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The Eye. 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.

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Presentation Transcript
a model for sensing
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.
the visual system
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
adaptation
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.
parts of the eye
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
more parts of the eye
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
types of photoreceptors
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
transduction
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
bipolar cell receptive fields
Bipolar Cell Receptive Fields
  • The receptive field is the area of the retina capable of changing the bipolar cell’s 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
edge detection
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.
color contrast
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.