Visual System Mod 3

• Light


  Waves of electromagnetic energy that are between 380 and 760 nanometers (billionths of a meter) in length
• Light


  • Wavelength plays an important role in the perception of color
  • Intensity plays an important role in the perception of brightness
• How light enters the eye and reaches the retina

  1. Iris regulates the light that reaches the retina

  2. Pupil is the hole where light enters the eye

  3. Lens focuses incoming light on the retina

  4. Accommodation - process of adjusting the lens to bring images into focus on the retina
• Bright light


  Constricted pupils, sharper image, greater depth of focus
• Low light


  Dilated pupils, less acuity and depth of focus

  
• Eyes


  • Vertebrates have two eyes on the front of their heads (predators) or on the sides (prey)
  • Eyes must converge (turn inward) to project the same point in the visual world onto corresponding points on the two retinas
  • Binocular disparity - difference in the position of the same image on the two retinas, greater for close objects

  
• Retina


  Converts light to neural signals, conducts them to the CNS, and participates in processing the signals
• Retinal cell types


  • Photoreceptors (rods and cones)
  • Amacrine cells
  • Horizontal cells
  • Bipolar cells
  • Retinal ganglion cells

  
• Rods


  Rod-like receptors which provide twilight vision but do not give color vision (scotopic vision)
• Cones


  Cone-shaped receptors which provide color vision (photopic vision)
• Lack of rhodopsin in rods may cause night blindness
• Lack of iodopsin in cones may cause color blindness
• From retina to visual cortex

  1. Signals from each retina are conducted to the primary visual cortex via the lateral geniculate nuclei of the thalamus

  2. Signals from the left visual field reach the right primary visual cortex, and vice versa

  
• Edge perception


  • Edges are the most informative features of a visual display, defining the extent and position of objects
  • Mach Bands - nonexistent stripes of brightness and darkness running adjacent to edges, enhance contrast and make edges easier to see
• Contrast enhancement


  Process that highlights the edges and makes them easier to see
• Contrast enhancement occurs in the nervous system, not just in books
• Mach Bands


  Phenomenon where the nervous system highlights edges
• Black


  Experienced when there is an absence of light
• White

  Produced by an intense mixture of a wide range of wavelengths in roughly equal proportions
• Gray


  Produced by the same mixture as white but with lower intensities
• Hues


  The correct term for colors
• Color perception


  Depends on the wavelengths of light that an object reflects into the eye
• Trichromatic Theory


  • There are three different kinds of color receptors (cones): red, blue, and green
  • Blue receptors are the most sensitive and red the least
  • The ability to perceive color requires interaction between at least two types of photoreceptors
  • These three colors can then be combined to form any visible color in the spectrum

  
• Opponent-Process Theory


  • There are two different classes of cells in the visual system; one for encoding color and another class for encoding brightness
  • The opponent process theory suggests that our ability to perceive color is controlled by three receptor complexes with opposing actions: red-green, blue-yellow, and black-white
  • These cells can only detect the presence of one color at a time because the two colors oppose one another
  • Complementary colors are pairs of colors that cancel each other out and produce white or gray when combined in equal measures
• Neither component nor opponent processing can account for color constancy
• Color constancy


  The perceived color of an object is not a simple function of the wavelengths reflected by it, but a feature of the human color perception system that ensures perceived color remains relatively constant under varying illumination conditions
• Types of visual cortex

  • Primary Visual Cortex
  • Secondary Visual Cortex
  • Visual Association Cortex
• Primary Visual Cortex


  • Receives most of its input from the visual relay nuclei of the thalamus
• Secondary Visual Cortex


  • Receives most of their input from the primary cortex
• Visual Association Cortex


  • Receives input from the areas of secondary visual cortex as well as the secondary areas of other sensory systems
• As one moves up the visual hierarchy, the neurons have larger receptive fields and the stimuli to which the neurons respond are more specific and more complex
• Scotoma


  An area of blindness in the corresponding area of the contralateral visual field of both eyes, caused by damage to the primary visual cortex

  
• Hemianopsia


  Having a scotoma covering half of the visual field
• Agnosia


  A failure of recognition, where a person can see visual stimuli but doesn't know what they are
• Types of visual agnosia

  • Movement agnosia
  • Object agnosia
  • Color agnosia
• Prosopagnosia


  Visual agnosia for faces, where a person recognizes a face as a face but has problems recognizing whose face it is
• Prosopagnosia


  • Often associated with damage to the ventral stream in the area between the occipital and temporal lobes, known as the fusiform face area
  • This area also selectively responds to classes of visual stimuli other than faces

  
• Akinetopsia


  A rare condition where a person experiences difficulty in seeing movement progress in a normal smooth fashion, associated with damage to the middle temporal (MT) area of the cortex