Download The Eye

The Eye
A Model for Sensing
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Three components:
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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
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Retina
Optic nerve (axons of ganglia in eye)
Layers in the thalamus (LGN)
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Secondary paths to SCN (circadian rhythms) and
superior colliculi to control eye movement
Primary visual cortex
Adaptation
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Adaptation -- diminishing receptiveness of a
sensory receptor neuron.
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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
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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
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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
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Rods – used for brightness perception and motion
Cones – used for color and form (shape) perception
Cones are wavelength-specific:
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Blue = 430 nm
Green = 530 nm
Red = 560 nm
Mixing all three equally produces white
Transduction
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Photoreceptors release the neurotransmitter
glutamate (glu) when depolarized.
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Only ganglion cells have action potentials.
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Depolarized in the dark.
Hyperpolarized by light.
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
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The receptive field is the area of the retina
capable of changing the bipolar cell’s
membrane potential
Two kinds of receptive fields:
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OFF cell – excitatory
ON cell – inhibitory
OFF and ON refers to light, not the cell
Center and surround are opposites
Edge Detection
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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
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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.