4.2—the visual system Flashcards Preview

🚫 PSY100H1: Introduction to Psychology (Winter 2016) with J. Vervaeke > 4.2—the visual system > Flashcards

Flashcards in 4.2—the visual system Deck (26):
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4.2 Learning Objectives

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4.2 Focus Questions

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How the Eye Gathers Light

  • wavelength: the distance between peaks of a wave; long wavelengths correspond to our perception of reddish colours and short wavelengths correspond to our perception of bluish colours.
    • if a large portion of the lightwaves are clustered around one wavelength, you'll see an intense, vivid colour.
    • if there are a large variety of wavelengths being viewed at the same time, the colour wlil appear washed out.
  • amplitude: the height of a wave; low amplitude waves are seen as dim colours, whereas high-amplutide waves are seen as bright colours.

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The Structure of the Eye (Image)

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Sclera

the white, outer, surface of the eye.

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Cornea

is the clear layer that covers the front portion of eye and also contributes to the eye's ability to focus.

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Pupil

regulates the amount of light that enters by changing its size; it dilates (expands) to allow more light to enter and constricts (shrinks) to allow less light into the eye.

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Iris

a round muscle that adjusts the size of the pupil; it also gives the eyes their characteristic colour.

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Lens

  • lens: a clear structure that focuses light onto the back of the eye.
  • accomodation: the lens changing its shape to ensure that the light entering the eye is refracted in such a way that it is focused when it reaches the back of the eye.
  • transduction: when light reaches the back of the eye, it stimulates a layer of specialized receptors that convert light into a message that the brain can then interpret.

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Retina

lines the inner surface of the eye and con- sists of specialized receptors that absorb light and send signals related to the properties of light to the brain. 

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Photoreceptors

  • photoreceptors: where light will be transformed into a neural signal that the brain can understand.
  • having the photoreceptors wedged into the back of the eye protects them and provides them with a constant blood supply, both of which are useful to your ability to see.
  • ganglion cells: gather information from the photoreceptors, which will alter the rate at which the ganglion cells fire; they then send information out of the brain through the optic nerve.

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Optic Nerve

  • optic nerve: a dense bundle of fibres that connect to the brain.
  • optic disc: an area on the retina with no photoreceptors.
  • blind spot: a space in the retina that lacks photorecepters (as a result of the optic disk).
  •  

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Rods

  • rods: photoreceptors that occupy peripheral regions of the retina; they are highly sensitive under low light levels.
  • limited to the periphery of the retina.
  • have a ten-to-one ratio with ganglion cells.
  • explains why colourful stimuli are often perceived as sharp images while shadowy grey images are perceived as being hazy or unclear.

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Cones

  • cones: photoreceptors that are sensitive to different wavelengths of light that we perceive as colour.
  • cones tend to be clustered around the fovea.
  • have a one-to-one ratio with ganglion cells.

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Fovea

the central region of the retina.

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Dark Adaptation

is the process by which the rods and cones become increasingly sensitive to light under low levels of illumination.

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Trichromatic Theory (Young-Helmholtz Theory)

maintains that colour vision is determined by three different cone types that are sensitive to short, medium, and long wavelengths of light.

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Opponent-Process Theory

  • opponent-process theory: we percieve colour in terms of opposing pairs: red to green, yellow to blue, and white to black.
  • a cell that is stimulated by red is inhibited by green; when red is no longer perceived (as when you suddently look at a white wall), a "rebound" effect occurs.

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Common Visual Disorders

  • nearsightedness: (or myopia) occurs when the eyeball is slightly elongated, causing the image that the cornea and lens focus on to fall short of the retina.
  • farsightedness: (or hyperopia) occurs when the length of the eye is shorter than normal, causing the image to be focused behind the retina.

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Optic Chiasm

  • optic chiasm: the point at which the optic nerves cross at the midline of the brain.
  • ipsilateral: half of the nerve fibres travel to the same side of brain.
  • contralateral: half ot the nerve fibres travel to the opposite side of the brain.
  • the left half of your visual field is initially processed by the right hemisphere of your brain, whereas the right half of your visual field is initially processed by the left hemisphere of your brain.
  • having both eyes send some information to both hemispheres increases the likelihood that some visual abilities will be preserved.
  • lateral geniculate nucleus (LGN): specialized for processing visual information; fibres from this nucleus send messages to the visual cortex.
  • feature detection cells: these cells respond selectively to simple and specific aspects of a stimulus, such as angles and edges.
    • researchers can map the areas feature detection cells respond by measuring the firing rates of groups of neurons in the visual cortex in lab animals.

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The Ventral Stream

  • the ventral stream of vision extends from the visual cortex in the occipital lobe to the anterior (front) portions.
  • the division of our visual system serves for object recognition.
  • prosopagnosia: face blindness; individuals are able to recognize voices and other defining features of individuals, but not faces.
  • perceptual constancy: the ability to perceive objects as having constant shape, size, colour despite changes in perspective.
    • shape constancy: we judge the angle of the object relative to our position.
    • colour constancy: allows us to reocgnize an object's colour under varying levels of illumination.

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The Dorsal Stream

  • the dorsal stream of vision extends from the visual cortex in our occipital lobe upwards of the parietal lobe.
  • it locates an object in space and allows you to interact with it.
  • e.g. you need the dorsal stream to recognize that the object in front of you is a cup, the liquid inside is caffeine, and that you can drink it. you decide you want to have a sip, and thus require your arm to move your hand to grasp the mug.

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Binocular Depth Cues

are distance cues that are based on the differing perspectives of both eyes.

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Convergence

occurs when the eye muscles contractso that both eyes focus on a single object.

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Retinal Disparity

  • retinal disparity: the difference in relative position of an object as seen by both eyes, which provides information to the brain about depth.
  • stereoscopic vision: overlapping fields of vision, which most primates have.

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Monocular Cues

  • monocular cues: are depth cues that we can perceive with only one eye.
  • one is accommodation.
  • motion parallax: used when you or your surroundings are in motion.

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