Sensory Processing: Vision

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Sensory Processing Vision
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Sensation Perception

• Pattern recognition
• Multiple senses
• Other neural processes
• (memory, emotion)

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Brain Doesnt Analyze Sensory Input as a Uniform
Thing

• Different aspects analyzed in specialized
  regions.
• EX color of object processed in one place, shape
  of an object is analyzed somewhere else.

Thus, the brain gives perception of unified
object.
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Sensation
Stimulus
Thalamus
Sensory Receptor (modifiedneuron)
Primary Sensory Cortex
Perception
Other Cortical Areas
Response
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Sensory Receptors
Are Key Translators
Stimulus ? Neural Signal Receptor Potential
Sensory Transduction
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Vision
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Stimulus
light
Thalamus
LGN
Sensory Receptor (modifiedneuron)
Primary Sensory Cortex
occipital lobe
photoreceptors
Perception
Other Cortical Areas
Response
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Anatomy of the Visual System
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LGN
Visual Field
Primary Visual Cortex
Retina
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The Eye Works Like a Camera
Fig 8.3

• Lens focuses light onto photoreceptors in retina
  which convert it into membrane potentials
• Lens projects inverted image, brain adjusts
  inversion so we see world in correct orientation

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Photoreceptors - specialized neurons
Rods Dim light Black, white 120
million Cones Bright light Color (R, G, B) Fine
detail Most dense - FOVEA 6 million
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Fig 8.5
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Visual Path within the Eye
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Some Processing Starts in the Retina
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• Ganglion axons converge at optic disk before
  exiting eye
• forming optic nerve.
• Optic disk Blindspot. For fun try blindspot
  demo Fig 8.4

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Right visual to Left side of brain Left Visual to
Right side of brain.
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Visual Pathways show Parallel Processing
Reflex processing
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The LGN
Visual info from retina to brain is mapped in an
orderly fashion onto neurons in the thalamus
This topographic mapping continues when LGN sends
info to visual ctx
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Occipital Cortex
Different areas process different aspects of
visual info. V1 motion, shape color info that
is segregated
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This segregation remains throughout visual ctx
Fig 8.19
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Perception interpretation? Other brain areas
beyond visual ctx put
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• Info now moves out of visual cortex into parietal
  temporal regions forming dorsal ventral
  streams

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So, how do we "see"?
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Location

• Ganglion cell receptive field represents region
  on retina that activates it. i.e. location info
• Each ganglion sees a small portion of world

Location info is maintained in LGN
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Receptive fields get larger but maintain
topographic map
Fig 8.23

• Topographic map is maintained all the way up to
  V1 such
• that each neuron has a receptive field which
  corresponds to a specific part of the retina.

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Line
Retina
Lateral geniculate nucleus
Primary Visual Cortex (V1)
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Shape

• Retina sends signals about edges (edges are
  what form shape)
• Brain there are specific cells that are
  sensitive to specific characteristics of the
  visual world (shape, movement, color)

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V1 Cells respond to more complex stimuli
Fig 8.30
Simple Cells - bar of light oriented In
particular direction
Complex - bar traveling in a Particular direction
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COLOR

• 3 types of cones
• RED
• BLUE
• GREEN
• Ratio of activity of 3 cones creates impression
  of different colors

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Opponent- Process Theory
Starts in Retinal Ganglion cells
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Dorsal Ventral Streams
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Ventral - identifies objects... what

• Neurons excited by complex visual stimuli (faces,
  expressions, paintings)
• stimulus equivalence can see something as the
  same object under different conditions
• Temporal lobe

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Dorsal- visual guidance of movement how

• V5 - 95 neurons selective for motion. Many types
  of cells, but all respond to visual information
  when movement of body to take place.
• Parietal lobe - cortical area for spatial
  awareness.

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DAMAGE!!
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What happens when various parts of the visual
pathway are injured?

• Type of deficit indicates where in pathway damage
  is.
• Destruction of retina or optic nerve of one eye
  produces monocular blindness (loss of sight in
  one eye).
• Injuries beyond also cause blindness, but they
  are different

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Homonymous Hemianopia

• Complete cut of optic TRACT, LGN, V1
• Blindness to one entire side of visual field

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Injuries to the What Pathway

• Agnosia - not knowing
• Visual agnosia - inability to recognize objects
• visual perception general intellectual function
  is normal

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DF

• posterior ventral pathway lesion (WHAT)
• Cannot recognize objects
• Cannot estimate size, orientation, copy drawings
• Can draw objects from memory but cant recognize
  what she was drawing!
• Can reach for objects with appropriate grasp

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GL

• Lesion more rostral (V5)
• Cannot detect movement
• Objects that move either vanish or appear frozen

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The Colorblind painter

• Lesion to V4
• Cannot detect color
• Tomato juice is black. Color TV is a
  hodge-podge
• Foods disgusting due to grayish, dead appearance

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Prosopagnosia - Face Blindness

• Can identify that a face is a face
• Have no sense of familiarity from faces of
  co-workers, family, famous people.
• Thought to result problems in accessing memory
  associated w/ faces that make feeling of
  familiarity possible
• Lesion to occip -temporal
• Evidence that unconscious recognition of face