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					General Principles of Perception
Each Receptor is Specialized
to Absorb One Kind of
Energy & Transduce it into
an Electrochemical Pattern in
the Brain
Coding of visual information in the brain
does not duplicate the shape of the
object

Law of Specific Nerve
Energies
Any activity by a particular nerve
always conveys the same kind of
information to the brain
The Eye/Brain Connection
              Structure
              Light enters the eye through the Cornea
              & the Pupil
              It is focused by the Cornea & Lens &
              projected on to the Retina
              Retina
              The rear surface of the eye which is
              lined with visual receptors
              The Route
              Receptors send messages to the Bipolar
              Cells, which send messages to Ganglion
              Cells
              Amacrine Cells are important for
              complex processing of visual
              information
              Ganglion Cells join together to form the
              Optic Nerve
                The Fovia & the Periphery of the Retina

Macula
Portion of the Retina with the greatest
ability to resolve detail
Fovea
Central potion of the Macula specialized for
acute, detailed vision
Fovea has the least impeded vision
Each receptor connects to a single Bipolar
Cell which connects to a single Ganglion
Cell
Midget Ganglion Cells
Receive input from a single cone
Each cone has a direct line to the brain
        Visual Receptors
Rods
Abundant in the periphery of the Retina
For Periphery & Night Vision
Cones
Primarily in the Fovea
For Visual Acuity & Color Vision
Photopigments: Chemicals that release energy when struck by light
Color Vision
       Requires Comparing Responses of
       Different Kinds of Cones
       Shortest to Longest Wavelengths
       Shortest wavelength seen as
       Violet, longest wavelengths seen
       as Blue, Green, Yellow, & Red
       Two Main Theories
       Trichromatic Theory
       Opponent-process Theory
               Color Vision
Retinex Theory
(The Land
Effect)
Proposed to account for Color
Constancy
When information from various
parts of the retina reaches the
cortex, the cortex compares
each of the inputs to determine
the brightness & color
perception for each area
Colorblindness
        Color Vision
        Deficiency
        Seen mostly in Males
        Red-Green colorblindness is most
        common
        On the X-chromosome
        X-linked disorder
The Visual System
Rods & Cones Synapse
with Horizontal & Bipolar
Cells
Horizontal cells make inhibitory contact
onto bipolar cells which synapse with
amacrine and ganglion cells
Axons of the Ganglion Cells
Form the Optic Nerve
Optic nerves from both eyes meet at the
optic chiasm where ½ of the axons from
each eye cross to the opposite side of the
brain
Most of the ganglion cells go to the Lateral
Geniculate Nucleus of the thalamus
Mechanisms of Visual Processing
Receptive Fields
Visual Field
     The area of the world that you can see at any time
Receptive Field
     The portion of the visual field to which any neuron
     responds
Lateral Inhibition
     The reduction of activity in one neuron by activity in
     neighboring neurons
     This is the retinal technique that sharpens the
     boundaries of visual objects
       Neurons in the Visual
            Pathways
Parvocellular                                    Koniocellular
Neurons                                          Neurons
Small cell bodies located in or near the fovea   Similar in size to Parvocellular
with small receptive fields & respond best to    Neurons, but distributed throughout the
details & color                                  retina
They synapse only onto cells of the LGN
                                                 They have several different functions &
Magnocellular                                    their axons connect to the LGN, other

Neurons
                                                 areas of the Thalamus, & the Superior
                                                 Colliculus
Larger cell bodies distributed
throughout the retina & have a larger
receptive field responding best to
moving stimuli
Most synapse onto cells of the LGN, but
a few connect to other areas of the
Thalamus
In the Cerebral Cortex
Most Axons from the
LGN go 1st to the Primary
Visual Cortex (V1)
V1 sends information to the
Secondary Visual Cortex (V2)
Connections between V1 & V2 are
reciprocal
In the cortex, Parvocellular &
Magnocellular pathways split from
2 to 3 pathways
Parvocellular is sensitive to shape
Magnocellular is sensitive to
movement
The mixed pathway is sensitive to
brightness & color
Object Recognition
          Ventral Stream
          Made up of parvocellular &
          magnocelluilar pathways
          Goes through V1, V2, V4 & areas of the
          Inferior Temporal Lobe
          Sensitive to shape, movement & color
          brightness
          Specialized for object recognition &
          identification

          Dorsal Stream
          Mostly magnocellular pathways
          From V1 to Parietal & to Temporal
          Lobes
          Integrates vision & movement leading
          to the Parietal Lobe
Categories of Neurons in the
      Cerebral Cortex
Simple Cells
Neurons with fixed excitatory & inhibitory zones in their receptive fields
Found only in the Primary Visual Cortex (V1)

Complex Cells
Receive input from a combination of Simple Cells
Have receptive fields that respond to particular orientations of light but cannot be
mapped into fixed excitatory & inhibitory zones
Located in V1 or V2

End-stopped (Hyper-complex) Cells
Strongly resemble complex cells but have an inhibitory area at one end of its bar-
shaped receptive field
Recognition of Shape
Cells in the Visual
Cortex are in Columns
Set perpendicular to the surface
according to response orientation

Feature Detectors
Neurons whose responses indicate
the presence of a particular
feature

Inferior Temporal
Cortex
Provides information about complex
shaped stimuli
Important in Shape Constancy
Disorders of Object Recognition
                 Visual Agnosia
                 The inability to recognize objects
                 despite otherwise normal vision
                 Prosopagnosia
                 The inability to recognize faces
                 without an overall loss of vision or
                 memory
                 The Fusiform Gyrus in the Inferior
                 Temporal Cortex is specialized for
                 face recognition
                 This area is also activated when
                 identifying car models, bird species,
                 and so on
Color, Motion, &
     Depth
Color Perception Depends
on Parvocellular &
Koniocellular Pathways
Blobs
Patches of cells in V1 highly
sensitive to color areas
Includes Parvocellular &
Koniocellular neurons for color &
Magnocellular for brightness
Output is sent to V2, V4, &
Posterior Inferior Temporal Cortex
Creating Stereo Images
             Anaglyph 3-D
             Uses red/blue lenses on
             glasses

             Cross-eyed 3-D
             Must cross eyes to create a
             single image or use lenses that
             create the image

             Polarized Lens 3-D
             Use of polarized lenses on
             glasses
Motion Detection
Medial Temporal Cortex
Middle Temporal Cortex & Medial
Superior Temporal Cortex important in
motion detection
Mechanisms to Distinguish
between Moving Objects & Head
Changes
Damage to Medial Temporal Cortex
results in motion blindness
Importance of V1 Area
Activation & feedback to V1 area necessary for attention or conscious awareness of a
stimulus

Binding Necessary for
Consciousness
Synchronized activity of the 2 hemispheres necessary to see something that crosses
the midline of vision as a single object
A limited amount of visual processing takes place without being conscious
       Blindsight
       Some people with extensive damage to V1 can localize visual objects with a
       blind visual field
            Development of the Visual System

Infant Vision
Infants have better vision than once imagined
Spend more time looking at faces, circles, or stripes than at patternless
displays
They have trouble shifting their gaze until about 6 months
The Effects of Experience
Lack of Early                             Sensitive or Critical
Stimulation                               Period
In One Eye: Most neurons in the Visual    A stage of development when
Cortex receive binocular input.           experiences have a particularly
Deprivation leads to blindness in the     strong & long-lasting influence
one eye                                   Effects of abnormal experiences on
In Both Eyes: If both eyes are            cortical development depend on the
deprived of stimulation, cortical cells   length of the sensitive period
will remain sluggishly responsive in      In humans, even a brief abnormal
both eyes                                 experience can result in deficits
People born blind but acquiring
vision later have trouble identifying
shapes & objects & find newly
gained vision almost useless
Restoring Response after Early
          Deprivation
                Depends on When
                If normal experiences begun soon
                enough, sensitivity can be restored
                Amblyopia
                Lazy Eye, can be treated by putting
                a patch over the active eye
Stimulation in Both Eyes
Retinal Disparity
The discrepancy between the
left & the right eye sees
It is necessary for
stereoscopic depth perception
The fine-tuning of binocular
vision depends on experience
Strabismus
The eyes do not point in the
same direction
Cannot perceive depth better
with 2 eyes as opposed to 1
Astigmatism
      Blurring of Vision in
      One Direction
      Caused by an asymmetric curvature of
      the eyes
      Corrective lenses in early childhood
      improve the vision

      Early Blind
      Certain portions of the Visual Cortex in
      people blind early in life become
      responsive to auditory or touch stimuli

				
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posted:4/9/2011
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