the trick. But that does not end the photon-provoked parade.

Depolarization of the cell membrane opens voltage-gated calcium channels in the photoreceptor neuron. An increased intracellular concentration of Ca²⁺ causes vesicles with neurotransmitters to merge with the cell membrane.

This incites neurotransmitter release into the synaptic cleft: the area between the end of one nerve cell and the beginning of another. Glutamate – a neurotransmitter whose receptors are often excitatory – is released.

In the chemical cascade depolarization causes a shift in sodium-potassium cell balance. Cell hyperpolarization happens as sodium channels close while potassium current continues. This hyperpolarization causes voltage-gated calcium channels to close.

Because calcium is required for the glutamate-containing vesicles to fuse with a cell membrane and spill the contents, glutamate release lessens as the calcium level in the photoreceptor cell drops. The glutamate drop depolarizes the “on” center bipolar cells and hyperpolarizes the “off” surround bipolar cells.

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The human eye has an ostensible perceptive wavelength range between 380–780 nanometers. Within those limits, imagery produced via the eye is practically perfect; an unfathomable physiological feat. Of course, what we call visible light is a narrow band in the electromagnetic spectrum.

A black light is a lamp that emits relatively long-wave ultraviolet (UV) light, just outside the boundary of human vision. Many substances fluoresce: reflect light at a different wavelength, typically longer than that absorbed. Greeneye fish and other deep-sea animals use florescence to see.

While the UV of a black light cannot be seen by humans, reflected black light is visible, and so useful for spotting the otherwise unseeable. Applications include forensic finding of fingerprints and blood, dermatological detection of skin conditions, various non-destructive tracking of biological molecules, and for gazing at trippy psychedelic posters.

By contrast to human vision range, reptiles can see in the infrared spectrum, while spiders and many insects, including bees, see ultraviolet light.

Plants evolved their flowers to please their pollinators, with accent and guides that let insects readily collect their reward for providing plant pollination service. Colors are particularly striking in the yellow spectrum which butterflies are especially sensitive to, and in the ultraviolet spectrum, where bees have fine visual discernment. Flowers that look a consistent yellow to humans have a much more striking appearance to butterflies and bees, which can perceive subtler gradations. A dash of ultraviolet gives a bee good guidance.

Vision Processing

Photonic waves bounce off objects willy-nilly based upon the ability of individual objects to absorb light or allow its escape. Refracted photons stray into the eyes. In humans, a dilating iris controls how much light passes through the pupil.

The incoming spray of photons are bent by the lens and strangely splayed on the retina. Before being absorbed by the rods and cones that convey the message which light has to offer, photons must first pass through nerve tissues that partly block incoming light patterns before doing their job as signal transmitters.

To facilitate peripheral vision and afford the ability to see objects larger than the pupil, the light patterns put on the retina are upside-down and reversed from how they entered the eye. Without reversal, the view of the world would be like looking through a drinking straw.

Stimulated by coherent light energy, neurons on the optic nerve disc pass their excitement to the brain. By this time, the incoming photonic pattern has been translated into chemical signatures that supposedly magically convey the precise spatial arrangement and atomic intensity of the light that breached the eyeball. Thus, luminous spectral input is transmitted to the mind-brain. But this is just the beginning of visual discombobulation. The optic nerve bundle from each eye crosses over to the opposite side of the brain at a crossing called the optic chiasma.

The visual cortex is at the back of the brain. Each hemisphere of the brain receives optical input from the eye on the opposite side of the head. The input images are mysteriously reoriented, mixing them together into a single mental image.

Once the chemical ferment from the retina reaches the visual processing part of the brain, a stunningly accurate facsimile of the received light pattern is painted by the mind in less than 150 milliseconds. In the process, the extreme distortion created by the eye’s lens is eliminated. Many other corrections must transpire to attain a discernable image with the clarity we associate with sight.

 Light & Color

Rods are sensitive to dim light but cannot detect sharp edges. Thus, rods provide scotopic (dark-adapted) vision.

Cones require more light to operate in most animals, including humans. Hence, color washes out in low light. But cones detect details in brighter light that rods can. The 6–7 million cones that a human eye has may be divided by their spectral sensitivity: red (64%), green (32%), and blue (2%).

The additive color model is based upon colored light mixtures. Red, green, and blue mixed create white light. More precisely, light activating the 3 different cone types in the proper proportion gives the mind-brain the impression of white light.

Scottish physicist James Clerk Maxwell is credited with experimentally discovering additive color, though the concept was articulated earlier by German physician and physicist Hermann von Helmholtz ~1850. Maxwell is best known for formulating classical electromagnetic theory, demonstrating that electric and magnetic fields travel as waves.

Calling the long-wave cone receptors “red” is something of a misnomer. The normalized wavelength peak of red cone response is around 580 nm, which is yellow light, not red. Red light kicks in at over 600 nm. The mind reddens sensed yellow light. While hominids evolved red color reception for social signaling, most other mammals do not see red.

The light response of rods peaks sharply at higher wavelengths: bluish light. Rods scarcely respond to slower wavelength red light.

With only 2% of the cone population, blue cones are grossly outnumbered, even though they are much more light-sensitive than red or green cones. This numerical disadvantage is overcome by mental image processing, which provides a compensatory blue amplifier.

The retina has a static contrast ratio of 100:1. By contrast, a typical clean film print at a movie theater may be 500:1, and flat-panel television displays are commonly 1000:1 on up.

While high contrast is desired for any display, it creates a biological trade-off for vision between image clarity and motion adjustment. The need for adjustment wins, hence low contrast reception.

As the eye constantly moves (saccades), it adjusts light exposure both chemically and geometrically via the iris, which regulates pupil size. Low contrast reception is mentally compensated for during visual processing: heightening contrast by successive image comparison at slightly different light levels and focal planes.

It takes 4 seconds of utter darkness for the eyes to initially adjust. Full adaptation by alterations in retinal chemistry (the Purkinje effect) takes a half hour or more.

Czech anatomist Jan Evangelista Purkynĕ (aka Purkinje) often meditated at dawn during long walks in blossoming Bohemian fields. He noticed that his favorite flowers appeared vivid red in bright sunlight, but dark at dawn. He correctly surmised in 1819 that the eye has 2 adaptive techniques to detect colors: one for sunshine, and the other for crepuscular light.

The Purkinje effect occurs at the transition between primary use of cones (photopic) to rods (scotopic). This is the mesopic state: as luminance dims, the rods take over; but before color disappears completely, sight shifts toward the rods’ top sensitivity. The upshot is that human eye color reception shifts toward blue at lower light level, because rods, which are more light-sensitive but color-insensitive, respond best to green-blue light.

Red lighting is the answer to having both the photopic and scotopic systems working. Submarines are dimly lit to preserve the night vision of the crew; so, for example, to be able to see through a periscope at night. But the control room must be sufficiently lit to afford reading instrument panels. By employing red lights, cones receive enough light to provide photopic vision: the high-acuity vision requisite for reading, while not saturating the rods, which are not sensitive to long-wavelength red light.

Eye adaptation results in a dynamic contrast ratio range of about 1,000,000:1 (about 20 ƒ-stops). Adaptation, like most biological mechanisms, is nonlinear and multifaceted.

Full adaptation depends on good blood flow. Dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

Physiological activity related to post-reception processing transpires in the visual cortex. Glia are processor and memory cells, as well as the maker and keeper of nerve cells which serve as signal transceivers.

The 3 cone types have some response wavelength overlap. Given various levels of input intensity, owing to discrepancies in population and distribution by cone type, it is more mathematically efficient to process the differences in cone responsiveness rather than direct intensities.

Cone wavelength overlap provides sampled comparative baselines of light level. This sampling is supplemented by secondary light level input information from bipolar and ganglion cells. The resultant data matrix provides relative input upon which to apply an inscrutably sophisticated, near-instantaneous algorithmic process that approximates the scene in view within the physical tolerances of photoreceptors. The acuity of vision operates at the outermost bounds of the known laws of physics. Just as rods have been honed by evolution to detect a single photon, so biological vision processing is a mathematical wonder beyond comprehension.

For decades it was thought that cones in the retina send their respective blue, green, and red color signals to the mind-brain, whereupon the mind combines colors, like a color printer (albeit using additive color). Instead, color vision processing works much like filling in a coloring book or colorizing a black-and-white film.

Prior knowledge, such as object coloration, is mentally projected in the earliest stages of visual processing. ~ German vision zoologist Andreas Bartels

Each type of cone comes in 1 of 2 varieties: color and value (light level or whiteness). Most cones detect value, which is employed to create a high-resolution picture defined by edges. Meanwhile, the color signal that the mind-brain receives is low resolution. These color splotches are combined and applied to edge-delineated areas to render an image.

Color perception is learned, not congenital, though the faculty to learn and apply colorization is innate. The mind takes color information from sensory inputs and colorizes according to heuristics acquired through practice.

Infant monkeys were reared for nearly a year in a room where the illumination came from only monochromatic lights. After extensive training, they were able to perform color matching. But their judgment of color similarity was quite different from normal animals. Furthermore, they had severe deficits in color constancy. Their color vision was very much wavelength dominated, so they could not compensate for changes in wavelength composition. Early visual experience is indispensable for normal color perception. ~ Japanese psychologist Yoichi Sugita

 Physical Defect Compensation

The optic nerve blind spot is located below the macula. As the macula is full of cones requiring brightness, that dimple becomes a 2nd blind spot in low light.

Full-picture night vision is autonomically created by constantly shifting the eyes 4° to 12° in saccades so that rods catch some photons.

With the intense processing required for moving image comprehension, the visual system cannot process information moving across the retina at more than a few degrees a second. For humans to see while moving, subconscious compensation for head motion is made by turning the eyes.

 Image Fabrication

Each incoming light pattern is magically aligned to produce a coherent image: a physiological unattainability that is taken for granted as being physiologically accomplished.

Creating the mental movie that constitutes vision requires mixing a hodgepodge of sequential images into what seems a static frame, then connecting frames. Human vision processing is so demanding that 1/3rd of the brain is intensely active during image fabrication.

A different picture is projected in each eye, offset by the space between them; binocular vision. Spatial orientation is derived from processing these separate images. Those projections are simultaneously passed to the visual cortex for assimilation, accounting for the parallax from eye placement.

One aspect of that assimilation is depth perception, which arises from a variety of cues. The impression of depth (stereopsis) as a distance cue most readily arises from binocular vision. The eyes converge on a single object for stereopsis. Visual convergence is the binocular oculomotor cue for perceiving distance. This convergence stretches extraocular muscles, providing kinesthetic sensations which frother cue the mind-brain.

The cue of stereoscopic convergence is effective for a focal plane less than 10 meters. The angle of convergence is less when the eyes focus on faraway objects.

There are many more depth perception cues that are monocular, including motion parallax, kinetic depth, crash distance, blurring, perspective, relative size, familiar size, interposition, shading, aerial perspective, texture gradient, peripheral vision, and accommodation.

Motion parallax: indicates distance by discrepancy in relative motion when one is moving. Many animals move their heads to gain different viewpoints to get better depth perception from motion parallax.

Kinetic depth perception: the opposite of motion parallax in accounting for objects moving without one moving. Receding objects typically become smaller, and vice versa.

Crash distance: countdown to contact at a certain velocity. The mind can even account for variances in velocity, which is instantaneous 2nd-derivative calculus; hence the ability play “catch” and drive in traffic.

Blurring: moving objects blur. The mind calculates size and distance information from this cue by incorporating offset from the instant focal plane.

Perspective: when parallel lines converge in the distance.

Relative size: when 2 objects known to be of absolute selfsame size show their distance by relative size.

Familiar size: as the visual angle of an object decreases with distance the mind instantaneously performs algebraic trigonometry to determine depth.

Interposition: occlusion hints at distance as objects in front block ones behind.

Shading: the relative way light falls on an object, and the shadow cast, is an effective cue of shape and position.

Aerial perspective: owing to atmospheric light scattering, objects at distance have lower luminance contrast and color saturation. Objects in the foreground appear in higher contrast.

Texture gradient: textures are sharper close-up, becoming blurry at distance.

Peripheral vision: like a photo from a fish-eye lens, parallel lines curve at the outer extremes of the visual field. Though not of objects in direct visual focus, this distortion yields visual information of depth.

Accommodation: this oculomotor cue for depth perception comes in from the mind-brain accounting for the ciliary muscles, which stretch to change lens focal length, allowing focus on faraway objects.

In short, depth perception is a convergence of multi-vectored mathematical factor analysis simultaneously performed by the mind instant by instant. This is on top of rod/cone input-filtering and image-mixing.

Vision is an extensive exercise in working memory. The details of known objects are filled in from experience, supplemented with current awareness. Discrepancies focus visual awareness, as do new objects for which no memories exist.

The tendency to shortcuts in object recognition creates an illusion of detail without actually sensing that level of detail. This accounts for the vagaries of visual memory that are common in later recall.

Dreams are illustrative. Engaging the sensation system used for vision, dreams are visually sketchy, with only focal objects rendered with any detail. Yet dreams can create such an immersive experience as to give the impression of reality. This occurs because dreams mix sensation with emotion-laden content. Dreams feel real.

Awareness while awake is similar in this regard: a pastiche of perceptions mixed into a sensory stew. Emotions create an especial intensity to an experience that renders it vivid.

Mentally processing visual text does not require vision for its input. It doesn’t matter if the person is reading with the eyes or the hands. The mind treats the pattern as if it were seen.

As with other senses, touch and sight are intertwined in the mind-brain. When looking at an object, the mind not only processes what the object looks like, but also how it feels.

Given all this, it is easy to understand why vision processing is the most intensive constant cognitive activity, and why the respite of sleep is necessary. Yet, during dreaming, the visual processing part of the mind-brain is given a workout. Perhaps dreaming provides practice for the mind to make its vision processing more efficient – learning how to better render the visual world ‘real’. That infants and children spend so much time dreaming lends support to this hypothesis.

A supposed visual snapshot is actually a time-varying composite, as the eyes constantly move (saccade), even staring at a single spot. Hence, what is mentally construed as a static visual image is a pastiche of collated patterns.

While anatomical study has rendered a detailed picture of the tissues and cells involved, and studies on visual acuity have discerned processing factors and tradeoffs, how detailed panoramic imagery is construed from a deficiently sampled collage of light input remains an enigma. Physiologically, sight is impossible.

 Flying

Hummingbirds are one of nature’s most agile fliers. They can fly 50 kilometers per hour and stop on a dime to maneuver through dense vegetation. To do so, their vision must accommodate what their wings can deliver.

The most important facet of flying fast is judging distance. Insects and mammals process distance by how quickly an object moves past their field of vision: a technique termed image velocity.

Birds take a different tack. They rely on specific object sizes to determine distance, especially vertical size. This approach demands greater mental processing than image velocity, as it involves rapid point-to-point comparisons in real-time. Such precision is essential, as birds cannot survive collisions at relative speeds that would only stun insects.

Birds sense altitude in the same way that flying insects do, using image velocity in the vertical axis. Thus, avian vision processing takes 2 different algorithmic approaches for flight navigation. This is a most impressive exhibition of mental virtuosity, especially in tiny hummingbirds. Hummingbirds also have as good an episodic memory as any animal, including corvids. (Human episodic memory is pathetic compared to that of birds.)

To accommodate the cognitive workload, hummingbirds have the largest relative brain of all birds: 4.2% of total body weight. (Though hummingbirds have relatively large avian brains, their cranial real estate is still far too meager to explain their cognitive acumen as a purely physical phenomenon.) Hummingbirds also have (relatively) the biggest hearts and most powerful breast muscles, which take 30% of total body weight.

Eye Signals

Infants are sensitive to eye expressions of fear and direction of focus. These responses operate without conscious awareness. ~ German psychologist Tobias Grossmann

Both the pupil and sclera – the blacks and whites of the eyes – act as social signals.

Pupils marginally dilate when viewing something of extraordinary interest. Women tend to be more aware of this social signal than men, though that may be said for most subtle social signals.

The sclera provides a powerful social signal. Humans are the only one of 221 primates that have whites of the eyes that are easily seen.

By ~7 months, babies know to follow the direction a person is looking by the sclera, not the head.

Apes and humans readily follow the gaze direction of others. But, owing to the high-contrast white sclera, only humans can discriminate between the eyes and the head. Apes follow the direction of the head, not the eyes.

Coordinating visual attention is important in cooperation. Infants acquire language through joint activities with others. Sharing a focus of attention lets a little one readily learn a word for an object or activity in question.

Circadian Rhythm

“The body is essentially a collection of clocks.” ~ Indian biologist Satchindananda Panda

“Every single cell in our body has its own molecular clock, including all the machinery required for the body to know what time it is.” ~ American neurobiologist Andrew Gaudet

All organisms coordinate their biology and behavior according to an endogenous circadian clock which is based upon Earth’s daily rotation. The circadian clock is an evolutionarily highly conserved physiological timing mechanism.

One way that bacteria manage antibiotic tolerance is by slowing their biological clock, allowing dormancy during dosing, thereby letting toxicity flow past without taking it in.

The many biological processes that oscillate via the circadian clock are under sway of circadian rhythm. All manner of functions are timed, including the wake/sleep cycle and even blood production. The common clock-winder is exposure to light.

 Biorhythms

“By the Law of Periodical Repetition, everything which has happened once must happen again and again….” ~ American humorist Mark Twain

The body has biological rhythms, which should not be confused with the pseudoscience labeled biorhythms, which has been around at least since the late 19th century.

According to the biorhythm hypothesis, the human system goes through 3 biological cycles: a 23-day physical strength cycle; a 28-day emotional cycle; and a 33-day intellectual prowess cycle.

As a practice, biorhythms supposedly can be calculated from date of birth to predict susceptibility to accidents, or conversely, opportunities of achievement.

However congruent with the concept of circadian rhythm, biorhythms are to biological rhythms as astrology is to astronomy. Its clockwork cogency may appeal, but biorhythms scientifically flounder for lack of evidence.

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Photosensitive proteins and circadian rhythms originated in the earliest single-celled life, protecting replicating of DNA from high ultraviolet radiation during daytime. Nighttime is the right time: genic replication relegated to the dark.

Light-based rhythm regulation varies widely among species. Tadpoles and other amphibians detect light via pigmented skin cells, thus adapting camouflage to different backgrounds. Sparrows sense light through their feathers, skin, and bone.

It is the color of light, not its brightness per se, that the mammalian system uses to set its circadian rhythms. In the eyes of mammals, retinal ganglion cells (RGCs) detect higher wavelength (blue) light as part of the mechanism for maintaining biological rhythms. RGC cells may work even in blind animals.

Like many animals, the human body relies upon sunlight to maintain its biological clock. Without sunlight, there is a natural tendency for biological rhythms to adjust such that sensed time shortens: 3 hours may seem as 1. This temporal condensation lessens stress, as being without sunlight for extended periods when the body expects it is stressful.

Most bodily functions are regulated by bio-clockwork. Through genetically encoded protein production, the body ramps up metabolism in the morning: rise and shine.

Levels of bioclock proteins rise and fall during the day, slowing biological functions at night. Blood pressure drops, heart rate slows, mental processes wind down: time for bed.

The biological clock gets jittery with age, often causing the elderly to have trouble sleeping. Shift workers, who have jobs outside the biological norm, are at a much higher risk for certain diseases because their circadian rhythms are chronically out of whack.

Jet lag is a common phenomenon for long-distance air travelers. One of the best ways to set the bioclock to the current time zone is to eat at local mealtime. Insulin release helps put hormones on the right time track.

Daily cell cycles are fundamental to the biomechanics that control cell growth. Egg cells have a biological clock with different biomechanical responses than ordinary somatic cells.

Bad circadian rhythm has profound effects on health. Cancer and diabetes are related to metabolic cycles that are bio-rhythmically controlled.

 Cryptochrome

Cryptochrome is a light-sensitive protein used for circadian rhythm regulation. Cryptochrome is also sensitive to magnetism.

Many animals rely upon magnetic field detection for spatial orientation. Magnetic sensitivity is essential for migratory animals such as birds and monarch butterflies. A gene for cryptochromes ostensibly encodes the ability.

Magnetic field sensitivity, which relies upon extra-dimensional (ed) dynamics for biological functionality, is tied to the vision system. Magnetic fields create subtly detectable visual effects that are processed by the mind-brain into useful information.

Humans have a cryptochrome gene that is highly active in the eye. But men have created continuous clouds of electromagnetic interference by their wireless communication devices, which do have a biological effect. Chronic cell phone use, for example, alters brain functioning.

 Coral Rhythm

Coral form a compact colony. The hard calcium carbonate that builds reefs comprises the exoskeleton for hundreds of thousands of tiny polyps. Each polyp is only a few millimeters in diameter.

Although some coral catch small fare using stinging cells, most live symbiotically with zoox, a photosynthetic unicellular algal protist. Zoox are endosymbiotic, living within coral polyps. Requiring sunlight so the zoox can provide nutrition, coral typically live shallower than 60 meters.

Coral polyps on a reef reproduce by simultaneous release of eggs and sperm. This happens on just 1 night, or a very few consecutive nights of the year, shortly after sunset, just after a full moon, for 20–30 minutes, beginning at 9:20 pm local time.

The synchronicity is astonishing, especially considering that coral have no eyes, and so supposedly cannot see. But polyps have photoreceptors that detect hue shifts in the twilight sky.

Prior to a full Moon, the Moon hits the sky before sunset. Reflecting the ruddy light of a setting sun, the mixture with moonlight sets the sky slightly redder. Just after a full moon, sunset precedes moonrise. With no Moon reflecting pinkish twilight, there is a slightly bluer glow.

As scuba divers know, underwater, red is the first color to go. That’s because red is the color of long wavelength, and relatively easily disrupted by water movement.

Blue and green are at short wavelengths and carry farther underwater. While blue is the shortest visible wavelength for humans, green is the last color to disappear underwater. At extremity of color detection, the mind-brain cannot compensate for the relatively few blue cone receptors in the human eye, leaving green the most visible color in an aquatic murk.

In the lunar cycle blue light is rarer, appearing only after a full Moon. As with all things biological, the coherent solution is derived via evolution: polyps detect the higher concentration of blue light.

How coral polyps manage their calendar for release only once a year is not yet understood. The smart money is on temperature variation averaging: coral calendar calculus.

Evolution of Sight

The visual system evolved to identify targets for behavior. ~ Israeli neurobiologist Yorum Gutfreund

English naturalist Charles Darwin had “much difficulty” in understanding the origin of something “as perfect and complex as the eye.” Darwin would have been shocked to learn how quickly life attained what he referred to as “an organ of extreme perfection.”

Algae can detect the entire human-visible spectrum, including red, which seawater absorbs. The ability to detect colors evolved exceedingly early.

Erythropsidinium is a single-celled alga with an organelle eye that it uses to catch prey and avoid predators. Erythropsidinium’s eye, a modified chloroplast, detects polarized light.

 Lookout from the Dung

Pilobolus is a fungus that starts its life cycle as a black sporangium let loose in the grass. A grazing herbivore eats the fungal bit.

The sporangium survives digestion intact, emerging in excrement. Once back out in the open air, embedded in fertilizer, the spores in the sporangium germinate, growing as a mycelium on the dung.

The fungus then fruits to produce more spores. The sporangiophore – asexual fruiting structure – is a transparent stalk (hypha) which rises above the excrement, ending in a balloon-like globe. A single sporangium forms on top.

Sporangiophore stalks like this are common in simple fungi. Pin mold on stale bread employs the same technique.

In Pilobolus, the globe (subsporangial vesicle) acts as a lens, focusing the light onto a carotene light receptor. If not properly focused, the fungus adjusts its hypha to point it at the light.

Pilobolus anticipates the human eye: a focusing element that acts as a lens, a photoreceptor that acts as a retina, and the means to detect unacceptable reception and adjust accordingly.

Pilobolus employs its eye for more than aiming its spores. The vesicle swells until it bursts, shot-gunning spores at up to 50 km per hour.

One tiny nematode appreciates the physics of this. The nematode slips into the sporangium. When the capsule bursts, ejecting the spores, the nematode rides along, traveling much farther than it otherwise could.

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Early animal eyespots were protein photoreceptors, similar to receptor patches for smell and taste in single cell organisms. Such eyespots could distinguish light from dark, but not direction. Their original function was regulating circadian rhythm.

All animal eyes have a common origin: a proto-eye that could resolve something more than light from dark. Sophisticated optical systems appear abruptly in the fossil record ~540 million years ago during the Cambrian explosion of life forms.

Rapid eye evolution was dictated by competition between predators and prey. Good eyesight quickly became essential. The Cambrian period witnessed nimble innovations in fine-scale anatomy as well as gross morphology.

In the animal kingdom, there are ~38 different body plans, or phyla. Only 6 have eyes. But those 6 body plans account for 96% of all animal species. There are 10 different structural forms of eyes with resolving power: the ability to perceive detailed images.

These different eye layouts vary considerably in architectural aspects. In some instances, similar forms evolved multiple times. But some things about vision are consistent. With rare exceptions animals rely upon sight as a primary sense.

2 eyes are ubiquitous. But starfish have a compound eye at the tip of each arm which can see a low-resolution image.

Of the 10 eye forms, apposition eyes are the most common. The human eye is an apposition eye: individual images from each eye are combined in the mind-brain. Apposition eyes are presumed to be the ancestral form of compound eyes.

Compound eyes are a collection of ommatidia: clusters of photoreceptor cells suffused with support and pigment cells. Each ommatidium is innervated by a single axon, providing the brain with a single picture element.

There are at least 7 distinct forms of optics in the various types of compound eyes. These may form either a single image or multiple inverted images. How multiple images are meaningfully interpreted by the mind-brain is not known.

Arthopods are the phylum that includes slugs, snails, insects, spiders, and crustaceans. There may have been a billion species of arthropod in Earth’s history, with 5 to 10 million still extant.

515 mya the Cambrian seas were home to Anomalocaris, a predator that was a cousin of arthropods. Anomalocaris had compound eyes with snug-fitting hexagonal lenses; 16,000 in each eye. The eyes were on movable stalks. That allowed high-resolution vision equivalent to dragonflies today.

Arthropod species in those same seas had highly advanced compound eyes, each eye with over 3,000 ommatidial lenses and a specialized zone for handling bright light. Such eyes may have had 28,000 photoreceptors, arranged hexagonally, able to provide a full 360° field of vision.

Mental integration of pixelated or partial scene elements, while differing in details, applies to all vision systems. Motion detection is another facet of memory-based visual integration.

Insect eyes and their visual system may account for up to 30% of the body’s mass; far more than most other animals.

Compound eyes are excellent for detecting fast movement, good vision at low light levels, and, in many instances, the polarization of light.  Polarized light waves are aligned in a plane. Fruit flies, foraging ants, and bees navigate by the polarization of natural light. Monarch butterflies and locusts migrate thousands of miles, across continents. Even a patch of sunlight on a cloudy day provides sufficient polarization information to navigate.

Detecting polarized light is not solely the province of compound eyes. With simple eyes bats use polarization at sunset to calibrate their internal compass. We have no perception of light polarization.

Cuttlefish are known to be able to detect polarization changes as small as 1°: an incredible acuity. Octopi, shrimp, and other crustaceans manipulate polarized light to send messages.

Flying insects, such as flies and bees, or prey-catching insects, like praying mantis or dragonflies, have specialized zones of ommatidia, organized into a fovea area which affords acute vision. Fovea cells are flatter and the facets larger. Flattening allows more ommatidia to receive light from a specific spot, creating higher resolution.

 Simple Eyes

Some arthropods have simple eyes, also called ocelli. Each ocellus has up to 1,000 sensory cells behind a single lens. Simple eyes convergently evolved at least 7 times.

Ocelli often do not form images but are sensitive to light direction and intensity. Flying insects typically have 3 ocelli on the top of their head, as well as a pair of compound eyes.

Auxiliary ocelli help maintain flight stability. Flying insects with compound and simple eyes have 2 physiologically distinct but mentally integrated vision processing systems: the most complex of any animal.

 Spider Eyes

Spiders do not have compound eyes. Instead, they have several pairs of simple eyes. Each pair is adapted for specific tasks. Spiders have principal and secondary pairs of eyes.

Only the principal eyes have movable retinas. Principal eyes may have compound lenses that give a wide field of view while efficaciously gathering available light.

Secondary eyes have a reflector (tapetum) at the back of the eye that affords detection of direct and reflected light. Tapeta improve light sensitivity.

Hunting and jumping spiders have 4 pairs of eyes. The large forward-facing principal pair have the best resolution. Some eyes may have telescopic ability to see small prey at considerable distance.

Most wolf spiders hunt at dusk and by moonlight. Besides a front principal pair, they have 2 pair of large posterior eyes with well-developed tapeta, letting them spot prey movement anywhere around them.

Deinopis, a genus of net-casting spiders, has 2 enormous night-hunting eyes which give a wide field of view as well as effectively gathering light. Unlike other night hunters, Deinopis‘ eyes lack tapeta. Yet the lenses of Deinopis‘ eyes detect light better (F 0.58) than cats (F 0.9) or owls (F 1.1). That is possible because a large area of light-sensitive membrane grows within the eyes each night. This membrane rapidly disintegrates at dawn.

 Jumping Spiders

Propelled by their back legs, jumping spiders can leap over 20 times their body length. But when pouncing on prey, they make short, accurate leaps.

Jumping spiders are most active during the day. They have excellent vision: able to hunt prey as well as recognize mates and enemies.

Physically separate sets of eyes cooperate. ~ American arachnologist Elizabeth Jakob

While hunting, jumping spiders can see in 3 ways using 3 different pairs of eyes. 1) Movement of a distant prey may be detected by side eyes or rear eyes, which give a blurry wide-angle image. 2) Once a prey is sensed, a jumping spider turns in that direction, locking on with its large, middle front eyes. These main anterior eyes can only see what is front of them. With 4 distinct layers of light-sensitive receptors, these eyes provide a clear, telephoto-focused, rich color image.

The spider tracks prey both by moving its body and by using muscles to swivel the elongated eye capsules so that the light-sensitive retina stays locked on target. 3) As the spider closes in, its side eyes judge distance. Once within 2–3 cm, the spider jumps its prey.

The principal eyes of jumping spiders have a unique retina with 4 tiered photoreceptor layers, on each of which light of different wavelengths is focused by a lens with appreciable chromatic aberration. ~ Japanese biologist Takashi Nagata et al

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Judging distance via depth perception is an important aspect of sight. Many animals, including humans, sense depth via by having 2 binocular eyes which see stereoscopically.

2 separate types of monocular cues provide depth perception for some animals. Some insects sense depth via motion parallax: image changes on the retina, the amount of which depends on the distance to an object. An insect typically gets motion parallax information by moving its head side-to-side and interpreting the differential.

Contrastingly, chameleons and other vertebrates use accommodation: focal adjustment and contrast. Accommodation is a technique distinct from motion parallax but to the same end: judging distance via snapshot differences. How much an image defocuses can be used as an absolute depth cue by comparing a defocused image with other images in which the same object is sharply focused or distinctly defocused. People use image defocus for a rough estimate of relative depth.

With their front eyes having 4 layers, jumping spiders have accommodation built in. Each layer presents a different focal plane, letting the spider mind to tell depth by comparing the different planes.

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Biological trade-offs abound in eye form. Those trade-offs are not always readily apparent. Various vision systems evolved in specialized ways, adaptive to lifestyle.

Anableps have eyes specially adapted to their surface-dwelling lifestyle. Anableps float on the surface, with the top of their eyes exposed to the air, while the bottom half, along with their bodies, are submerged.

Each Anableps eye has a single lens but 2 pupils and corresponding retinas. One pupil keeps an eye on the sky while the other watches the water below. This give Anableps an unparalleled range of vision.

 Box Jellyfish

Anableps’ bifurcated vision system is an advanced application of the same principle employed by the box jellyfish Tripedalia cystophora. This 1-cm jellyfish lives swims around by vigorously expanding and contracting its bell-shaped body.

Cystophora is fond of feeding on tiny copepods which it finds in sunny spots among mangrove roots. At night, a jellyfish takes its ease by moving away from the mangroves and sinking to the bottom of the shallow lagoon where it lives.

To see, cystophora has 6 eyes of 4 types. 2 eyes have lenses which give it a decent view. The uppermost lensed eye looks up. The lowest looks down. The lower eye spots obstacles; mostly the mangrove roots that the jellyfish swims among. The upper one keeps its eye on the prize.

If the top eye sees starkly bright light, it means that the jellyfish has strayed into open water, and risks starving. If instead the light gives the jellyfish the sense that it is within the mangrove canopy, lunch is in sight.

The box jellyfish is a brainless blob, but it has the wits to get around, find food, and get a good night’s sleep.

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An obvious significant variance in vision systems is eye placement, which depends upon whether the species is predator or prey. Predators have eyes front, to optimize focus and distance estimation. The trade-off is limited peripheral vision.

The strategic placement of the eyes of crocodiles and alligators allows them to stalk prey while keeping their bodies submerged and hidden. Eye placement is to the side, more optimized for field of vision. Crocodiles have keen hearing, albeit at a limited range of 50–4,000 hertz.

By contrast to predators, the survival of prey animals is greatly aided by field of vision. So, prey animals have eyes on the side of their heads – their minds create a panoramic mental image, optimized for motion detection but with suboptimal vision directly in front and poor depth perception. Prey animals often have a sensitive sense of smell and acute hearing to compensate for poor visual focal acuity.

The distribution of photoreceptors in an eye matches the area where visual acuity is needed. Animals that live on the African plains need an excellent line of sight, and so have a horizontal line of high-density photoreceptors. Tree-dwellers need good vision all around, and so sight tends to a symmetrical distribution, such that acuity decreases from the center.

Flight is especially taxing on the vision system. Males that mate mid-air have tremendous visual acuity, as they need to spot and assess potential mates against a flowing panorama.

At the other extreme, animals active at night tend to have larger eyes to increase light capture. Nocturnal animals tend to methodical movements as low light limits sight.

Many animals see far more of the world than humans can. Some moths can see color at night when we can hardly see at all. The color sense of moths is limited, as they are dichromats: able to see 2 color pigments and thus able to distinguish yellow, green, and blue, but unable to perceive red, which is a longer wavelength. Dichromats can see ~10,000 colors.

Most land mammals are thought to be dichromats. Marine mammals are cone monchromats: unable to perceive different color hues as they have only one type of cone cell. More particularly, both orders of sea mammals – pinnipeds (seals, sea lions, and walruses) and cetaceans (dolphins and whales), have cones that perceive mid-wavelength light, which corresponds to green, which travels best underwater.

Humans and closely related primates are trichromats: color conveyance from 3 different cone types, thus able to see red as well as green and blue. Each of the 3 different cone types in a human eye can perceive about 100 distinct color gradations. The combinatorial effect allows trichromats to see millions of colors. The human eye can distinguish over 7 million colors. Red cones predominate in the human eye as an evolutionary adaptation to see subtle changes in skin tone and facial expression.

There are exceptions to human trichromacy. Females have 2 X chromosomes, as contrasted to male XY chromosomal configuration. Via an anomaly in X inactivation (normally only 1 X chromosome is active), a tiny percentage of females carry cone cell pigment variants which provide for 4 kinds of cone cells. This aberration affords tetrachromacy, and thereby discrimination of 100 million shades of color.

 Monkey Vision

Old World monkeys are trichromats, as are female New World monkeys. Most male New World monkeys are red-green colorblind.

Howler monkeys are an exception. Thanks to a duplicated gene on their X chromosomes, both female and male howlers see red. The reason is leaves. Howlers graze on leaves when fruit cannot be found. The monkeys prefer younger, more nutritious leaves. The reddish hue of new leaves makes them more pronounced with trichromatic vision: hence the adaptation, and trichromacy in Old World monkeys, where leaves are common fare.

Other New World monkeys usually go for insects instead of leaves when there’s no fruit, so dichromatic vision is a better lifestyle fit.

Color can impede ability to see patterns, borders, and textures. Insects hide and camouflage. ~ Canadian biological anthropologist Amanda Melin

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Marsupials are trichromats. Some insects, including the honeybee, are also trichromats, but are receptive to a higher wavelength range: ultraviolet, blue, and green, instead of blue, green and red.

Some cephalopods, including certain octopi, can produce vibrant color displays that adroitly match their desires, whether blending into the environment to disguise themselves or in acts of bodacious mimicry. This remarkable color shifting is achieved with eyes that are ostensibly color-blind.

Mental color construction in cephalopods is achieved via odd vision hardware: an off-axis u-shaped pupil that affords chromatic aberration, where prismatic colors are perceived because different light wavelengths come into focus at different distances behind the lens. The resultant visual imagery is not sharp but does give a sense of color. There may be unknown means for mental sharpening.

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Theory predicts that 2 to 4 color receptors are ideal for the best color reception. Theory never met mantis shrimp.

 Mantis Shrimp Vision

Mantis shrimp are awfully fierce predators considering the seeming serenity of the clear, aquamarine tropical waters in which they live. They have what are widely considered the most complex eyes. Each eye has 3 separate sections, mounted on a flexible stalk that can be swiveled any which way.

Humans have 4 types of light-sensitive pigment cells, including 3 for color. Mantis shrimp have 16 kinds of photocells, 12 of which sense color.

A mantis shrimp eye can perceive a broad color palette in a frequency range from ultraviolet to infrared, including polarized light. The characteristics of ambient light vary in seawater depending upon depth.

Mantis shrimp vision is tunable. Different individuals can express different light filters appropriate to the depth at which the shrimp lives. Received images are clear, with excellent depth perception befitting their predatory lifestyle.

Not surprisingly, mantis shrimp are dappled with fluorescent markings attuned to maximize conveyance to other shrimp, affording clear signaling of the postural gesturing that the shrimp use in mock combat between males, as well as more subtle courtship rituals.

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The last known ancestor shared between humans and octopi might have been a worm of some sort; but humans and octopi have eyes that bear more than passing resemblance. Both roughly resemble a camera, in having a single lens at front, with a light-sensitive sheet – the retina – at the rear. The only reasonable conclusion, based upon a myriad of data relating to development and structural details, is that octopus and human eyes evolved independently, but converged to similar solutions. Similar, but with significant differences. Octopus eyes are more straightforward. The light-sensitive receptors point out to light, with neural pathways running to the optic nerve.

Human eyes have the receptors at the back of the neural cluster, requiring light to pass through a neuronal mesh to reach rods and cones. This arrangement makes the optic nerve a blind spot.

It is difficult to account for the convoluted arrangement of the human eye. While retinal tissues are largely transparent, various nerve cells in front of the rods and cones obstruct incoming photons, changing the wave energy. There is no evidence that this is some sort of sublimation process, such as acting as a wave guide. There is no geometric precision in the arrangement of the cells in front of the rods and cones.

Though not a primary photoreceptor, ganglion cells are sensitive to light level. There may be some optimality in the ganglion layer getting an account of overall brightness before the residual photons terminally deposit themselves in rods and cones; but that is purely speculative.

The human retina is an integrated cellular construct. The light reception cells are embedded within a support structure that has an ample blood supply immediately available. This arrangement supports a continuous regeneration of photosensitive pigments, facilitating rapid adjustment to changing light conditions.

The human eye is the most energetic organ in the body, consuming, on a per-gram basis, more oxygen than the brain. Sustaining such a high metabolic rate is likely beyond the capability of an octopus eye. But then, octopi live underwater, at lower light intensity, and probably do not have to maintain a high photo-pigment turnover.