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Chapter 6
Vision
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Vision
 Sensory receptor:
• A specialized neuron that detects a particular
category of physical events.
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The Stimulus
 Perceived color of light is determined by:
 Hue:
 Determined by wavelength
 Saturation:
 Determined by the purity of the light wave
 Brightness:
 Determined by the intensity of the
electromagnetic radiation
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Anatomy of the Visual System
 The Eyes
 Orbits:
 Bony pockets in the front of the skull
 Sclera:
 The white tissue of the eye
 Conjunctiva:
 Mucus membranes that line the eyelid and
protect the eye
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Anatomy of the Visual System
 The Eyes
 Cornea:
 Transparent outer covering of the eye that
admits light
 Pupil:
 Adjustable opening in the iris that regulates
the amount of light that enters the eye
 Iris:
 Pigmented ring of muscles situated behind
the cornea
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Anatomy of the Visual System
 The Eyes
 Lens:
Consists of a series of transparent, onion-like
layers. Its shape can be changed by
contraction of ciliary muscles.
 Accommodation:
 Changes in the thickness of the lens,
accomplished by the ciliary muscles, that
focus images of near or distant objects on the
retina
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Anatomy of the Visual System
 Photoreceptors
 Retina:
 The neural tissue and photoreceptive cells
located on the inner surface of the posterior
portion of the eye.
 Rod:
 Photoreceptor cells of the retina, sensitive the
light of low intensity.
 Cone:
 Photoreceptor cells of the retina; maximally
sensitive to one of three different wavelengths
of light and hence encodes color vision.
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Anatomy of the Visual System
 The Eyes
 Fovea:
 Area of retina that mediates the most acute
vision. Contains only color-sensitive cones.
 Optic Disk:
 Location on retina where fibers of ganglion
cells exit the eye. Responsible for the blind
spot.
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Anatomy of the Visual System
 Photoreceptors
 Bipolar cell:
 A bipolar neuron located in the middle layer of
the retina, conveying information from the
photoreceptors to the ganglion cells.
 Ganglion cell:
 A neuron that receives visual information from
bipolar cells, its axons give rise to the optic
nerve.
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Anatomy of the Visual System
 Photoreceptors
 Lamella:
 A layer of membrane containing photopigments;
found in the rods and cones.
 Photopigment:
 A protein dye bonded to retinal, a substance
derived from vitamin A; responsible for the
transduction of visual information.
 Opsin:
 A class of protein that, together with retinal,
constitutes the photopigments
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Anatomy of the Visual System
 Photoreceptors
 Retinal:
 A chemical synthesized from vitamin A, joins
with an opsin to form a photopigment
 Rhodopsin:
 A particular opsin found in rods
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Anatomy of the Visual System
 Connections between Eye and Brain
 Dorsal lateral geniculate nucleus:
 Cell bodies within the lateral geniculate body
of the thalamus; receives inputs from the
retina and projects to the primary visual
cortex.
 Magnocellular layer:
 One the inner two layers in the dorsal lateral
geniculate nucleus; transmits information
necessary for the perception of form,
movement, depth, and small differences in
brightness to the primary visual cortex.
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Anatomy of the Visual System
 Connections between Eye and Brain
 Parvocellular layer:
 One of the four outer layers of neurons in the
dorsal lateral geniculate nucleus; transmits
information necessary for perception of color
and fine details to the primary visual cortex.
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Coding of Visual Information in the Retina
 Coding of Light and Dark
 Striate cortex:
 The primary visual cortex.
 Optic chiasm:
 A cross-shaped connection between the optic
nerves, located below the base of the brain,
just anterior to the pituitary gland.
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Coding of Visual Information in the Retina
 Coding of Light and Dark
 Calcarine fissure:
 Horizontal fissure on the inner surface of the
posterior cerebral cortex; the location of the
primary visual cortex.
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Analysis of Visual Information:
Role of the Striate Cortex
 Anatomy of the Striate cortex
 David Hubel and Torsten Wiesel
 1960’s at Harvard University
 Discovered that neurons in the visual cortex did
not simply respond to light; they selectively
responded to specific features of the visual world.
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Analysis of Visual Information:
Role of the Striate Cortex
 Modular Organization of the Striate Cortex:
 Striate cortex:
 The striate cortex is divided into approximately
2500 modules, each approximately 0.5 X 0.7 mm
and containing approximately 150,000 neurons.
 Ocular dominance:
 The extent to which a particular neuron receives
more input from one eye than from the other.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Extrastriate cortex:
 A region of the visual association cortex; receives
fibers from the striate cortex and from the
superior colliculi and projects to the inferior
temporal cortex.
 Regions respond to particular features of visual
information such as orientation, movement,
spatial frequency, retinal disparity, or color.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Perception of Color
 Achromatopsia:
 Inability to discriminate among different hues;
caused by damage to the visual association
cortex.
 Inferior temporal cortex:
 In primates the highest level of the ventral stream
of the visual association cortex; located on the
inferior portion of the temporal lobe.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Analysis of Form
 Agnosia:
 Inability to perceive or identify an object by
means of a particular sensory modality.
 Visual agnosia:
 Deficits in visual perception in the absence of
blindness; caused by brain damage.
 Aperceptive visual agnosia:
 Failure to perceive objects by their shape, even
though visual acuity is relatively normal.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Analysis of Form
 Prosopagnosia:
 Failure to recognize particular people by the sight
of their faces.
 Associative visual agnosia:
 Inability to identify objects that are perceived
visually, even though the form of the perceived
object can be drawn or matched with similar
objects.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Perception of Spatial Location
 Balint’s syndrome:
 A syndrome caused by bilateral damage to the
parieto-occipital region; includes optic ataxia,
ocular apraxia, and simultanagnosia.
 Optic ataxia:
 Difficulty in reaching for objects under visual
guidance.
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Analysis of Visual Information:
Role of the Visual Association Cortex
 Perception of Spatial Location
 Ocular apraxia:
 Difficulty in visual scanning.
 Simultanagnosia:
 Difficulty in perceiving more than one object at a
time.
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