Vision

• Nearly 70 percent of all the sensory receptors in your whole body are in the eyes
• Nearly half of your entire cerebral cortex has to get involved for you to see, perceive, and recognize something
• Vision is considered the dominant sense of humans
• Light is electromagnetic radiation traveling in waves
• Frequency of a light wave


  Determines its hue
• Amplitude of a light wave


  Relates to its brightness
• We register short waves at high frequencies as bluish colors
• Long, low frequencies look reddish to us
• The visible light we’re able to see is only a tiny chunk of the full electromagnetic spectrum
• Photoreceptors convert light energy into nerve impulses that the brain can understand
• Outer accessories of the eye

  • Eyebrows
  • Eyelashes
  • Eyelids
  • Lacrimal apparatus
• The eyeball is irregularly spherical, with an adult diameter of about 2.5 centimeters
• The eyeball is essentially hollow and full of fluids that help it keep its shape
• You can really only see about the anterior sixth of the whole eyeball
• The eyeball is tucked into a pocket of protective fat and tethered down by six straplike extrinsic eye muscles
• The wall of the eyeball is made up of three distinct layers
• Layers of the eyeball

  • Fibrous layer
  • Vascular layer
  • Inner layer
• Fibrous layer


  • Made of connective tissue
  • Includes sclera
  • Includes cornea
• Cornea


  The transparent part of the fibrous layer that lets light into the eye
• The middle vascular layer contains the posterior choroid, which supplies blood to all layers
• Vascular layer


  • Contains ciliary body
  • Contains iris
• Iris


  The distinctive colored part of the eye that changes the size of the pupil
• Pupil


  The opening in the iris that allows light to travel into the eye
• Light comes in through the cornea and pupil and hits the lens
• Lens


  The convex, transparent disc that focuses light onto the retina
• The retina makes up the inner layer of the back of the eyeball
• The retina contains millions of photoreceptors that convert light energy into electrical signals
• Types of photoreceptors


  • Rods
  • Cones
• Rods


  More numerous, light-sensitive, register grayscale
• Cones


  Detect fine detail and color, divided into red, green, and blue-sensitive types
• Rods are not very good at providing detailed images
• Each cone gets its own personal ganglion cell to hook up with
• Rods connect to ganglion cells in groups, which makes it hard for the brain to tell which individual rods have been activated
• Photoreceptors can make us see afterimages
• Some stimuli are so strong that photoreceptors will continue firing action potentials even after we close our eyes or look away
• Cones can get tired if you stare long enough at a brightly colored image
• The inner layer of the eye consists of the retina and its three kinds of neurons
• Neurons in the retina

  • Photoreceptors
  • Bipolar cells
  • Ganglion neurons
• Bipolar neurons form a kind of bridge between photoreceptors and ganglion neurons
• The optic nerve carries impulses from the retina to the thalamus and then to the brain’s visual cortex