The Eye

1
The Eye
2
A Model for Sensing

• Three components
• Stimulus detection a specialized sensory neuron
• Reception where neurons receive information
  from the sensory neurons
• Integration where information from receivers is
  processed
• All the human sensory systems have these
  components.

3
The Visual System

• Retina
• Optic nerve (axons of ganglia in eye)
• Layers in the thalamus (LGN)
• Secondary paths to SCN (circadian rhythms) and
  superior colliculi to control eye movement
• Primary visual cortex

4
Adaptation

• Adaptation -- diminishing receptiveness of a
  sensory receptor neuron.
• Habituation also diminishing receptiveness but
  at a different level (within a neuronal circuit
  not a cell).
• Adaptation is essential for the perception of
  change.
• Dark and light adaptation permit vision under
  different environmental conditions.

5
Parts of the Eye

• Cornea protects eye and initiates focusing
• Lens focuses light on the appropriate spot on
  the retina
• Iris adjusts focus by opening and closing pupil
  to admit light
• Pupil hole that admits light

6
More Parts of the Eye

• Aqueous and vitreous humor fluid inside eye
• Sclera whites of eye
• Retina layer of photoreceptors at the back of
  the eye, responsive to light
• Blind spot place where optic nerve exits the
  eye
• Fovea spot of best focus and densest cones

7
Types of Photoreceptors

• Rods used for brightness perception and motion
• Cones used for color and form (shape)
  perception
• Cones are wavelength-specific
• Blue 430 nm
• Green 530 nm
• Red 560 nm
• Mixing all three equally produces white

8
Transduction

• Photoreceptors release the neurotransmitter
  glutamate (glu) when depolarized.
• Depolarized in the dark.
• Hyperpolarized by light.
• Only ganglion cells have action potentials.
• Photoreceptors produce graded response that
  provides input aggregated by bipolar cells.
• Magno ganglion cells receive input from rods,
  parvo ganglion cells from cones

9
Bipolar Cell Receptive Fields

• The receptive field is the area of the retina
  capable of changing the bipolar cells membrane
  potential
• Two kinds of receptive fields
• OFF cell excitatory
• ON cell inhibitory
• OFF and ON refers to light, not the cell
• Center and surround are opposites

10
Edge Detection

• The center-surround organization of the receptive
  fields of ganglion cells exaggerates the contrast
  at borders.
• Visual processes fill in what occurs between
  borders (edges).
• Contrast effects occur because we notice
  variations, not absolute magnitudes of light.

11
Color Contrast

• Cones respond to specific wavelengths of light
  that determine hue.
• Color cells have complementary surrounds that
  heighten contrast and strengthen their signal.
• Opponents are red/green, blue/yellow.