Handbook of Language and Literacy Development - a Roadmap from 0 to 60 Months

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Vision Development (0-60 Months)click to print Print

accommodation – the ability of the crystalline lens inside the eye to alter the focus of the eye in order to view a near or distant object

acuity – clearness of eyesight

amacrine cells, retinal – interneurons in the retina that are responsible for 70% of input to retinal ganglion cells

amblyopia – a reduction in vision even through corrective lenses, with no detectable organic cause; often called “lazy eye”

astigmatism – an error of ocular focus in which a circular object is imaged as an ellipse due to unevenness in the optical power across the meridians of the eye., causing blurred vision; may be corrected with contact lenses, eyeglasses or surgery

axon – long, slender projection of a nerve cell that conducts electrical impulses away from the neuron’s cell body

binocular vision ability to use both eyes simultaneously to focus on the same object, fusing the two images into one

bipolar cells –specialized sensory neurons in the retina; transmit signals from the photoreceptors to the ganglion cells

cataracts form when proteins in the crystalline lens begin to denature leading to lens opacities which then cause blurry vision.

central retina – circular field approximately 6 mm around the fovea

cones a type of retinal nerve cell that is responsible for detailed vision and color vision

contrast sensitivity –the eye’s ability to distinguish between the lightness and darkness of objects

convergence – rotation of the eyes towards each other to look at an object closer by

cortex; cortical the outer covering of the cerebral hemispheres consisting mostly of nerve cell bodies and branches

cycles/degree – a unit of visual acuity which defines the frequency of alternating patterns of light and dark that is captured within an angle of one degree at the eye

dendrites – the branched projections of a neuron that act to conduct the electrical stimulation received from other neurons to the cell body of the neuron from which the dendrites project

diopter (D) – the unit used to designate the refracting power of a lens or the eye, equal to the reciprocal of its focal length (expressed in meters)

emmetropia – the normal refractive state of the eye, in which, with accommodation relaxed, parallel rays of light will converge to a sharp focus on the retina

emmetropization –a natural process that operates in the early years of life to produce a greater frequency of emmetropia by reducing the magnitude and variance of ocular errors of focus

farsightedness; hyperopia a condition of the eye in which objects are imaged behind the retina. Clear vision can be achieved in some cases through increased accommodation until the object becomes conjugate with the retina

fovea the very central part of the retina, used for fine detailed vision

frontal cortex – cortex covering the front regions of each of the cerebral hemispheres. Primarily active in processing motor responses such as eye movements and executive function such as planning, coordinating, controlling and executing behaviour

fusion –the process by which visual stimuli imaged on the two retinas are combined into a single percept

ganglion cells – neural cells which project from the inner surface of the retina to key visual centers in the brain

Gaussian distribution – also referred to as a normal distribution. A bell shaped distribution where the peak of the distribution contains the sample mean and values above and below the mean decline symmetrically in a manner defined by standard deviation of the sample.

geniculo–cortical – connections from the geniculate to the cortical areas

hemispherectomy – a surgical procedure where one cerebral hemisphere is removed; generally used in extreme cases to treat a variety of seizure disorders

horizontal cell – laterally connecting neurons in the retina

inferior temporal – area in the temporal lobe of the brain

isotropic – independent of direction

lateral geniculate body (LGN) – a thalamic relay nucleus of the brain

lens, crystalline –the lens located within the eye that acts to vary the eye’s focal power

leptokurtic – a distribution with an acute peak around the mean

magnocellular – cells in the brain concerned primarily with detecting motion or spatial configuration

monotonic – a function which preserves the given order; e.g. where each value is equal to or greater than the previous one or each value is less than or equal to the previous one

morphology – the study of the shape and form of things in general

myelin; myelination fatty insulation around an axon which protects the nerve fibers and improves the speed of conduction of nerve impulses

neural circuits; neural networks – connections of thousands of neurons with extraordinary specificity that is essential for normal function; a network or circuit of functionally related neurons that perform a specific physiological function

neuroanatomy – the anatomical organization of the nervous system

neurophysiology – study of the nervous system function

nucleus of the optic tract – nucleus in the pretectal area of the brain

occipital cortex the cortex covering the posterior lobe of the brain; involved with vision; also referred to as “visual cortex”

oculomotor – pertaining to the movement of the eye

opacity – the condition of being opaque, such as corneal or lenticular opacity

optic nerve – a bundle of ganglion cell axons which projects from a single eye to the joining fibres from the fellow eye at the optic chiasm and synapsing at the lateral geniculate body (LGN) optic tract – continuation of the optic nerve which runs from the optic chiasm to the lateral geniculate nucleus (LGN)

optics – branch of physics that describes the behaviour and properties of light

optokinetic nystagmus – physiological nystagmus (a repetitive, rhythmic involuntary movement of the eyes) induced by attempting to fixate objects traversing the visual field, as when viewing out the window of a moving train.

optotype – standardized symbol such as specially shaped letters or geometric symbols for testing vision

ophthalmologist – a medical doctor who specializes in the diagnoses and treatment of eye diseases, surgery of the eyes, and can prescribe glasses and contact lenses

optometrist – a primary eye care provider who prescribes glasses and contact lenses, and diagnoses and treats certain conditions and diseases of the eye

parietal cortex the cortical area covering the middle lobe of each cerebral hemisphere between the frontal and occipital lobes; it contains important sensory centers

parvocellular – neurons of the optic system that project from the retinal ganglion cells to the lateral geniculate nucleus; sensitive to colour and more capable of discriminating fine details than magnocellular neurons

peripheral retina – area beyond the central retina

photoreceptors – nerve cells in the retina that are specialized for capturing light; subdivided into rods and cones

pretectal nucleus – group of cells constituting several subnuclei in the pretectal area receiving input from retinal ganglion cells

pulvinar – nucleus of the thalamus of the brain

reticular activating systems – part of the brain believed to be the center of arousal and motivation

retina – thin sensory tissue which lines the back of the eye; contains millions of photoreceptors (rods and cones) that capture light and convert them into electrical impulses that are sent the brain by way of the optic nerve

retino-geniculate pathway – neural connections from the retina to the geniculate

rods – receptor cells in the retinas that are sensitive to light and are primarily responsible for peripheral and night vision

saccade an eye movement where each eye moves in the same direction in order to quickly and voluntarily shift gaze from one target to another

sinusoidal pattern – a pattern that varies in space or in time in a sinusoidal fashion

smooth pursuit eye movements – the ability of the eyes to smoothly follow a moving object; the object’s velocity is detected and matched by the resulting eye movement

spatiotopic – spatial organization of the neuronal receptive fields, such that adjacent neurons have receptive fields that include slightly different, but overlapping portions of the visual field

stereopsis the ability to perceive a three dimensional depth which requires adequate fusion of the images from each eye followed by disparity detection

stereo acuity – the smallest angular subtense of a retinal disparity that can be seen in depth.

sub-cortex; subcortical – the portion of the brain immediately below the cerebral cortex

superior colliculus – a paired subcortical structure in part of the brain’s tectal area which processes visual information

synapse – the junction between the axon terminals of a neuron and the receiving cell

temporal cortex the cortex covering the lobe located below the frontal and parietal lobes; involved in perception and recognition of auditory stimuli and memory

trichromacy – the condition of possessing three independent channels for conveying colour information derived from the three cone types which possess distinct visual pigments

uniocular – pertaining to one eye

vergence – the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision

vestibular ocular reflex – a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement

visual acuity –the ability to distinguish object details and shape; the ability to resolve two separate objects in terms of their angular subtense at the eye

visual cortex located in the occipital lobe; involved in detection of simple visual stimuli

visual evoked potential – an evoked potential caused by sensory stimulation of a subject’s visual field

visual pathway route of the nerve impulses from the retina along the optic nerve, and optic nerve radiations to the brain's sensory cortex that is located at the base of the skull

wavelength – the distance in a periodic wave where a complete cycle of modulation has occurred