Perception Exam 1

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albinism

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also called achromia, achromasia, or achromatosis) is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin. Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. The most common term used for an organism affected by albinism is "albino". Additional clinical adjectives sometimes used to refer to animals are "albinoid" and "albinic".

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amblyopia (and varieties of)

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is a disorder of the visual system that is characterized by poor or indistinct vision in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities. It has been estimated to affect 1–5% of the population.[2]
Amblyopia means that visual stimulation either fails to transmit or is poorly transmitted through the optic nerve to the brain for a sustained period. It often occurs during early childhood, resulting in poor or dim vision. Amblyopia normally affects only one eye, but it is possible to be amblyopic in both eyes if both fail to receive good, clear visual images. Detecting the condition in early childhood increases the chance of successful treatment.
While the colloquialism "lazy eye" is frequently used to refer to amblyopia, the term is inaccurate because there is no "laziness" of either the eye or the amblyope involved in the condition. "Lazy brain" is a more accurate term to describe amblyopia. The term "lazy eye" is imprecise because it is also a layman's term for strabismus, particularly exotropia. [3]

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Anableps anableps

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Anableps, of fishes in the family Anablepidae. They have eyes raised above the top of the head and divided in two different parts, so that they can see below and above the water surface at the same time. Like their relatives, the onesided livebearers, four eyed fishes only mate on one side, right-"handed" males with left-"handed" females and vice versa. These fish inhabitat freshwater and brackishwater and are only rarely coastal marine. They originate from lowlands in southern Mexico to Honduras and northern South America.[1]

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Anton’s syndrome

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a rare symptom of brain damage occurring in the occipital lobe. People who suffer from it are "cortically blind", but affirm, often quite adamantly and in the face of clear evidence of their blindness, that they are capable of seeing. Failing to accept being blind gets dismissed by the sufferer through confabulation. It is named after Gabriel Anton and Joseph Babinski.

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aphakia, aphake

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A child is not born being able to see. Vision develops as an image is formed in the eye and is transmitted to the brain. If the brain is denied an image, pathways are not formed which will allow the brain to process the picture. This is called amblyopia. If the brain is denied vision in both eyes from birth, the eyes may develop a jumpy, searching movement. This is called nystagmus. There is a critical period in a child’s life in which these pathways can be formed. This time is most critical during the child’s first year of life. After the first year it becomes increasingly difficult to improve vision. Limited improvement can be made after 8 years of age.

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astigmatism

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An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".[1]

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bipolar cell

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a type of neuron which has two extensions. Bipolar cells are specialized sensory neurons for the transmission of special senses. As such, they are part of the sensory pathways for smell, sight, taste, hearing and vestibular functions.
Common examples are the bipolar cell of the retina, the ganglia of the vestibulocochlear nerve,[1] and the extensive use of bipolar cells to transmit efferent (motor) signals to control muscles.
Bipolar cells are also found in the spinal ganglia, when the cells are in an embryonic condition.

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blindspot

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also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the optic nerve passes through it. Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain fills in with surrounding detail and with information from the other eye, so the blind spot is not normally perceived.

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brightness constancy

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the tendency for a visual object to be perceived as having the same brightness under widely different conditions of illumination

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brightness contrast

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The difference in light intensity between an object and its immediate surroundings; also, the intensity difference between adjacent bars in a grating.

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cataract

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Clouding that reduces the lens's transparency and, hence, degrades the quality of the retinal image.

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circadian rhythm

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Fluctuation in biological or behavioral activity that coincides with the day/night cycle.

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cones vs. rods

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Photoreceptors that are specialized for daylight and color vision. See rods.
rods Photoreceptors that are specialized for vision under dim light. See cones.

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cornea

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The transparent portion of the eye's front surface, which refracts light and allows it to pass into the eyeball.

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cortical column

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also called hypercolumn or sometimes cortical module,[1] is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields. Neurons within a minicolumn encode similar features, whereas a hypercolumn "denotes a unit containing a full set of values for any given set of receptive field parameters"[2]. A cortical module is defined as either synonymous with a hypercolumn (Mountcastle) or as a tissue block of multiple overlapping hypercolumns (Hubel&Wiesel).

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cortical hypercolumn

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An aggregation of columns of cortical cells whose receptive fields overlap on the same restricted region of the retina.--- A cortical column, also called hypercolumn or sometimes cortical module,[1] is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields. Neurons within a minicolumn encode similar features, whereas a hypercolumn "denotes a unit containing a full set of values for any given set of receptive field parameters"[2]. A cortical module is defined as either synonymous with a hypercolumn (Mountcastle) or as a tissue block of multiple overlapping hypercolumns (Hubel&Wiesel).

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cortical magnification

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The mapping of the retina onto the visual cortex so that the representation of the fovea is exaggerated or magnified.

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critical period (sensitive period)

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In general, a critical period is a limited time in which an event can occur, usually to result in some kind of transformation. A "critical period" in developmental psychology and developmental biology is a time in the early stages of an organism's life during which it displays a heightened sensitivity to certain environmental stimuli, and develops in particular ways due to experiences at this time. If the organism does not receive the appropriate stimulus during this "critical period", it may be difficult, ultimately less successful, or even impossible, to develop some functions later in life.[1]
For example, the critical period for the development of a human child's binocular vision is thought to be between three and eight months, with sensitivity to damage extending up to at least three years of age[1]. Further critical periods have been identified for the development of hearing and the vestibular system.[2] There are critical periods in childhood in which imprinting can occur, such as when a greylag goose becomes attached to a parent figure within the first 36 hours after hatching. A young chaffinch must hear an adult singing before it sexually matures, or it will never properly learn the highly intricate song.[3] These observations have led some to hypothesise a critical period for certain areas of human learning, particularly language acquisition.

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dark current

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Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because cyclic GMP (cGMP) is bound to them. Hence, positively charged ions (namely sodium ions) enter the photoreceptor, depolarizing it to about −40 mV (resting potential in other nerve cells is usually −65 mV). This depolarizing current is often known as dark current.

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Doctrine (or Law) of Specific Nerve Energies

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The doctrine that the qualitative nature of a sensation depends on which particular nerve fibers are stimulated.

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duplex retina

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a retina consisting of both rod cells and cone cells. In contrast to duplex retinas, pure rod and pure cone retinas have only rods or cones, respectively. Referring to the co-existence within the eye of two different systems, scotopic and photopic. The scotopic system provides high sensitivity in dim light; the photopic system provides high resolution under daylight conditions.

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emmetropia

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the state of vision where an object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. This condition of the normal eye is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina resulting in perfect vision. An eye in a state of emmetropia requires no correction.
Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 meters or 20 feet away. The light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort (an emmetropic eye does not need corrective lenses). If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort. In other words, the eye is automatically focused on things in the distance unless a conscious effort is made to focus elsewhere. For a wild animal or human prehistorical ancestors, this arrangement would be adaptive because it allows for alertness to predators or prey at a distance.

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eye cup

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a photoreceptive organelle found in the flagellate (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light (positive phototaxis) or away from the light (negative phototaxis). A related response ("photoshock" or photophobic response) occurs when cells are briefly exposed to high light intensity, causing the cell to stop, briefly swim backwards, then change swimming direction. Eyespot-mediated light perception helps the cells in finding an environment with optimal light conditions for photosynthesis. Eyespots are the simplest and most common "eyes" found in nature, composed of photoreceptors and areas of bright orange-red pigment granules.[1] Signals relayed from the eyespot photoreceptors result in alteration the beating pattern of the flagella, generating a phototactic response.[2]

Front 24

fMRI

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Functional magnetic resonance imaging, a for localizing regions of neural activity within the brain. See PET.

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fovea

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Pit or depression in the retina; the region of sharpest vision. Most Cones.

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fundus

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The fundus of the eye is the interior surface of the eye, opposite the lens, and includes the retina, optic disc, macula and fovea, and posterior pole.[1] The fundus can be viewed with an ophthalmoscope.[1] The term may also be inclusive of Bruch's membrane and the choroid.[citation needed]
The color of the fundus varies both between and within species. In one study[2] of primates the retina is blue, green, yellow, orange, and red; only the human fundus (from a lightly pigmented blond person) is red. The major differences noted among the "higher" primate species were size and regularity of the border of macular area, size and shape of the optic disc, apparent 'texturing' of retina, and pigmentation of retina.

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glaucoma

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A relatively common ocular disorder in which fluid pressure builds up within the eyeball, eventually causing blindness if not corrected.

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Hermann grid

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A regular, geometric pattern within which illusory spots are seen; the presence and strength of the illusory spots depend on the spacing of the grid's elements.

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Hubel, David and Wiesel, Torsten

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of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W. Sperry for his independent research on the cerebral hemispheres. In 1978, Hubel and Wiesel were awarded the Louisa Gross Horwitz Prize from Columbia University.

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hyperopia

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also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or the lens cannot become round enough), causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves toward the eye, the eye must increase its optical power to keep the image in focus on the retina. If the power of the cornea and lens is insufficient, as in hyperopia, the image will appear blurred.

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intrinsically-photosensitive RGC (ipRGC)

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also called photosensitive Retinal Ganglion Cells (pRGC), intrinsically photosensitive Retinal Ganglion Cells (ipRGC) or melanopsin-containing ganglion cells, are a type of neuron (nerve cell) in the retina of the mammalian eye. They were discovered in the early 1990s[1] and are, unlike other retinal ganglion cells, intrinsically photosensitive. This means that they are a third class of retinal photoreceptors, excited by light even when all influences from classical photoreceptors (rods and cones) are blocked (either by applying pharmacological agents or by dissociating the ganglion cell from the retina). Photosensitive ganglion cells contain the photopigment melanopsin. The giant retinal ganglion cells of the primate retina are examples of photosensitive ganglion cells.

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ion channels

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are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells (see cell potential) by allowing the flow of ions down their electrochemical gradient.[1] They are present in the membranes that surround all biological cells. The study of ion channels involves many scientific techniques such as voltage clamp electrophysiology (in particular patch clamp), immunohistochemistry, and RT-PCR.

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iris

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The two-layered ring of tissue that gives the eye its
characteristic color.

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layers of eyeball (functions and specializations)

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Cornea: The transparent part of the eyeball which covers the iris and pupil. A tear film normally coats the cornea, keeping the eye moist. Directly behind the cornea is the aqueous chamber, which is filled with a clear fluid for the purpose of maintaining the pressure of the eye.

Iris: The colored diaphragm in the anterior chamber of the eyeball which contracts and expands to adjust for light intensity.

Pupil: The opening in the center of the iris through which light passes.

Lens: The transparent, dual-convex body which focuses light rays onto the retina. It is normally capable of changing shape to allow the eye to focus on both near and distant images.

Retinal Cell Layers: The membrane on the inner wall of the eyeball which receives the image from the lens and converts it into nerve impulses.

Vitreous Gel: A clear jelly-like substance which fills the posterior chamber of the eyeball. Normally attached to the retina, it can become detached in clumps or strings, called "floaters," which are usually harmless, but can cast annoying shadows on the retina.

Fovea: The center of the retina. The region of highest visual acuity and cone cell density.

Macula: The area in and around the fovea. Made up mostly of cone cells, it is responsible for central vision and contains the fovea. The macula is less than 5% of the total retina.

Sclera: The white, dense, fibrous outer coating of the eyeball.

Optic nerve: Transmits nerve impulses from the retinal cell layers to the brain.

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lens

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a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina. This adjustment of the lens is known as accommodation (see also Accommodation, below). It is similar to the focusing of a photographic camera via movement of its lenses. The lens is flatter on its anterior side.

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LGN

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A group of nerve cell bodies arranged in layers in the thalamus, each layer receiving input from either the left eye or the right eye; the major relay station between the eye and the visual cortex.----- the primary processing center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain, and is thus part of the central nervous system.
The LGN receives information directly from the ascending retinal ganglion cells via the optic tract and from the reticular activating system. Neurons of the LGN send their axons through the optic radiation, a pathway directly to the primary visual cortex (or V1), also known as the striate cortex. The primary visual cortex surrounds the calcarine fissure, a horizontal fissure in the medial and posterior occipital lobe.[1] In addition, the LGN receives many strong feedback connections from the primary visual cortex. In mammals and humans the two strongest pathways linking the eye to the brain are those projecting to the LGNd (dorsal part of the LGN in the thalamus), and to the Superior Colliculus (SC)[2]

Front 37

Magnocellular vs. Parvocellular (M, P)

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In the lateral geniculate nucleus, layers containing large cells; layers 1 and 2. In the lateral geniculate nucleus, layers containing small cells; layers 3 through 6.

Front 38

melanopsin

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a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of G-protein-coupled receptor. A melanopsin based receptor has been linked to the association between light sensitivity and migraine pain. [3]
Melanopsin differs from other opsin photopigments in vertebrates. In fact, it resembles invertebrate opsins in many respects, including its amino acid sequence and downstream signaling cascade. Like invertebrate opsins, melanopsin appears to be a bistable photopigment, with intrinsic photoisomerase activity,[1] and to signal through a G-protein of the Gq family.

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myopia

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"shortsightedness" (Brit. Eng.))[1] is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.
Eye care professionals most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, but this does have many risks and side effects. The corrective lenses have a negative optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

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naive realism

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The philosophical view that perception accurately portrays all objects and events in the world.

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neural code

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a neuroscience-related field concerned with how sensory and other information is represented in the brain by networks of neurons. The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or ensemble neuronal responses and the relationship among electrical activity of the neurons in the ensemble [1]. It is thought that neurons can encode both digital and analog information.[2]

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OFF-Center/ON-Surround neuron

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An off-center cell is stimulated by activation of the surround and inhibited by stimulation of the center

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ON-Center/OFF-Surround neuron

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An on-center cell is stimulated when the center of its receptive field is exposed to light, and is inhibited when the surround is exposed to light. Off-center cells have just the opposite reaction.

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optic chiasm

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The point at which nerve fibers from the two eyes are rerouted to higher visual centers, with some fibers from each eye projecting to the same side of the brain (ipsilateral fibers) and the remainder projecting to the opposite side of the brain (contralateral fibers).

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optic nerve

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The bundle of axons of retinal ganglion cells that carries visual information from the eye to the brain. Also known as the second cranial nerve

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optic radiation

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a collection of axons from relay neurons in the lateral geniculate nucleus of the thalamus carrying visual information to the visual cortex (also called striate cortex) along the calcarine fissure.
There is one such tract on each side of the brain.

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optic tract

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The two bundles of axons of retinal ganglion cells formed after the nerve fibers exit the optic chiasm.

Front 48

optical power

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the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device.[1] The dioptre is the most common unit of measurement of optical power. The SI unit for optical power is the inverse metre (m-1).
For two or more thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens. Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface. These approximations are commonly used in optometry.

Front 49

orientation-tuning (also known as orientation-selectivity)

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A unique property of visual cortical cells, whereby they respond best to contours of a particular orientation, with the response decreasing as the orientation deviates increasingly from the preferred value.

Front 50

outer segment (of photoreceptor)

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The distal part of a photoreceptor that contains photopigment.

Front 51

PDE (not partial differential equation)

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Phosphodiesterase 6 (PDE6) is highly concentrated in the retina. It is most abundant in the internal membranes of retinal photoreceptors, where it reduces cytoplasmic levels of cyclic guanosine monophosphate (cGMP) in rod and cone outer segments in response to light.

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photoisomerization (cis-retinal into trans-retinal)

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While attached to the opsin molecule in the form of rhodopsin, the retinal has a shape called 11-cis, being somewhat folded, while on conversion to prelumirhodopsin the retinal has a straighter shape called all-trans; the process is called one of photoisomerization, the absorption of light energy causing the molecule to twist into a new shape. Having suffered this alteration in shape, the retinal presumably causes some instability in the opsin, making it, too, change its shape, and thereby exposing to the medium in which it is bathed chemical groupings that were previously shielded by being enveloped in the centre of the molecule. It may be assumed that these changes in shape induce alterations in the light-absorbing character of the molecule that permit the recognition of the new forms of molecule represented by lumirhodopsin, metarhodopsins I and II, and so on.

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photopsin(s)

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(also known as iodopsins) are the photoreceptor proteins found in the cone cells of the retina that are the basis of color vision. Photopsins are very close analogs of the visual purple rhodopsin that is used in night vision. Photopsins consist of a protein called opsin and a bound chromophore, the retinal.

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proximal stimulus, distal stimulus

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the proximal stimulus refers to physical stimulation that is available to be measured by an observer's sensory apparatus. It can also refer to the neural activity that results from sensory transduction of the physical stimulation. The proximal stimulus is usually contrasted with the distal stimulus, which is the state of objects in the world that were the cause of the proximal stimulus. The distal stimulus is the stimulus of an object as it actually exists in the real world.
The distal stimulus provides information for the proximal stimulus. The proximal stimulus registers (onto sensory receptors) the information given by the distal stimulus, and may also refer to the neural activity which results from the sensory transduction of the physical stimulation. A mental recreation of the distal stimulus in the mind of the perceiver is the percept.

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pupil

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The aperture in the eye formed by two sets of concentric bands of muscle; the constriction and dilation of these muscles vary the diameter of the pupil.

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receptive field

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The region of a sensory surface within which the activity of a sensory neuron can be influenced. In the case of vision, this constitutes a region of the retina which, itself, represents a given region of visual space. In the case of touch, the receptive field corresponds to a delimited region on the skin's surface.

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retinal ganglion cell

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Cells within the retina with center/surround receptive field organization; axons of the retinal ganglion cells constitute the optic nerve. See bipolar cell, amacrine cell, horizontal cell.

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retinal image

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The distribution of light falling on the retina; the quality and overall intensity of this image influence visual perception.

Front 59

rhodopsin

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known as visual purple, is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions.[1] Exposed to light, the pigment immediately photobleaches, and it takes about 30 minutes[2] to regenerate fully in humans.

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RPE

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retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells.[1][2] [The RPE is composed of a single layer of hexagonal cells that are densely packed with pigment granules.[1]
At the ora serrata, the RPE continues as a membrane passing over the ciliary body and continuing as the back surface of the iris. This generates the fibers of the dilator. Directly beneath this epithelium is the neuroepithelium (i.e., rods and cones)passes jointly with the RPE. Both, combined, are understood to be the ciliary epithelium of the embryo. The front end continuaton of the retina is the posterior iris epithelium, which takes on pigment when it enters the iris[4]
When viewed from the outer surface, these cells are smooth and hexagonal in shape. When seen in section, each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light.]

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rTMS

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a noninvasive method to cause depolarization in the neurons of the brain. TMS uses electromagnetic induction to induce weak electric currents using a rapidly changing magnetic field; this can cause activity in specific or general parts of the brain with minimal discomfort, allowing the functioning and interconnections of the brain to be studied. A variant of TMS, repetitive transcranial magnetic stimulation (rTMS), has been tested as a treatment tool for various neurological and psychiatric disorders including migraines, strokes, Parkinson's disease, dystonia, tinnitus, depression and auditory hallucinations.

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“Sea gypsies”

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Sea "gypsies" is the name given to the Moken, a tribe that lives near Burma and Thailand. They are famed for their knowledge of the sea, and their ability to harvest food from it. Incidentally, there are reports that their knowledge of the sea allowed them to get advanced warning that a tsunami was imminent, which allowed them to get to high ground before it hit.

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simple cortical neuron

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Visual cortical cells that exhibit clearly defined ON and OFF regions within their receptive fields. See complex cells.

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Snellen notation for acuity

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Visual acuity is a measure of the spatial resolution of the visual processing system and is usually tested in a manner to optimise and standardise the conditions. To this end, black symbols on a white background are used (for maximum contrast) and a sufficient distance allowed to approximate infinity in the way the lens attempts to focus. Twenty feet is essentially infinity from an optical perspective (the difference in optical power required to focus at 20 feet versus infinity is only 0.164 diopters). Whilst in an eye exam, lenses of varying powers are used to precisely correct for refractive errors, using a pinhole will largely correct for refractive errors and allow VA to be tested in other circumstances. Letters are normally used (as in the classic Snellen chart) as most people will recognise them but other symbols (such as a letter E facing in different directions) can be used instead.

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suprachiasmatic nucleus

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abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regulate many different body functions in a 24-hour cycle, using around 20,000 neurons.[1]
The SCN, pine cone-shaped and the size of a grain of rice, interacts with many other regions of the brain. It contains several cell types and several different peptides (including vasopressin and vasoactive intestinal peptide) and neurotransmitters.

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tilt after-effect

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Observation of a temporary change in the perceived orientation of lines after having adapted to lines tilted in another direction. If, for example, you stare at white and black bars tilted to the left for a minute or so, then look at vertical bars, these will now appear to tilt slightly to the right. This is an example of the adaptation of orientation-specific cells in the visual system which become fatigued and therefore temporarily less responsive. See adaptation; waterfall after-effect; orientation-specific cell.

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visible portion of EMR spectrum

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the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm.[1] In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function). The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can only be made by a mix of multiple wavelengths.

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Visual Angle

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Unit of measure expressing the size of an image on the retina; the visual angle subtended by an image is determined by the size of the object casting that image and the viewing distance from eye to object.

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visual field

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The extent of visual space over which vision is possible with the eyes held in a fixed position.