Recent discoveries in astronomy, physics, biology, paleontology, and geology show that a complex web joins us with our planet, our solar system, our galaxy, and with our universe. The universe is not a cold or hostile void. Instead, the earth is a focal point where intricate forces have come together and spun the web of life.
There is a purpose and an underlying intelligence behind the Universe – the Universe is becoming aware of itself through brain-sensory systems it evolves.
All life forms reflect in their design and function the properties of the physical environment in which they live. This is driven by evolutionary adaptation. Bird’s wing reflects aerodynamic properties (physical laws) of the air, the body shape of the dolphin reflects hydrodynamic properties of the water, an eye reflects physical laws governing rays of light…
Eye “design” of crocodile, stingray, shark, cat, owl and human.
The Human Eye
The eyes are the two organs of sight. They are located in the front upper part of the skull and consist of structures that focus an image onto the retina at the back of the eye which is a network of nerves that convert this image into electrical impulses to be recorded in a region of the brain. The eyeball lies in pads of fat within the orbit, a bony socket that provides protection from injury. Each eyeball is moved by six delicate muscles which are activated and coordinated by nerves in the brain stem. The eyeball has a tough, outer coat called the “sclera,” or white part of the eye.
The front, circular part is the “cornea” and is transparent. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the “iris” (colored part) with the “pupil” (center). The pupil is black and its diameter is changed by light intensity to control the amount of light which enters the eye. Immediately behind the iris, and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms.
The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels which lie just under it called the choroid plexus. The eyeball is sealed off from the outside by a flexible membrane called the “conjunctiva,” which is attached to the skin at the corners of the eye and forms the inner lining of the lids and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness. [ http://www.innerbody.com/image/nervov.html ]
The Formation of Images on the Retina
Because light rays diverge in all directions from their source, the set of rays from each point in space that reach the pupil must be focused. The formation of focused images on the photoreceptors of the retina depends on the refraction (bending) of light by the cornea and the lens. The cornea is responsible for most of the necessary refraction, a contribution easily appreciated by considering the hazy out-of-focus images experienced when swimming underwater. Water, unlike air, has a refractive index close to that of the cornea; as a result, immersion in water virtually eliminates the refraction that normally occurs at the air/cornea interface. The lens has considerably less refractive power than the cornea; however, the refraction supplied by the lens is adjustable, allowing objects at various distances from the observer to be brought into sharp focus on the retinal surface.
We now know the process of how we can perceive light at a molecular level. But how is the image formed?
Light from our surroundings enters our eye through the dioptric media — cornea, lens, aqueous humour and vitreous body. Among these, the anterior part of the cornea accounts for providing nearly 2/3 of the refractive power, because it has a highly curved surface and high refractive index.
Light stimulates the photoreceptors on our retina to produce nerve impulses, which will travel along the optic nerve to the visual cortex of our brain.
The visual cortex of the brain is that part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain. There is a visual cortex in each hemisphere of the brain. The left hemisphere visual cortex receives signals from the right visual field and the right visual cortex from the left visual field.
The image formed on the retina is real, inverted and smaller. However, on interpretation by the brain, the images will be upright. This is an inborn ability. Some people put it in this way: we ‘see’ with our brain, not our eyes. [ Sources: Wikipedia and webschoolsolutions.com ]
The Eye is not the only way to “see” the World
How Dolphins Perceive Their World
Whales and dolphins (collectively called cetaceans) live in a world in which it is often difficult to see very far. Even in the clearest tropical water the visibility is less than a few hundred feet. This means that cetaceans cannot rely on their vision to communicate or forage. Instead they use sound to explore their water world. The dolphin can work out how far away the fish is from the time it takes the click that was sent out to return. The click returns when it bounces back of an object. If the dolphin keeps producing clicks and receiving the echos, it will get information back about the speed and direction that the fish is moving.
An echolocating dolphin can detect a 2.5cm object from 72 metres away! If the dolphin is far from the target it will produce clicks at a slow rate. The closer the dolphin gets to a target, the faster the clicks bounce back and the faster the dolphin sends out more clicks to detect the object the clicks are bouncing off. [ http://www.cbmwc.org/education/echo.asp]
The phonic and aural systems of the bottlenose dolphin have evolved well beyond that of other chordates, including the bat.
1. Sound is generated primarily in the nasal passages (and does not emanate from the mouth)
2. Sound generation has been broadened in frequency (into the UHF region of 150 kHz or higher)
3. Sound generation has evolved into two parallel systems that can operate independently or simultaneously
4. Sound reception is supported by two conformal lens-type receivers (unrelated to the “outer ear”)
5. The overall system measures range and azimuth using two distinct operating modes
* pulse mode for short range and high precision (less than 100 meters)
* swept continuous tone for longer range (out to about 600 meters)
* The system can determine the azimuth of active sources at much longer ranges
There is growing neurological evidence that the dolphin can “see” sound patterns just as it and other chordates see light patterns.
* It is likely that it can merge the information that is sees in the visual and acoustic bands.
A front view of the bottlenose dolphin showing the forward looking lens-type external ears and the general location of the melon used for echolocation (dotted overlays) . The caption of the Caldwell & Caldwell figure hint that the eyes and possibly the eye sockets of the dolphin may rotate to support their apparently good stereoscopic vision. The eyes are seen more clearly in the original art. Modified from Caldwell & Caldwell, 1972. http://www.neuronresearch.net/hearing/files/dolphinbiosonar.htm
The marine world is a dynamic environment filled with a myriad of life forms, noises, topographical features, differing substrates, varying water qualities and movements, temperature variations, pressure variations, chemical variations, and objects of various sorts. Many or all of these things may be important for the dolphin to sense, interpret, and act on to protect or enhance its well-being. To achieve that favorable outcome, dolphins have evolved or adapted sensory specializations that enable it to monitor its environment and take advantage of opportunities. Oceanic dolphins have retained the basic senses of vision, hearing, taste and touch, but smell has been abandoned. The olfactory receptors and the olfactory lobes of the brain were apparently lost during the migration of the nostrils to the top of the head, and because olfaction was no longer useful to an air-breathing mammal that spent most of its time underwater and no longer sniffed the air. Other, more useful, senses evolved—echolocation and, apparently, magnetic-field detection.
Hearing and echolocation
The underwater world is filled with sound, providing information to the listener on such diverse things as vocalizing schoolmates, shoaling water, and prey locations. In response to the advantages of perceiving and interpreting underwater sounds, the dolphin’s hearing and sound production systems have undergone extensive modifications. The external ears have disappeared, resulting in a more streamlined body shape better suited to rapid swimming. New sound pathways to the inner ear have evolved, including broad area around the sides of the dolphin’s head and the fat-filled spaces of the lower jaw. Each of the two inner ears is isolated acoustically from the other, enabling the dolphin to precisely locate the sources of underwater sound, something that is very difficult for humans immersed in water. Hearing is remarkably acute throughout a broad range of frequencies, and the dolphin is capable of distinguishing small differences in the frequency (pitch) of sounds. One of the types of sounds produced by the bottlenosed dolphins and some other dolphin species, is the whistle, a narrow-band continuous sound that varies in its frequency. Individual dolphins tend to have unique whistle sounds, called “signatures,” whose changing frequency characteristics can be easily detected by the listening dolphin and often identify who among the schoolmates is whistling. Additionally, the inner ear has been modified to allow for the perception of high-frequency sounds, reaching some ten times or more above the upper limit of adult human hearing. The ability to sense these high-frequency sounds is vital for the dolphin’s echolocation sense, sometimes called its active sonar system, because it allow the dolphin to detect very small objects. Series of very short duration, high-intensity, broad-band clicks containing frequencies as high as 120-kHz are projected in a narrow beam from the region of the dolphin’s melon and broadcast in front of the dolphin into the adjoining waters. When the clicks strike an object, echoes are returned and sensed by the dolphin through its special pathways for hearing. Recent research suggests that these echoes may preserve the spatial structure or shape of the reflecting object and be interpreted by higher center of the dolphin’s brain as an image of the object. This echolocation sense seems to be closely integrated with the dolphin’s visual sense, allowing it to easily relate things heard to things seen. The echolocation signals of dolphins can penetrate through many objects, revealing their inner structures. Spotted and bottlenosed dolphins, for example, have been observed echolocating into the sand bottom of the ocean, searching through sound for hidden fish and rooting them out with their snout.
Echolocation is a sensory sonar system that dolphins use for communication and for locating things in their environment.
Dolphins release a focused beam of clicking sounds (sound waves) and then listen to the echo. From this they can determine the following about an object (such as a fish).
* internal structure (depending on the object)
Echolocation enables the dolphin to see in a much more complex way than it might seem. In fact, the information available from echolocation includes things that we would not notice or see with the naked eye. Depending on the object, sound waves can enter beneath the surface therefore giving feedback and information of the internal structure of an object.
The dolphin’s echolocation sensory system is fascinating and complex and remains a mystery in many ways. There are theories that dolphins may view their surroundings holographically and are able to transfer these holographic images to other dolphins. We do know that echolocation is extremely sensitive and allows dolphins to examine small objects hundreds of yards away!
Because of their exceptional hearing capabilities, their echolocation sense, and the importance of sound in the underwater world, all dolphins were thought of as primarily “auditory animals.” Vision was believed to be not particularly well developed, even in oceanic dolphins, and of secondary importance at best. We now know, however, that oceanic dolphins, such as the bottlenosed dolphin, have excellent vision, and that vision plays an important part in their natural world. Research has shown that dolphins can see equally well both underwater and in air. In contrast, we humans are visually handicapped underwater unless we wear a facemask or goggles. The dolphin instead seems to rely on adaptations of the optical qualities of its cornea and lens to allow it to see well in air or underwater. The dolphin eye, though lacking color vision, is highly sensitive to light in the blue region of the visible spectrum, which is in keeping with its off-shore blue-green underwater world. Dolphins, like dogs and cats, have “eye shine,” produced when light reflects back out of the eye from “mirror cells” located behind the receptor cells of the retina. Mirror cells act to amplify the light entering the eye, making it possible for the dolphin to see well at night or in the dim light of its underwater world. Dolphin eyes are laterally placed, providing a wide field of view enabling the dolphin to see forward, laterally, and even rearward. Also, unlike our eyes, the dolphin’s eyes act independently. For example, as the dolphin swims on its side, the pupil of the downward looking eye, gazing into the dark, may be fully open, while the pupil of the upward looking eye, gazing toward the bright surface, may be tightly constricted. In the wild, underwater vision helps the dolphin to capture swift-moving prey and to keep in contact with schoolmates swimming nearby. It also helps the dolphin to identify its own species, recognize familiar individuals from its social group, and to interpret behavior in the context of age, size, and sex differences of individuals. In-air vision helps the dolphin to keep in contact with more distant leaping members of its group, to detect circling birds that may indicate the presence of prey fish, and to recognize land features in its coastal habitat. Laboratory studies have confirmed that vision is an important information source for the dolphin. Dolphins can attend to and interpret human gestures, follow the direction in which a human is pointing, monitor rapidly occurring visual symbols appearing on a television screen and report the occurrence of certain key symbols, and easily recognize the same objects across the senses of vision and echolocation. It is clear, therefore, that we should regard dolphins as both auditory and visual specialists, making their way though their world through both sound and vision, just like us.
Copyright © 2002, The Dolphin Institute