Third Eye Blind
Third Eye Blind performs at SUNY Geneseo on November 17, 2007
Background information
Origin	San Francisco, California, United States
Genres	Alternative rock, post-grunge, power pop
Years active	1993–present
Labels	Warner, Elektra, Sony RED Distribution
Associated acts	Bully, Apocrypha, Kevin Cadogan, Year Long Disaster, Radio Angel, Megasapien
Website	3eb.com
Members
Stephan Jenkins Brad Hargreaves Abe Millett Kryz Reid
Past members
Kevin Cadogan Arion Salazar Tony Fredianelli Michael Urbano Jason Slater

Third Eye Blind is an American alternative rock band formed in the early 1990s in San Francisco. The songwriting duo of Stephan Jenkins and Kevin Cadogan signed the band's first major label recording contract with Elektra records in 1996 resulting in two multi platinum albums. The band's lineup changed in 2000 ending the songwriting partnership of Kevin Cadogan and Stephan Jenkins. The band's current line-up is Stephan Jenkins (vocals, guitar), and Brad Hargreaves (drums, percussion). Guitarist Kryz Reid and bassist Abe Millett have replaced the departed Tony Fredianelli and Arion Salazar respectively.

After the success of their eponymous debut album in 1997, the band released one more album, 1999's Blue, before guitarist Kevin Cadogan was released under controversial circumstances.^[1] In 2003, the band released Out of the Vein. In 2008, the band released the digital EP Red Star. After a six-year hiatus from releasing a full-length album, Ursa Major was released on August 17, 2009. A fifth album is also being worked on, without a tentative release date.

The band is best known for the singles "Semi-Charmed Life", "How's It Going to Be", "Graduate", "Jumper", "Never Let You Go" and "Deep Inside of You". Third Eye Blind has sold over 10 million copies worldwide.

History[link]

Beginnings (1993–1996)[link]

Third Eye Blind recorded their first demo in 1993.^{[citation needed]} In 1994, the band recorded its 2nd demo with band members: Kevin Cadogan on guitar, Steve Bowman on drums, Arion Salazar on bass, and Stephan Jenkins on vocals.^{[citation needed]} From late 1995 through early 1996, the band recorded its third demo with funds from RCA records to record with producer engineer Eric Valentine, which resulted in the band's gaining major label attention, including that of Clive Davis, who invited the band to perform a showcase for Arista Records in New York City.^[2] During Third Eye Blind concerts at the time, it was customary for the band to have a piñata release candy above their mosh pits, yet at the showcase for the record executives, lead singer Stephan Jenkins released live crickets from the piñata instead.^[2] With regard to the name of the band, Jenkins indicated during a radio interview that the name came from the metaphysical idea of a mind's eye, the topic of a book he had read. The other group members liked it and chose it as the official name. In April 1996, after Jenkins had challenged Epic Records executive Dave Massey in a meeting, the band landed an opening gig for Oasis at the San Francisco Civic Auditorium.^[2] In an unlikely scenario for an opening act, the band was invited back for an encore after playing their initial set^[3] and was paid double by the concert promoter.^[4] In addition, Stephan Jenkins' production of The Braids' cover of Queen's "Bohemian Rhapsody" gained major-label attention.^[5] Afterwards, the band found themselves in a bidding war among record labels, and after a showcase in Los Angeles, Kevin Cadogan and Stephan Jenkins signed as artists professionally known as Third Eye Blind with Sylvia Rhone of Elektra Records because they believed it offered the most artistic freedom.^[4]

Success (1997–2000)[link]

Third Eye Blind's first album, Third Eye Blind was released in 1997. The album had 5 singles: "Semi-Charmed Life", "Graduate", "How's It Going to Be", "Losing a Whole Year", and "Jumper". "Semi-Charmed Life" peaked at No. 4 on the Billboard Hot 100 and was number 1 on the Modern Rock Tracks for 8 weeks. The band also performed "How's It Going to Be" on Saturday Night Live. To date their eponymous debut has been the group's most successful album, selling 7 million copies in the U.S. alone.^[6] Smash Mouth drummer Michael Urbano played drums on 4 songs on the album. During this period they also opened a number of shows on U2's PopMart Tour.

In 1999 the band released their second album, Blue. Although not received as well as Third Eye Blind, the album sold 75,000 copies the first week of release and by 2003 had sold 1.25 million in the U.S.^[7] Four singles were released from the album, "Anything", "Never Let You Go", "10 Days Late", and "Deep Inside of You". In early 2000, shortly after the release of the album, Kevin Cadogan was released from the band. Cadogan filed suit, alleging wrongful termination, adding that his production, recording, and songwriter royalties were withheld since being kicked out of the band.^[8] The lawsuit was settled out of court in June 2002, with the terms of the settlement undisclosed.^[9]

[edit] Out of the Vein and A Collection (2001–2006)

After extensive international touring, the band took a break from performing, appearing only at charity events. They put on shows for the Tiger Woods Foundation and the Breathe Benefit Concert in Los Angeles after Jenkins' mother was diagnosed with breast cancer.^[10] During the four year gap between albums, the band also built a recording studio in anticipation of their next album.

In 2003 the band released Out of the Vein. Two singles were released from the album; "Blinded", and "Crystal Baller". Out of the Vein did not sell as well as its predecessors, with numbers estimated around 500,000 copies as of March 2007.^[11] Elektra Records was being absorbed into Atlantic Records at the time,^[12] and the only music video created from the album was for the single "Blinded". Due to the merger, the band found themselves without label support, as Jenkins said, "Our record company ceased to exist. The month the record was released, Elektra Records imploded."^[11]

In May 2004, Warner Music cut Third Eye Blind, along with over 80 other acts, from its roster.^[12] While no specific reason was given for Third Eye Blind being cut, Atlantic co-chairman Craig Kallman said the cuts were made to get Atlantic's roster down to an appropriate size where "we can give each of our acts top priority."^[12]

It would be over six years after the release of Out of the Vein until the band would release another full-length album. In the meantime, the band did release "A Collection", in 2006. This album was a collection of songs from the first three albums. Jude Gold, associate editor of Guitar Player Magazine, recognized that the liner notes falsely credited guitarist Tony Fredianelli with the creative work of former guitarist Kevin Cadogan, who was completely omitted from the bands biography included in the liner notes, which state: "As always, the band profited from the musical interplay between Tony Fredianelli, Stephan Jenkins, Arion Salazar and Brad Hargreaves". In regards to this, Gold stated "Its like saying Gun's N Roses music always profited from the interplay between Axl Rose and guitarist Bucket Head".^[13]

[edit] Red Star and Ursa Major (2007–2009)

For the 10th anniversary of the release of Third Eye Blind's debut album, the band performed at the Fillmore on March 13 and 14, 2007. The shows were filmed for broadcast on HDNet on December 2, as well as normal broadcast and release on DVD and as a live album tentatively to be released in early 2008, as announced by Jenkins on November 9, 2007, on DC101's "Elliot In The Morning".^[14] However, as of early 2011, neither the DVD or live album have been released.

A single, "Non-Dairy Creamer", was released in November 2008 and was part of the internet exclusive digital EP Red Star.

Third Eye Blind's fourth studio album titled Ursa Major was released on August 17, 2009.^[15] The album had been anticipated since mid-2007 and was previously expected to be named The Hideous Strength.^[16] The album was released under their own label Mega Collider Records.^[17]

On June 5, 2009, Third Eye Blind released their first single off Ursa Major, "Don't Believe a Word". The band later released a second single, "Bonfire". Neither were able to chart on any radio formats.

Also announced was a possible fifth album titled Ursa Minor,^[18][19] that is planned to follow Ursa Major. Jenkins said that Ursa Minor will consist of songs that did not make the cut for Ursa Major.^[19] It was later revealed that the title of the fifth album will not be Ursa Minor, and it will consist of new songs rather than songs that did not make the cut.

Fifth album (2010–present)[link]

Longtime guitarist Tony Fredianelli was fired from the band in early 2010.^[20] According to an article which quotes a lengthy letter of his, Fredianelli "... is suing the band for apparently being denied songwriting credits and benefits that he allegedly was entitled to".^[21][22] Irish musician Kryz Reid played guitar on the 2010 tour.^[23]

Third Eye Blind co-headlined The Bamboozle Roadshow 2010 between May and June 2010. In an interview with Stephan Jenkins and Brad Hargreaves at The Rave in Milwaukee, Wisconsin, Jenkins stated that Ursa Minor was going to be recorded in July, and would be released in 2011.^[24] During 2010 Bamboozle Roadshow, Jenkins elaborated on the expected sound of Ursa Minor in an interview with WreckedRadio.net, "In the third album, we probably had use of too much technology, and then on the fourth album, we got rid of the technology and went back to just recording things on tape. And, that's what we're going to do on the fifth album that we are going to do this summer." ^[25] According to a November 2010 blog by Jenkins, the fifth album will no longer be titled Ursa Minor.^[26]

On February 23, 2011, it was revealed that former guitarist Fredianelli had filed a federal lawsuit against Jenkins for over eight million dollars in damages for not giving him credit for past work in the band.^[27][28]

On March 16, 2011, a song entitled "Ladies and Gentlemen" was released on Japanese singer and actress Chiaki Kuriyama's debut album. The track was written by Jenkins and Reid.^[29]

On November 16, 2011, Third Eye Blind posted a new song, "If There Ever Was a Time" in support of the Occupy Wall Street movement, to their Facebook page, for free download.^[30] On November 28, 2011, Jenkins was on VH1's Big Morning Buz and he played the song "If There Ever Was a Time" and "Jumper" acoustically.^{[citation needed]}

On March 13, 2012, Third Eye Blind began recording their fifth studio album in London.^[31]

Band members[link]

Current members[link]

• Stephan Jenkins – lead vocals, rhythm guitar, percussion, keyboards, drums (1993–present)
• Brad Hargreaves – drums, percussion, piano, backing vocals (1995–present)
• Abe Millett – bass guitar, backing vocals, piano (2008–Present)
• Kryz Reid – lead guitar, backing vocals (2010–present)

Former members[link]

• Tony Fredianelli – lead guitar, backing vocals, keyboards (1993, 2000–2010)
• Adrian Burley – drums, percussion (1993–1994)
• Jason Slater – bass guitar, backing vocals (1993–1994)
• Kevin Cadogan – lead guitar, backing vocals, keyboards, autoharp (1993–2000)
• Arion Salazar – bass guitar, backing vocals, guitar, piano (1994–2007)
• Steve Bowman – drums, percussion (1994–1995)
• Tim "Curveball" Wright – drums, percussion (1995)

Touring members[link]

• Leo Kremer – bass guitar, backing vocals (2007–2008)
• Joe Fredianelli – guitar (performed as a guest, various performances from 2007–2010)

Session musicians[link]

• Michael Urbano – drums, percussion on Third Eye Blind (1995)
• Jon Evans – bass guitar on Ursa Major (2007)
• Juan Alderete – bass guitar on pre-production of Ursa Major (2007)

Timeline[link]

 id:vocals      value:red         legend:Lead_Vocals
 id:drums       value:brightblue  legend:Drums,_percussion
 id:guitars     value:green       legend:Lead_Guitar
 id:bass        value:purple      legend:Bass_Guitar
 id:lines       value:black       legend:Studio_Albums
 id:lines2      value:gray(0.7)   legend:EPs

 at:04/08/1997 color:black  layer:back
 at:11/23/1999 color:black  layer:back
 at:05/13/2003 color:black  layer:back
 at:08/18/2009 color:black  layer:back
 at:11/18/2008 color:lines2 layer:back

 pos:(225,220) text:Third Eye Blind
 pos:(350,220) text:Blue
 pos:(460,220) text:Out of the Vein
 pos:(670,220) text:Red Star
 pos:(720,220) text:Ursa Major

 bar:Stephan text:Stephan Jenkins

 bar:Brad    text:Brad Hargreaves

 bar:Tony    text:Tony Fredianelli
 bar:Kevin   text:Kevin Cadogan
 bar:Kryz    text:Kryz Reid

 bar:Arion   text:Arion Salazar
 bar:Abe     text:Abraham Millett
 bar:Jason   text:Jason Slater

 width:10 textcolor:black align:left anchor:from shift:(10,-4)
 bar:Stephan from:01/01/1993 till:end        color:vocals
 bar:Brad    from:01/01/1994 till:end        color:drums
 bar:Tony    from:01/01/1993 till:11/30/1993 color:guitars
 bar:Kevin   from:01/01/1994 till:01/31/2000 color:guitars
 bar:Tony    from:04/01/2000 till:01/31/2010 color:guitars
 bar:Kryz    from:04/01/2010 till:end        color:guitars
 bar:Arion   from:01/01/1994 till:11/30/2006 color:bass
 bar:Abe     from:11/01/2009 till:end        color:bass
 bar:Jason   from:01/01/1993 till:01/01/1995       color:bass

A Cambodian Shiva head showing a third eye.

The third eye (also known as the inner eye) is a mystical and esoteric concept referring in part to the ajna (brow) chakra in certain dharmic spiritual traditions, in particular Hinduism. This concept was later adopted by Christian mystics and spiritualists as well as people from other religious faiths. It is also spoken of as the gate that leads within to inner realms and spaces of higher consciousness. Among Christian mystics, the term is used in a broad sense to indicate a non-dualistic perspective. In New Age spirituality, the third eye may symbolize a state of enlightenment or the evocation of mental images having deeply personal spiritual or psychological significance. The third eye is often associated with visions, clairvoyance (which includes the ability to observe chakras and auras),^[1] precognition, and out-of-body experiences. People who have allegedly developed the capacity to utilize their third eyes are sometimes known as seers.

In the Western Wisdom Teachings[link]

According to Max Heindel's Rosicrucian writings, called Western Wisdom Teachings, the third eye is localized in the pituitary body and the pineal gland. It was said that in the far past, when man was in touch with the inner worlds, these organs were his means of ingress thereto.

Christian teachings[link]

According to the teaching of Fr. Richard Rohr the concept of the 'third eye' is a metaphor for non-dualistic thinking, the way the mystics see. In Rhohr's concept, mystics employ the 'first eye' (sensory input such as sight) and the second eye (the eye of reason, meditation, and reflection), "but they know not to confuse knowledge with depth, or mere correct information with the transformation of consciousness itself. The mystical gaze builds upon the first two eyes—and yet goes further."

"It happens whenever, by some wondrous “coincidence,” our heart space, our mind space, and our body awareness are all simultaneously open and nonresistant. I like to call it presence. It is experienced as a moment of deep inner connection, and it always pulls you, intensely satisfied, into the naked and undefended now, which can involve both profound joy and profound sadness at the very same time." Rohr refers to this level of awareness as "having the mind of Christ".^[2]

Gnostic teachings[link]

According to the gnostic teachings of Samael Aun Weor, the third eye is referenced symbolically and functionally several times in the Book of Revelation,^[which?] which as a whole he sees as a work describing Kundalini and its progression upwards through three and a half turns and seven chakras. This interpretation equates the third eye with the sixth of the seven churches of Asia detailed therein, the Church of Philadelphia.^[3]

Taoism[link]

In Taoism and many traditional Chinese religious sects such as Chan (a cousin to the Zen school), "third eye training" involves focusing attention on the point between the eyebrows with the eyes closed in various qigong postures. The goal of this training is to allow students to tune into the correct "vibration" of the universe and gain a solid foundation into more advanced meditation levels.

In this theory, the third eye, also called the mind's eye, is situated between the two eyes, and expands up to the middle of the forehead when opened. Taoism says it is one of the main energy centers of the body located at the sixth chakra, with the third eye forming a part of the main meridian, the line separating left and right hemispheres of the body.^[4]

Other interpretations[link]

The third eye is used in many meditation schools and arts, such as in yoga, qigong, Aikido.

In terms of Kabbalah, the Ajna chakra is attributed to the sphere of Chokmah,^[5] or Wisdom, although others regard the third eye as corresponding to the non-emanated sephirah of da'ath (knowledge).

Some writers and researchers, including H.P. Blavatsky^[6] and Rick Strassman, have suggested that the third eye is in fact the partially dormant pineal gland, which resides between the two hemispheres of the brain. Various types of lower vertebrates, such as reptiles and amphibians, can actually sense light via a third parietal eye—a structure associated with the pineal gland—which serves to regulate their circadian rhythms, and for navigation, as it can sense the polarization of light.

C.W. Leadbeater claimed that by extending an "etheric tube" from the third eye, it is possible to develop microscopic and telescopic vision.^[1] It has been asserted by Stephen Phillips that the third eye's microscopic vision is capable of observing objects as small as quarks.^[7]

Notes[link]

References[link]

Wikimedia Commons has media related to: Ajna chakra (third eye)

• Hale, Teresa (1999). The Book of Chakra Healing. New York: Sterling Publishing Company. ISBN 0-8069-2097-1.
• Radha, Siviananda (2004). Kundalini Yoga for the West. New York: Shambhala. ISBN 1-932018-04-2.
• Sagan, Samuel, M.D. (2007). Awakening the Third Eye (3rd ed). Roseville, N.S.W., Australia: Clairvision. ISBN 0-9586700-5-6.
• Sharp, Dr. Michael (2005). Dossier of the Ascension: A Practical Guide to Chakra Activation and Kundalini Awakening. St. Albert, Alberta: Avatar Publications. ISBN 0-9735379-3-0.

Eye
Schematic diagram of the vertebrate eye.
Compound eye of Antarctic krill

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.^[1] Image-resolving eyes are present in molluscs, chordates and arthropods.^[2]

The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms.^{[citation needed]} From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.

Overview[link]

Eye of the wisent,
the European bison

Complex eyes can distinguish shapes and colours. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision.

The first proto-eyes evolved among animals 600 million years ago about the time of the Cambrian explosion.^[3] The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the thirty-plus^[4] main phyla.^[1] In most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye,^[5] and reducing aberrations when there is enough light.^[6]

The eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.^[7]

Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360° field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.

Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system.^[8] Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.

In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents–in this way the bearers can spot hot springs and avoid being boiled alive.^[9]

Evolution[link]

Evolution of the eye

Photoreception is phylogenetically very old, with various theories of phylogenesis.^[10] The common origin (monophyly) of all animal eyes is now widely accepted as fact. This is based upon the shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago,^[11][12][13] and the PAX6 gene is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race".^[14] Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel.

Eyes in various animals show adaptation to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eye in, for example, vertebrates and molluscs are often cited as examples of parallel evolution, despite their distant common ancestry.

The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.^[1]

Through gradual change, as the eyespot depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.^[15]

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.^[16]

The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.^[16]

Types of eye[link]

There are ten different eye layouts—indeed every way of capturing an optical image commonly used by human beings, with the exceptions of zoom and Fresnel lenses. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface.^[1] Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures.^[1] Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. These two groups are not monophyletic; the cnidaria also possess cilliated cells, ^[17] and some annelids possess both.^[18]

Non-compound eyes[link]

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa.^[19]

Pit eyes[link]

Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light.^[1] Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm.^[1] The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.^[1]

Pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates.

Spherical lensed eye[link]

The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable.^[1] The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina.^[1] This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration.^[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.^[1]

Heterogeneous eyes have evolved at least eight times: four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.^[1] No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".^[1]

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles.^[1]

The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain.^[20] Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity.^[20] This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).^[20]

Multiple lenses[link]

Some marine organisms bear more than one lens; for instance the copepod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilia, has two lenses in each eye, arranged like those in a telescope.^[1] Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.^[1]

Refractive cornea[link]

In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air.^[1] In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see Luneburg lens), or with an aspheric shape.^[1] Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens.^[1]

As mentioned above, a refractive cornea is only useful out of water; in water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water–penguins and seals, for example–lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.^[1]

Reflector eyes[link]

An alternative to a lens is to line the inside of the eye with " mirrors", and reflect the image to focus at a central point.^[1] The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.^[1]

Many small organisms such as rotifers, copepods and platyhelminths use such organs, but these are too small to produce usable images.^[1] Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimetre-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.^[1]

There is at least one vertebrate, the spookfish, whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of guanine crystals.^[21]

[edit] Compound eyes

An image of a house fly compound eye surface by using scanning electron microscope

Anatomy of the compound eye of an insect

Arthropods such as this Calliphora vomitoria fly have compound eyes

A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarisation of light.^[22] Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their heads.^{[citation needed]}

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.^[23] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.^[24]

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.^[25]

Apposition eyes[link]

Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum.^[1] Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.^[1] (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information.

The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels.^[1]

Superposition eyes[link]

The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also have a transparent gap but use corner mirrors instead of lenses.

Parabolic superposition[link]

This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.^[9]

Other[link]

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialised zones of ommatidia organised into a fovea area which gives acute vision. In the acute zone the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution.

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organised in a way that resembles a true compound eye.

The body of Ophiocoma wendtii, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons. The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.^[26]

Nutrients[link]

The ciliary body is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.^[27] It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.

Relationship to life requirements[link]

Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.^[1]:23-4 Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia.^[28] Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope.^[29] An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey.^[28] In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.^[28] Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.^[9]

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop.^[28] On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.^[28]

It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.^[28]

Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.^[9]

Visual acuity[link]

The eye of a red-tailed hawk

Visual acuity, or resolving power, is "the ability to distinguish fine detail" and is the property of cone cells.^[30] It is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD^[31] (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.^[32] A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.^{[citation needed]}

Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.^[33] A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.

However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia. Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than mammalian eyes.^[34]

Perception of colors[link]

"Color vision is the faculty of the organism to distinguish lights of different spectral qualities."^[35] All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between wavelengths of 400 and 700 nm.^[36] This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.^[37]

The most sensitive pigment, rhodopsin, has a peak response at 500 nm.^[38] Small changes to the genes coding for this protein can tweak the peak response by a few nm;^[2] pigments in the lens can also filter incoming light, changing the peak response.^[2] Many organisms are unable to discriminate between colours, seeing instead in shades of grey; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved.^[38] Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.^[2]

Most organisms with color vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.^[38]

Rods and cones[link]

The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones.

Rods cannot distinguish colours, but are responsible for low-light (scotopic) monochrome (black-and-white) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for colour vision. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibres to the brain.^[38]

Pigmentation[link]

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.^[38]

Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.^[2]

There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.^[39] The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes.^[39] Likewise, c-opsins have been found to be expressed in the brain of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form.^[39] C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins.^[39] Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group.^[39] Incidentally, the melanin produced in the raticate is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.^[39]

See also[link]

	Book: Eye
Wikipedia books are collections of articles that can be downloaded or ordered in print.

• Arthropod eye
• Cephalopod eye
• Emission theory (vision)
• Eye colour
• Eyelid
• Eye movement (sensory)
• Human eye
• Mammalian eye
• Mollusc eye
• Nictitating membrane
• Simple eye in invertebrates
• Tapetum lucidum
• Tears

References[link]

Notes[link]

Bibliography[link]

• Ali, Mohamed Ather; Klyne, M. A. (1985). Vision in Vertebrates. New York: Plenum Press. ISBN 0-306-42065-1. 

External links[link]

Wikimedia Commons has media related to: Eyes

• Evolution of the eye
• Diagram of the eye
• Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most toward graduate students.
• Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows
• A collection of resources for eye anatomy.
• Anatomy of the eye - flash animated interactive.
• Eyes - Eyes are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain.
• Detection of weak optical signals by the human visual system : Perspectives in Neuroscience and in Quantum Physics
• Are Your Eyes Right February 1944 Popular Science

v t e Sensory system – visual system – globe of eye (TA A15.2.1–6, TH 3.11.08.0-5, GA 10.1005) Fibrous tunic (outer) Sclera Episcleral layer • Schlemm's canal • Trabecular meshwork Cornea Limbus • layers (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) Uvea/vascular tunic (middle) Choroid Capillary lamina of choroid • Bruch's membrane • Sattler's layer Ciliary body Ciliary processes • Ciliary muscle Iris Stroma • Pupil • Iris dilator muscle • Iris sphincter muscle Retina (inner) Layers Inner limiting membrane • Nerve fiber layer • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer Outer plexiform layer • Outer nuclear layer External limiting membrane • Layer of rods and cones • Retinal pigment epithelium Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retina ganglion cell (Midget cell, Parasol cell, Bistratified cell, Giant retina ganglion cells, Photosensitive ganglion cell) → Diencephalon: P cell, M cell, K cell Muller glia Other Macula (Foveola, Fovea centralis) • Optic disc (Optic cup) Anterior segment Anterior chamber • Aqueous humour • Posterior chamber • Lens (Capsule of lens, Zonule of Zinn) Posterior segment Vitreous humour Other Ocular immune system • Tapetum lucidum • Keratocytes M: EYE anat(g/a/p)/phys/devp/prot noco/cong/tumr, epon proc, drug(S1A/1E/1F/1L)