retina

(noun)

The thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain.

Related Terms

Examples of retina in the following topics:

  • Anatomy of the Eye

    • However, light does not enter the retina unaltered; it must first pass through other layers that process it so that it can be interpreted by the retina .
    • Presbyopia occurs because the image focuses behind the retina.
    • (b) A blowup shows the layers of the retina.
    • The retina contains photoreceptive cells.
    • Rods and cones are photoreceptors in the retina.

  • Nearsightedness, Farsidedness, and Vision Correction

    • In order for the human eye to see clearly, the image needs to be formed directly on the retina; if it is not, the image is blurry.
    • For the vision to be clear, the image has to be formed directly on the retina.
    • Nearsightedness, or myopia is a vision defect that occurs when the focus of the image is in front of the retina.
    • Farsightedness, or hyperopia, is a vision defect that occurs when the focus of the image is behind the retina.
    • Rays from the top and bottom of the object are traced and produce an inverted real image on the retina.

  • Perceiving Motion

    • First-order motion perception occurs through specialized neurons located in the retina, which track motion through luminance.
    • An object must be directly in front of the retina, with motion perpendicular to the retina, in order to be perceived as moving.
    • The motion-sensing neurons detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighboring point on the retina after a short delay.
    • Second-order motion perception occurs by examining the changes in an objects' position over time through feature tracking on the retina.

  • Optic (II) Nerve

    • The optic nerve (cranial nerve II) receives visual information from photoreceptors in the retina and transmits it to the brain.
    • It transmits visual information from the retina to the brain.
    • The eye's blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.

  • Perceiving Depth, Distance, and Size

    • Visual stimuli enter as light through the photoreceptors in the retina, where they are changed into neural impulses.
    • When an image is projected onto a single retina, cues about the relative size of the object compared to other objects are obtained.
    • Depth perception relies on the convergence of both eyes upon a single object, the relative differences between the shape and size of the images on each retina, the relative size of objects in relation to each other, and other cues such as texture and constancy.
    • This is useful when you cross the street: as you watch a car come toward you, your brain uses the change in size projected on your retina to determine how far away it is.

  • Convergent Evolution

    • There is, however, one subtle difference: the cephalopod eye is "wired" in the opposite direction, with blood and nerve vessels entering from the back of the retina, rather than the front as in vertebrates .
    • In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot (4) where the nerves pass through the retina.
    • In the octopus version, the eye is constructed the "right way out," with the nerves attached to the rear of the retina.

  • Development of Vision

    • The optic cup then delaminates into two layers: the neural retina and the retinal pigment epithelium.
    • The lens placode is affected by the chordamesoderm making it invaginate and form the optic cup composed by an outer layer of neural retina and inner layer of pigmented retina that will unite and form the optic stalk .
    • The pigmented retina is formed by rods and cones and composed of small cilia typical of the ependymal epithelium of the neural tube.

  • The Human Eye

    • Innermost Layer - the retina, which can be seen with an instrument called the ophthalmoscope.
    • An image is formed on the retina with light rays converging most at the cornea and upon entering and exiting the lens.
    • Rays from the top and bottom of the object are traced and produce an inverted real image on the retina.
    • The cornea and lens of an eye act together to form a real image on the light-sensing retina, which has its densest concentration of receptors in the fovea and a blind spot over the optic nerve.
    • The power of the lens of an eye is adjustable to provide an image on the retina for varying object distances.

  • Transduction of Light

    • Light is tranduced in rods and cones; visual information is processed in the retina before entering the brain.
    • A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.
    • Photoreceptors in the retina continuously undergo tonic activity.
    • Exposure of the retina to light hyperpolarizes the rods and cones, removing the inhibition of their bipolar cells.

  • Resolution of the Human Eye

    • As a conscious sense organ the human eye allows vision; rod and cone cells in the retina allow conscious light perception and vision, including color differentiation and the perception of depth.
    • The retina of human eye has a static contrast ratio of around 100:1 (about 6.5 f-stops).