visual system

• 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

  -->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
• Vertebrates-are designed to have two eyes because they have two sides: left and right.

  
  

  According to evolutionary perspective:

  • Predators have front-facing eyes to perceive distance of prey
  • Prey animals have side-facing eyes to see predators approaching from different directions
• Binocular disparity


  Difference in the position of the same image on the two retinas, greater for close objects
• Retina


  • Converts light to neural signals
  • Composed of 5 layers of different neurons
• Rods


  Rod-like receptors that provide twilight vision but no color vision (scotopic vision)
• Cones

  Cone-shaped receptors that provide color vision and high-acuity perception (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


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

  
• Signals from the left visual field reach the right primary visual cortex, and vice versa
• Edge perception


  Edges are the most informative features as they define the extent and position of objects
• Mach Bands

  • Nonexistent stripes of brightness and darkness running adjacent to edges that enhance the contrast and make edges easier to see
• Contrast enhancement at edges is a mechanism of the nervous system
• 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 and makes them more visible
• 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
• Perception of an object's color

  • Depends on the wavelengths of light that it reflects into the eye
  • Not the entire story
• Trichromatic theory


  There are three different kinds of color receptors (cones): red, blue, and green
• Opponent-process theory


  • There are two different classes of cells in the visual system; one for encoding color and another class for encoding brightness
  • Cells can only detect the presence of one color at a time because the two colors oppose one another
• Complementary colors


  Pairs of colors that cancel each other out (lose hue) 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
• Types of visual cortex


  • Primary Visual Cortex - the area of the cortex that receives most of its input form the visual relay nuclei of the thalamus
  • Secondary Visual Cortex - the area of the cortex that receives most of their input from the primary cortex
  • Visual Association Cortex - the area of the cortex that receives input from areas of secondary visual cortex as well as the secondary areas of other sensory system.
• Scotoma


  An area of blindness in the corresponding area of the contralateral visual field of both eyes

  
• Hemianopsia


  Having scotoma covering half of the visual field

  • May see an entire face when they focus on a person's nose, even when the side of the face in the scotoma has been covered by a blank card.
• Agnosia


  A failure of recognition
• Types of visual agnosia

  • Movement agnosia - difficulty in recognizing movements
  • Object agnosia - difficulty in recognizing objects
  • Color agnosia - difficulty in recognizing colors
• Prosopagnosia


  Visual agnosia for faces
• Akinetopsia


  Difficulty in the ability to see movement progress in a normal smooth fashion
• Two properties of light

  • Wavelength
  • Intensity
• Iris


  is the donut-shaped bands of contractile tissue which gives our eyes their characteristic color. It is also responsible for the regulation of the light that reaches the retina.
• Pupil


  is the hole in the iris where light enters the eye.