Definitions of important terms related to the study
of the eye and the visual parts of the brain

A – B – C – D – E – F – G – H – I – J – K – L – M
N – O – P – Q – R – S – T – U – V – W – X – Y – Z

A

Achromatism
1) a person who is unable to perceive colours.
2) an optical system that has been corrected in order to minimize a chromatic aberration.

Age related macular degeneration
Late onset pathology of the photoreceptors of the macula. It usually arises after the age of fifty.

Aging
A variable which affects many processes, both cognitive and physiological, involved in visual information processing.
Agnosie
Neuropsychological disorder which affects the recognitions of objects, people and places. Patients retain their elementary sensory experience, as well as their general intellectual level of functioning.
Akinetopsia
Patients are unable to perceive moving objects or beings, despite the fact that they retain the capacity to perceive the same objects if they are stationary.
Amacrine cell
A cell found in the retina which is found between bipolar and ganglion cells. They have only one axon that terminates in the internal plexiform layer.
Amblyopia
It can cause either a severe or slight loss of vision. It is cortical in origin. It is known to reduce visual acuity and binocular vision. The cause is of cortical origin. It develops after an anisometropy, strabismus or a childhood cataract. In North America, it is the most frequent cause of childhood vision loss, with an incidence rate of 1-4% in the general population.
Angiography
Also known as an angiogram. A doctor inserts a catheter into an artery. An X-ray dye is then injected so that the blood vessels become visible on an X-Ray.

Anisometropy
When each eye has a different distance point refraction.
Aqueous humor
A fluid that fills the anterior chamber of the eye.
Area centralis
The part of the retina that contains the greatest number of photoreceptors and by extension the greatest acuity. It includes the fovea, parafovea and perifovea. It is the part of the retina solicited during active looking (e.g. reading).

B

Binocular vision
Vision achieved by the integration of information present in both eyes. It allows for vision in depth due to stereopsis which requires two eyes.

Biochemistry
The study of chemical processes of living organisms.

Bipolar cells
A retinal neuron with the soma (cell body) situated in the internal nuclear layer. Its function is to transmit information from the photoreceptors to the ganglion cells.

Blind spot
A part of the retina where there are no photoreceptors because all the axons from the ganglion cells exit at this point. Due to the lack of photoreceptors no light stimulation can produce a visual response in the retina.

Blindness
A total or partial loss of vision

Blindsight
Despite a demonstrable blindness, some patients retain some form of visual perception that is not conscious. For example, a patient may report that there is movement in the visual field but is unable to consciously experience or perceive that movement. This type of residual vision (to a lesion) can be explained by the fact that the visual system works in many parallel circuits. Although one part may be destroyed by a lesion, others areas retain their functions.

Brain imaging
A technique that permits the visualisation of brain structures and their relative level of activity expressed during different task.

C

Calcarine fissure
A deep fold of the cortex in the occipital lobe. The surrounding cortex is the primary visual cortex.

Cerebral dyschromatopsia
A disorder which affects the discrimination of colours. It is cerebral in origin.

Choroid
A vascularised surface in the posterior part of the eye. It is between the retina and the sclera. The choroid enables blood to get to the photoreceptors.

Chromatic
That which possesses one or more colors.

Chromatic aberration
An optical imperfection which separates a point of light according to the refractive index of the colour spectrum of the source of light.

Coloboma
A congenital abnormality that manifest itself either by the absence of or abnormal eyelids.

Colour constancy
A perceptual phenomenon based on a relative chromatic comparison between elements in a visual scene rather than a comparison based on absolute colour. This allows for a uniform perception of colours regardless of the colour of the ambient light.

Complex cell
A type of neuron found in the primary visual cortex that, via is electrical discharge rate, can signal the orientation as well as the direction of motion of a stimulus. The part of the visual field that modulates its response to visual stimulation (receptive field) is comprised of superimposed ON and OFF regions.

Cones
Nervous cells within the retina that are sensitive to light. They are most prevalent in the fovea and their concentration decreases toward the periphery. They are most efficient at high light intensities and allow for photopic vision (i.e. day vision). There are three types of cones which differ according to their sensitivity to different light wavelengths. It is these differing sensitivities to wavelengths that permit colour vision.

Cortical area
The top layers of cells composing the brain. Its cellular architecture and function is distinct from that of the rest of the brain.

Critical period
A circumscribed period during development of the nervous system during which organisation of the visual pathways is susceptible to changes due to visual experience. A lack of proper stimulation during this period cannot be compensated for once the period is over. The effects are irreversible.

D

Daltonism
It is the abnormal perception of red and green, caused by an anomalous gene located on the X chromosome.

Dark adaptation
The time required for the visual system to increase its sensitivity due to a reduction of ambient light.

Deuteranopia
A form of dichromatism. It is characterized by a difficulty in distinguishing red, green and grey. All three of these colours are perceived as shades of grey. All colours of the visual spectrum are perceived on a blue-yellow continuum.

Diabetic retinopathy
A pathology of the retina that can be attributed to the effects of diabetes. Small haemorrhages in the posterior pole of the retina are characteristic of diabetic retinopathy.

Diffraction
The trajectory of a ray of light is deviated when it passes from one medium to the next. The second object is said to have a different diffraction index.

Direction selectivity
The capacity of some visual system cells to detect and modify their response pattern when a light stimulus with a proper direction of motion is presented.

Diurnal vision
Those processes involved in day vision or vision in a high luminance setting.

Dopamine
It is a neurotransmitter that belongs to the catecholamine family. In the visual system it is present in some amacrine cells of the retina, where it plays a role in the contrast sensitivity of visual cells upstream.

E

Electro-oculography
A measurement technique in which electrodes are placed around the orbit of the eye. It measures and records electrical potentials that are associated with eye movements.

Electrophysiology
The study of the electrical properties of nerve cells. It allows the characterization of the response properties of neurons when they are presented with different stimuli.

Extraocular muscles
Each eye has six muscles which are involved in the movement of the eye.

Extrastriate cortex
Visually responsive areas within the occipital cortex other than the primary visual cortex.

F

Fluorescin
A fluorescent compound that is used as a diagnostic tool.

Fovea
The part of the retina which has the highest density of cone photoreceptors. It is characterized by a slight depression at the centre of the retina. It is also the part of the retina which has the highest visual acuity. When actively observing an object it is this part of the retina that is used.

G

Ganglion cell
Retinal neurons that receive their input from bipolar cells. The axons of the ganglion cells come together to form the optic nerve (blind spot).

Ganzfeld
Half of a sphere which permits the diffusion of light. This in turn allows for an equal stimulation throughout the retina, which eliminates the perception of contours. This apparatus is mainly used in conjunction with an electroretinogram, or in order to determine the extent of the visual field.

Genes
Functional subsets of the DNA sequence. Genes encode all the information necessary to produce any element within the organism.

Genetic engineering
Within the domain of biotechnology, it allows a modification of hereditary information (DNA sequence) of a living cell. This changes either the function or the workings of the cell. Genetic engineering therefore leads to a reprogramming of the target cells.

Genetic therapy
A therapeutic modification of a cells genetic material in order to get the cell to stop performing pathological functions.

Glaucoma
A pathology within the eye which is caused by excessive pressure that compresses the retina. This in turn affects the function of the rods and the cones.

Glial cell
An important type of cell within the nervous system. The main function is to support the functions of the neuronal cell population.

H

Hemeralopia
A condition in which the patient is unable to perceive when there are low light levels or when the patient is unable to adapt to the dark. These patients do not show any abnormal signs during vision in brightly light conditions. It is caused by a dysfunction of the rod photoreceptors. It is often attributable to a lack of vitamin A.

Hemianopsia
Low of vision in half of the visual field.

Hemineglect
An attentional problem that is cortical in nature. It manifests itself by a lack of attention for hemifield that is contralateral to the lesion that causes it. The underlying sensory system is intact.

Hemispherectomy
The ablation (destruction) of a cerebral hemisphere.

Hereditary disease
A pathology, who’s principal cause is genetic in origin. It can be transmitted by one of both parents depending on the nature, dominant or recessive, of the genes involved.

Histoplasmosis
An infectious disease that affects the lungs but it can spread to other parts of the body. Up to 50% of the North American population is believed to have contracted it at least once during their lifetime. Treatment is not usually necessary.

Horizontal cells
Retinal neuron whose soma (cell body) lies within the internal nuclear layer. Its principal role is to modulate the activity of the cone photoreceptors.

Horner’s syndrome
A loss of oculosympathetic innervation that occurs somewhere between the hypothalamus and the eye. It is associated with ptosis, pupillary miosis.

Hyperoxia
An abnormal increase in blood oxygenation level that can cause lesions to different parts of the body, in particular the retina.

Hypertensive retinopathy
A pathology of the retina that is due to arterial hypertension. It appears with vasoconstriction of blood vessels in the retina which may cause haemorrhages and a macular oedema.

Hypoxia
Insufficient concentration of oxygen in the blood and in turn in the tissues.

I

Illuminance
The light that is reflected off of a surface. Units of measurement of illuminance are the lux and candela per metre squared, although many other units of measurement exist.

Illusion
Misinterpretation of external stimuli which leads to the perception of a non-existent or altered stimulus. The illusion occurs when the brain attempts to integrate contradictory signals concerning the stimulus. This serves as a way to create a percept of the real world, the resulting percept may at times be a misinterpretation.

Immunology
The study of processes involved in the defence of the body in the face of attack by pathogens.

In vitro

Any work performed on tissues extracted from a living being (i.e. cell cultures).

In vivo
Any work performed on or within a living being.

Ischemia
A lack of blood circulation circumscribed to a particular part of the body.

Isoluminance
Light sources of equal luminance, measured in a way that is independent of their wavelength.

K

Keratin
Water insoluble protein that is present in the epidermis, hair, and nails that gives these structures their strength and waterproof nature.

L

Lateral geniculate nucleus
A bilateral pair of densely grouped cells situated in the thalamus within the diencephalon. These nuclei receive direct projections via the axons of the ganglion cells from the retina. The neurons within the lateral geniculate nucleus in turn send their axons to the visual cortex. The lateral geniculate nucleus can thus be seen as a relay between the eye and the occipital lobe.

Light adaptation
The time requires for the visual system to decrease its sensitivity due to an increase of ambient light.

Lipid
Insoluble substance that forms the cellular membrane. It also acts as an energy source that is stored as fat.

Low vision
Despite a correction with a proper lens there is a persistent loss of visual acuity.

Luminance
The light emitted by a source of light. It is often measured in candela per metre squared or lux, although many other units of measurement exist.

M

Macula
A yellow spot in the retina which consist of the fovea and a ring surrounding it. It is the part of the retina that is in the centre of our visual field. It is also the part of the retina that has the highest visual acuity, with a peak in the fovea.

Macular degeneration
It is the progressive destruction of the central retina. It is characterized by the formation of blood vessels on the macula. Known factors that augment the chances of suffering from macular degeneration are myopia, diabetes, and the age of the patient.

Macular oedema
Abnormal accumulation of liquid at the macula.

Magnetic resonance imaging
A technique that creates a virtual representation of brain slices or of the body. This images are achieved by data-processing of changes in orientation of molecules that are exposed to a magnetic field.

Magnocellular
It refers to a number of different but related aspects of the visual system; cells, structures and pathways. The magnocellular pathway is implicated in motion processing, and in opposition to its counterpart, the parvocellular pathway, it integrates information over a greater spatial range.

Melanin
A pigment that is derived by metabolic activity. It gives colour to the skin, such as after sunbathing. It is also present in the choroids, iris, retina, and sclera.

Melanoma
Name given to malignant tumours originating in the melanocytes which are the cells which produce the pigmentation of the skin.

Mesencephalon (Midbrain)
A subdivision of the brain stem that is caudal to the diencephalons and rostral to the pons. Among other things, it is composed of the cerebral peduncle and the colliculi (corpora quadrigemina).

Microelectrode
An object that is microscopic and needle shaped. It is made of conductive materials in order to record the electrical activity of neurons.

Microsaccade
A rapid eye movement of small amplitude that occurs during fixation.

Microstrabismus
A strabismus that is on the order of minutes of arc.

Motion aftereffect
An illusion in which prolonged exposure to a moving stimulus causes other stationary stimuli to appear to move in a direction opposite of that of the original stimulus.

Müller cells
Retinal neuroglial cells whose soma (cell body) is situated in the internal nuclear layer. These cells support the metabolic functions of the other cells within the retina.

N

Neovascularisation
An abnormal increase in the vascular network.

Neurodegenerative disease
A pathology that progresses gradually. It is due to a loss of different functional aspects of the nervous system.

Neuron
The anatomical and functional unit of the nervous system. It is a specific type of cell that communicates with other cells via electrical discharge and neurotransmitters.

Neuronal plasticity
The nervous systems capacity to reorganize after a lesion or due to learning by experience.

Neuroophtalmology
A specialization of ophthalmology that centres around those aspects of vision that are related to neuronal function.

Neuroprotection
Refers to those substances which increase the chances of survival of neurons after a traumatic incident. For example; neural growth factor, brain derived neural factor, etc.

Neurotransmitter
A biochemical substance in synaptic vesicles that are liberated at the axonal terminals. These substances cross the synapse which permits the transmission of information between two neurons.

Neurotrophin
A biochemical substance that guides axonal growth during development and maturation of the nervous system.

Night blindness
A relative loss of vision when there is little ambient light (scotopic condition). It is attributed to a deficit in the function of the rod photoreceptors.

Nocturnal vision
Those processes involved in night vision or vision in a low luminance environment.

Nystagmus
An involuntary movement of eyes. It allows fixation to be maintained despite movement of either the observer of the object. It consist of a back and forth motion of the eyes.

O

Occipital lobe
A gross subdivision of the cortical surface of the brain. It is a subset of cortical regions directly involved in vision. It includes both striate and extrastriate cortex. The region is situated in the posterior quarter of the brain.

Oculomotor nerve
It is the third cranial nerve that acts as a relay between the oculomotor nucleus of the mesencephalon and the extraocular muscles. Hence, it is directly involved in the command for eye movements. A lesion of the third cranial nerve causes a divergent strabismus, diplopia, and areactive mydriasis.

Oncology
The study of cancer, or related pathologies.

Optic chiasm
A spatially restricted area where the optic nerve from each eye approach its counterpart. The proximity allows for the crossing of the axons of ganglion cells from each retina to the opposite hemisphere. The axons arising from ganglion cells from the temporal half of the retina remain uncrossed, while those from the nasal half of the retina do cross over.

Optic flow
The pattern of displacement of objects within the visual field caused by locomotion through the environment.

Optic neuritis
A lesion caused by the inflammation of the optic nerve.

Optic radiation
A group of axons, originating in the lateral geniculate nucleus, that project to the primary visual cortex.

Optic tract
The set of retinal ganglion cell axons between the optic chiasm and the brainstem.

Optical imaging
A technique that allows the measurement of electrical activity on the surface of the brain. This is done by using either voltage dependent dyes deposited on the surface of the cortex or by measuring the intrinsic activity of the neurons composing the cortex.

Orientation selectivity
The capacity of some visual system cells to detect and modify their response pattern when a light stimulus with a proper orientation is presented.

Oscillatory potential
Rapid waves that are superimposed on the B wave of the electroretinogram. They oscillate between 100 and 300Hz.

P

Palinopsia
The persistent perception of objects even when they are no longer present. This is distinct from an after image. It can be due to a tumour, vascular disease, trauma, of the use of LSD.

Panum’s area
The distance relative to each eye which causes an image to fall on corresponding points of the retina of each eye.

Parvocellular
It refers to a number of different but related aspects of the visual system; cells, structures and pathways. The parvocellular pathway is implicated in form and colour vision, and in opposition to its counterpart, the magnocellular pathway, it integrates information over a small spatial range.

Pathogenesis
The study of the origins of disease of the mechanisms that bring it about.

Phagocytosis
The capacity of white cells to absorb and destroy a foreign body.

Phagosome
The envelope around the foreign body that is the object of phagocytosis.

Phosphene
The perception of light induced by stimulation that is not visual in nature. For example; magnetic stimulation of the brain (transcranial magnetic stimulation).

Photons
The physical unit that is light. It is radiating energy that is perceived as light.

Photopic adaptation
See light adaptation.

Photoreceptors
Cells in the retina that transduce light into an electrical signal. There are two large subdivisions of photoreceptors, rods and cones. Rods are sensitive to light of all wavelengths and they are involved in vision with low ambient light. Cones work during high ambient light conditions and are involved in colour perception. There are three types, those which are maximally sensitive to short (blue), medium (green), and long (red) wavelengths.

Photosensitve pigments
Biochemical substance found in the photoreceptors. They change their configuration when they are in contact with light, which starts off the photoreceptors reaction to light.

Physiopathology
The study of abnormal function of organisms.

Pigmented epithelium
A monocellular layer placed behind the photoreceptors which aids the latter in their functions. It is pigmented epithelium which furnishes the photoreceptors with the necessary nutrients. It also plays a role in aiding the retina to remain attached to the back of the eye, it maintains the blood/retina barrier, and it synthesizes melanin.

Population genetics
The object is to determine the line of descent of genetically inherited pathologies. This is achieved by studying the family tree of an affected individual in order to isolate how the genetic anomaly is transmitted from one generation to the next.

Positron emission tomography (PET)
It is a technique that allows for an examination of cerebral structure and function in humans. It uses tomography and the injection of radioactive substances which are taken up by the brain. The level of take up is proportional to the level of activity of different brain areas. Thus it is possible to establish relative levels of activity amongst brain areas.

Proprioception
The bodies ability to perceive the position of each different part in space. The sensory information arrives from the muscles, tendons and joints.

Prosopagnosia
The inability to recognise people by looking at their faces. The patients are able to perceive and describe all the elements of the face.

Protanopia
Form of dichromatism. A form of Daltonism that is due to the loss of cones that optimally responsive to long wavelengths (red). This causes a loss of discrimination of colours between the medium and long wavelengths.

Protein
A molecule composed of a linear chain of amino acids.

Pulvinar
A anterior medio-lateral part of the thalamus. It has anatomical and functional links with most visual areas. It is implicated in processes underlying visual attention and motion.

Pupillary reflex
A non-voluntary act that causes the iris to change shape according to the intensity of the light.

Pursuit movement
A slow movement of the eyes during fixation of a moving object. The object thus remains the point of fixation.

Q

Quadranopsie
Loss of vision in one quadrant of the visual system. The quadrants of the visual field are divided by the vertical and horizontal axes.

R

Retina
A transparent membrane that lines the back of the eye. It is composed, among other things, of the photoreceptors which transduce light energy into electrical energy. The retina contains a number of different types of cells, namely photoreceptors, bipolar cells, horizontal cells, ganglion cells, amacrine cells and Müller cells.

Retinal disparity
It is the slight difference of the two images projected onto the retina of each eye. This difference acts as a cue that facilitates depth perception.

Retinitis pigmentosa
Hereditary in origin, it causes a reduction in the size of the visual field, which with time can lead to total blindness. The symptoms are due to an atrophy of the retina along with deposits of melanin pigment and a reduction in the size of blood vessels. There are many different forms of retinitis pigmentosa and there are more than six known abnormal genes involved. Most genes play some role in the junction between the external segment of the photoreceptors where light is transduced into an electrical impulse. Retinopathy
Any pathology that affects the retina.

Retinopathy of prematurity
A retinal pathology present in premature babies that is due to abnormal levels of oxygen in the blood and an immature blood oxygenation regulation system. Problems encountered include haemorrhages within the eye, accumulation of retinal waste materials in the vitreous humour, oedema and a separation of the retina from the back of the eye

Retinoschisis
A genetically caused pathology of the retina that mostly affects the male population. It causes fissures of the retina, in particular along the outer plexiform layer.

Retinoscope
An apparatus whose function is to aid the observation of the retina.

Rhodopsin
A photosensitive pigment of the retina whose molecular shape changes when exposed to light. This in turn brings about a change in electrical potential of the photoreceptor.

Rods
Nervous cells within the retina that are sensitive to light. They are not present in the fovea and their concentration increases toward the periphery. They are most efficient at low light intensities and allow for scotopic vision (i.e. night vision).

S

Saccade
Rapid movement of the eyes toward an object of interest. Saccades are mostly present when freely viewing and inspecting a visual scene. Saccades shift fixation from one point to the next.

Sclerotic
The outer white portion of the eye.

Scotoma
A part of the visual field where there is an abnormal reduction or loss of vision.

Scotopic
The light level that is present at night, which maximally excites the rod photoreceptors.

Scotopic adaptation
See dark adaptation.

Sensory substitution
A plethora of techniques used to give back a visual experience to blind patients by taping into another sensory modality (usually hearing and touch). For example; some systems allow blind patients to locate objects in space using echolocation.

Simple cell
A type of neuron found in the primary visual cortex that, via is electrical discharge rate, can signal the orientation as well as the direction (in certain cases only) of motion of a stimulus. The part of the visual field that modulates its response to visual stimulation (receptive field) is comprised of segregated ON and OFF regions. It thus becomes possible to pinpoint the position of a stimulus within the receptive field based on the response of the cell.

Stereopsis
It is the capacity to perceive depth based on the disparity of an image on the retina of each eye, therefore binocular vision is required.

Striate cortex
The first cortical area to receive massive visual input. Most of the visual information comes from the dorsal lateral geniculate nucleus of the thalamus. It is also known as primary visual cortex and V1.

Superior colliculus
A pair of regions of the brain, more precisely located in the midbrain. They receive neuronal projections from the visual system, both directly from the retina and the cortex. They are involved in eye movements.

T

Tapetum
It is a reflective layer found behind the retina in a number of nocturnal animals (ex: the cat). Animals living in low light conditions can thus use up a maximum number of photons for vision.

Thalamus
A part of the diencephalon that acts as a relay between cortical and other sub-cortical structures. The primary role of the thalamus is to relay sensory information to the brain and motor commands to the proper effectors.

Tritanopia
An abnormal perception of colours in which there is a confusion between crimson/blue, greenish yellow and grey. All three are perceived as shades of grey. The tritanope’s colour spectrum is limited to a red green continuum.

U

Ultraviolet
It is 100 to 400 nanometer waves of light energy that are beyond that which is visible to the human eye. There are three types of ultraviolet light, UVA,UVB,UVC.

Uvea
A set of vascularised structures that are the iris, the ciliary body, and the choroid.

Uveitis
An inflammation of the uvea that can be associated with an inflammation of the optic nerve.

V

Vergence
A movement of the eyes that occurs when fixating an object that is approaching the observer. The eye both move in a nasal direction.

Vernier acuity
A psychophysical task which establishes the smallest spatial extent that a subject or patient can resolve. The subject or patient must determine if two lines are perfectly aligned or not.

Visual cortex
The regions of the cerebral cortex that analyse visual information.

Visual deprivation
An experimental setup in which the subject receives no or limited visual stimulation. This is usually used in developmental studies of the visual system in order to measure the effects of abnormal development.

Visual field
That part of the world that is perceived by the viewer when the eye remains immobile.

Visual pathway
The set of structures involved in the analysis of visual information.

Visually evoked potential
The recording of electrical responses of the brain following visual stimulation.

Vitamin A
A liposoluble substance that is used in the production of rhodopsin. A lack of vitamin A produces a reduction in the quality of night vision.