HORMONES AND BEHAVIOR Psychology

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The Mammalian Visual System

• Within the eyeball
• rods and cones synapse to horizontal cells and
  bipolar cells
• horizontal cells make inhibitory synapse onto
  bipolar cells
• bipolar cells synapse to amacrine and ganglion
  cells
• axons of the ganglion cells leave the back of the
  eye

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The Mammalian Visual System cont.

• The inside half of the axons of each eye cross
  over in the optic chiasm
• most visual information goes through the lateral
  geniculate nucleus of the thalamus
• some goes to the superior colliculus
• lateral geniculate inputs to other parts of
  thalamus and to visual areas of cerebral cortex,
  which sends back axons to modify input
• number of neurons within this loop varies widely
  among people by a factor of 2 or 3

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Visual Fields
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Mechanisms of Processing in the Visual System

• Receptive field the point in space from which
  incoming light strikes a receptor
• receptors have both excitatory and inhibitory
  regions since receptive field is normally an
  array of light patterns
• Ex light in center of ganglion cell might be
  excitatory, with the surround inhibitory

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Figure 6.18

• Figure 6.18 Receptive fields. The receptive field
  of a receptor is simply the area of the visual
  field that strikes that receptor. For any other
  cell in the visual system, the receptive field is
  the collective receptors feeding the neural
  pathway to the cell.

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Mechanisms of Processing in the Visual System
cont.

• Lateral Inhibition each active receptor and its
  visual path tends to inhibit the visual path of
  neighboring receptors
• an active receptor excites both a bipolar and
  horizontal cell in turn, horizontal cell
  inhibits bipolar cell, but net potential is
  excitatory on bipolar
• but, horizontal cell does inhibit neighboring
  bipolar cells on border of visual field
• effect is to heighten contrast receptors inside
  visual field are excited and those on border tend
  to be inhibited

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Receptive Fields of Retinal Cells (Part 1)
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Receptive Fields of Retinal Cells (Part 2)
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Receptive Fields of Cells at Various Levels in
the Cat Visual System (Part 1)
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Receptive Fields of Cells at Various Levels in
the Cat Visual System (Part 2)
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Receptive Fields of Cells at Various Levels in
the Cat Visual System (Part 3)
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Receptive Fields of Cells at Various Levels in
the Cat Visual System (Part 4)
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Retina and Lateral Geniculate Pathways

• Parvocellular smaller ganglion cell bodies and
  small receptive fields, located near fovea
• detect visual detail and color
• all axons go to lateral geniculate nucleus
• Magnocellular larger ganglion cell bodies and
  receptive fields, distributed fairly evenly
  throughout retina
• respond to moving stimuli and patterns
• not color sensitive
• most axons go to lateral geniculate nucleus

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Retina and Lateral Geniculate Pathways cont.

• Koniocellular small ganglion cell bodies that
  occur throughout the retina
• many functions
• axons go to lateral geniculate nucleus, thalamus
  and superior colliculus
• Many different types of ganglion cells implies
  analysis of information from the beginning

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Cross Section of the Monkey Lateral Geniculate
Nucleus
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• Primary Visual Cortex
• primary visual cortex V1 striate cortex
• surrounds the calcarine fissure
• arranged in 6 primary layers (parallel to
  cortex)
• receives inputs from parvo/magno LGN (layer 4C)
• receives input from konio LGN (layers 2,3)
• projects to visual association areas (secondary
  visual cortex, temporal cortex,
• posterior parietal cortex

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Pathways in Cerebral Cortex

• Most visual information from lateral geniculate
  nucleus goes to primary visual cortex (V1)
• first stage of visual processing
• Output of V1 goes to secondary visual cortex (V2)
• second stage of visual processing which transmits
  visual information to additional areas
• feedback loop to V1
• V1 and V2 also exchange information with other
  cortical areas and thalamus
• 30-40 visual areas reported in brain of macaque
  monkey

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Pathways in Cerebral Cortex cont.

• Magnocellular and parvocellular paths split into
  three paths
• Magnocellular path
• ventral branch to temporal cortex is sensitive to
  movement
• dorsal branch to parietal cortex integrates
  vision with action
• Parvocellular path to temporal cortex is
  sensitive to details of shape
• Mixed parvo/magnocellular path to temporal cortex
  is sensitive to brightness and color

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• Figure 6.21 Three visual pathways in the cerebral
  cortex. (a) A pathway originating mainly from
  magnocellular neurons. (b) A mixed
  magnocellular/parvocellular pathway. (c) A mainly
  parvocellular pathway. Neurons are only sparsely
  connected with neurons of other pathways.
  (Sources Based on DeYoe, Felleman, Van Essen,
  McClendon, 1994 Tso Roe, 1995 Van Essen
  DeYoe, 1995.)

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Pathways in Cerebral Cortex cont.

• Visual paths in temporal cortex form the ventral
  stream
• the what path, specialized for identifying and
  recognizing objects
• if damaged, we can find and pick up objects but
  cannot describe them
• Visual path in parietal cortex is the dorsal
  stream
• the where or how path, helps motor system
  find objects, move toward them and pick them up
• if damaged, we can describe object but cant find
  and pick up object

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Parallel Processing Pathways in the Visual System
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Disorders of Object Recognition

• Visual agnosia inability to recognize some
  objects
• can describe object but doesnt know what they
  are, e.g., key, stethoscope, smoking pipe
• Prosopagnosia inability to recognize faces
• can still read and recognize person by their
  voice
• inferior temporal cortex area, fusiform gyrus,
  especially active in recognition of faces
• Also activated when recognizing other complex
  shapes, e.g., cars and birds

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Color, Depth and Motion Perception

• Cells sensitive to color are found in parts of V1
  known as blobs, which also have cells that
  contribute to brightness perception
• Area V4 or nearby is important for color
  constancy
• monkeys with damage here cant find yellow banana
  if light is changed from white to blue
• Cells of magnocellular path are specialized for
  stereoscopic depth perception

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Color, Depth and Motion Perception cont.

• Cells in area MT respond selectively to stimulus
  moving in a particular direction regardless of
  size, shape or color
• motion blind people who cannot determine if
  objects are moving may have damage here
• Cells on MST respond best to expansion,
  contraction or rotation of large visual scene

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• Figure 6.33 Stimuli that excite certain cells in
  the ventral part of area MST. Cells in this area
  respond when an object moves relative to its
  background. They therefore react either when the
  background is steady and the object moves or when
  the object and the background move.

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Visual Attention

• Attention is dependent on amount and duration of
  activity in a cortical area
• a brief response to stimulus produces activity in
  V1 area
• focused attention produces additional activity in
  V2 area
• similar focus on color or motion produces
  additional activity in visual cortex area
  responsible for color and motion perception

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The Binding Problem Revisited

• How does visual cortex bind color, shape and
  movement to an object, e.g., a rabbit and bring
  it into consciousness?
• evidence for synchronized activity in both
  hemispheres when an object is recognized
• Some visual processing without consciousness
• blindsight loss of visual field and person can
  still point out objects or light in the blind
  field
• some healthy tissue may remain to provide
  blindsight
• Dominant hypothesis is that consciousness is
  distributed over several cortical areas

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Infant Vision

• Infants two days old already prefer to look at
  faces, circles and stripes than patternless
  displays
• Great difficulty in shifting attention up until
  about 6 months
• In newborn mammals many properties will develop
  even if the eyes are damaged or raised in
  darkness
• but if darkness continues, these properties
  diminish
• visual experience is required to maintain and
  fine tune connections

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Early Development

• In newborn kitten, lack of stimulation
• of one eye for 4-6 weeks and that eye became
  blind
• of both eyes up to three weeks still left cortex
  responsive
• but if for a longer period of time, loss of sharp
  receptive fields is noted
• Sensitive or critical period for normal vision
• if congenital blindness is not restored for
  years, newly gained vision is almost useless
• removal of cataracts within 6 months of birth
  still leaves deficits

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Early Development cont.

• Vision can be restored when kitten is deprived of
  vision in one eye for only a few days
• but, kitten recovers better if normal eye is
  covered
• suggests using patch over normal eye for
  amblyopia
• Alternating normal stimulation one eye at a time
  and cortex learns to respond to one or the other,
  resulting in loss of binocular vision
• accounts for strabismus, where eyes point in
  different directions, in humans

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Early Development cont.

• Kitten became responsive only to horizontal lines
  when wearing goggles with horizontal lines during
  sensitive period
• similar to development of astigmatism in humans,
  which can be corrected if glasses are used early
• Kitten became motion blind when raised with only
  strobe lighting for 4-6 month period
• Kitten receiving no visual stimuli became more
  responsive to auditory and tactile stimuli than
  normal cats
• more true for children or infants than when
  blindness occurs in adults