Seeing

accommodation (focus)

In Vision, bringing objects located at different distances into focus by changing the shape of the lens.

Blind spot

The small area where the optic nerve leaves the back of the eye. There are no visual receptors in this area, so small images falling directly on the blind spot cannot be seen.

cones

cone-shaped photoreceptors in the retina that are primarily responsible for coloured vision and detailed vision

Cornea

The transparent focusing element of the eye that is the first structure through which light passes as it enters the eye. The cornea is the eye’s major focusing element.

Eccentricity

the distance between a position on the retina and the centre of the fovea

Electromagnetic spectrum

Continuum of electromagnetic energy that extends from very-short wavelength gamma rays to long-wavelength radio waves. Visible light is a narrow band within this spectrum.

Emmetropia

The condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball

Eye

the eyeball and its contents, which includes focusing elements, retina, and supporting structures

eyelid

a thin fold of skin that covers and protects the eye

Fovea

A small area in the human retina that contains only cone receptors. The fovea is located on the line of sight, so that when a person looks at an object, the centre of its image falls on the fovea.

Hyperopia

A condition causing poor vision in which people can see objects that are far away but do not see near objects clearly. Also called far-sightedness.

Iris

The coloured part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by changing the pupil size.

Illuminance

A photometric measure of the light incident on a surface (per unit area)

Lens

The transparent focusing element of the eye through which light passes after passing through the cornea and the aqueous humour. The lens’s change in shape to focus at different distances is called accommodation.

Luminance

A photometric measure of the luminous intensity per unit area of light travelling in a given direction

Myopia

An inability to see distant objects clearly. Also called near-sightedness

Optic nerve

Bundle of nerves that carry impulses from the retina to the lateral geniculate nucleus and other structures. Each optic nerve contains about 1 million ganglion cell fibres

Peripheral retina

the area of the retina outside the fovea

Photopic vision

Vision under well-lit conditions (i.e., with light intensities that are bright enough to stimulate cone receptors and bright enough so saturate the rod).

photoreceptor

a light sensitive receptor in the retina of the eye

Presbyopia

the inability of the eye to accommodate due to a hardening of the lens and a weakening of the ciliary muscles. It occurs as people get older

pupil

The opening through which light reflected from objects in the environment enters eye

Reflectance

the percentage of light reflected from a surface

Reflectance curve

a plot showing the percentage of light reflected from an object as a function of wavelength

Retina

A complex network of cells that covers the inside back of the eye. These cells include the receptors, which generate an electrical signal in response to light, as well as horizontal, bipolar, amacrine, and ganglion cells.

Rod

a cylinder-shaped photoreceptor in the retina that is responsible for vision at low levels of illumination

Scotopic vision

Vision under low light conditions (i.e., with light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors).

Selective reflection

When an object reflects some wavelength of the spectrum more than others. Selective reflection is associated with the perception of chromatic colour.

visible light

The band of electromagnetic energy that activates the visual system and that, therefore, can be perceived. For humans, visible light has wavelength between about 400 and 700 nanometres.

visible angle

The angle of an object relative to an observer’s eyes. This angle can be determined by extending two lines from the eye – on to one end of an object and the other to the other end of the object. Because an observer’s visual angle is always determined relative to an observer, its visual angle changes as the distance between the object and the observer changes.

wavelength

For light energy, the distance between one peak of a light wave and the next peak

absorption spectrum

a plot of the amount of light absorbed by a visual pigment versus the wavelength of light

Amacrine cell

a neuron that transmits signals laterally in the retina. Amacrine cells synapse with bipolar cells and the ganglion cells

Bipolar cell

A retinal neuron that receives inputs from the visual receptors and sends signals to the retinal ganglion cells.

Centre-surrounding organization

Arrangement of a neuron’s receptive fields in which one area is surrounded by another area, like the hole in a donut (corresponding to the centre) and the donut (the surround). Stimulation of the centre and surround causes opposite responses.

Ganglion cell

A neuron in the retina that receives inputs from bipolar and amacrine cells. The axon of the ganglion cells are the nerve fibres that travel out of the eye in the optic nerve.

Horizontal call

a neuron that transmits signals laterally across the retina. Horizontal cells synapse with receptors and bipolar cells

Lateral geniculate nucleus (LGN)

A structure in the thalamus that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.

Lateral inhibition

inhibition that is transmitted laterally across a nerve circuit. In the retina, lateral inhibition is transmitted by the horizontal and amacrine cells.

M cells

Ganglion cells with large centre-surround receptive fields that are rather indifferent to the wavelength of light and adapt rapidly

Optic chiasma

The part of the brain where the optic nerve partially cross. The optic chiasm is located at immediately below the hypothalamus

Optic nerve

a paired nerve that transmits visual information from the retina to the LGN and

outer segment

The part of the rod and cone visual receptors that contains light-sensitive visual pigment molecules (the retinal bound top membrane proteins).

P cells

Ganglion cells with small centre-surround receptive fields that are sensitive to the wavelength of light and adapt slowly.

phototransduction

The process by which light energy is transduced into electrical energy in the rod and cone cells in the retina of the eye.

retinal

A molecule bound to membrane proteins in photoreceptors that can absorb photons. It is hence the chemical basis of animal vision.

Retinotopic map

A map on a structure in the visual system, such as the lateral geniculate nucleus or the primary visual cortex that exhibits a spatial correspondence to the retina. In retinotopic maps, locations adjacent to each other correspond to locations adjacent to each other on the retina.

spontaneous activity

Neural activity in the absence of an explicit task, such as sensory input or motor output. It is also referred to as resting-state activity.

striate cortex

the primary visual cortex, also called V1 or Brodmann area 17

Superior colliculus

An area in the brain that is involved in controlling eye movements and other visual behaviours. This area receives about 10 percent of the retinal ganglion cell fibres that leave the eye in the optic nerve.