Visual Perception

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Visual Perception

• ????
• hallucinations
• hysterical blindness

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Structure of the Eye
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Retinal cells
rods and cones
bipolor cells
horizontal cell
ganglion cells
ganglion axon (OPTIC NERVE)
LIGHT
LGN
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Ganglion cells

• true nerve impulses (all-or-none)

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Other cells

• graded potentials (EPSP, IPSP)

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Transduction

• rhodopsin OPSIN (protein) retinal
• photosensitive
• found in rods

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Retinal

• several isomeric forms
• isomers same atoms, different arrangement
• photon striking retinal causes it to switch forms

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Retinal

• 11-cis isomer in the dark

bent tail
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dark current

• rods secrete INHIBITORY transmitter in the dark

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Let there beÖ.
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Retinal

• All trans-isomer in light

straight tail
I am a photon!
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in the light

• Na channels close
• hyperpolarization of -75 mV
• less inhibitory transmitter
• next cell fires

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Receptive Fields Cat Ganglion Cell
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Center
Surround
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On-center, Off-surround
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On Center
FIRE
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On Center
FIRE
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Off Surround
FIRE
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Off Surround
FIRE
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Visual Pathways
Retina
Optic nerve
Cerebral Cortex
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RVF
LVF
RVF
LVF
LEFT HEMISPHERE
RIGHT HEMISPHERE
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Development of Visual Perception
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Nature vs. Nurture

• How do we come to perceive a meaningful visual
  world?
• Does experience affect perception?

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William James, 1890

• ìThe baby, assailed by eyes, ears, nose, skin,
  and entrails all at once, feels it all as one
  great blooming, buzzing confusionî
• not exactly true

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Adult Experiments
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S.B.

• lost his sight at 10 months
• vision restored at age 52
• problems with distance, depth, visual illusions
  didnít fool him
• vision never completely normal
• learned to visually recognize objects he had
  known by touch faster than others (e.g. capital
  letters)

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experience plays an important role in visual
perception

• perceptionACTIVE process of constructing picture
  of the world

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The Role of Movement in Development of Perception

• subjects wore prisms that displaced the world to
  one side
• those who moved around freely adapted
• those who were passively wheeled did not

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George Stratton

• invented devise which restructured visual world
• reversed up and down, left and right
• adaptation to altered world not complete
• did learn to move around by 3rd day
• by 9th day, when lenses removed, the world looked
  ìbewilderingî

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Adult Experiments

• Conclusions adults adapt--but only partially--to
  a new visual world as they gain experience with
  it. Moving around in the world helps.

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Child Studies
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The Visual Cliff

• Gibson, 1960
• Eleanor Gibson concerned that her own children
  would crawl over edge of Grand Canyon

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glass sheet
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Visual Cliff

• Subjects 36 infants, aged 6-14 months
• Procedure placed on center board and called by
  mother from each end of apparatus
• Results 27 infants moved
• 3 crawled over the cliff
• 24 refused to cross cliff to reach mother

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Observations

• some infants stared over cliff and backed away
  from it
• some patted the glass suspiciously but refused to
  cross it

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Conclusions

• apparently, very young infants recognized the
  visual cues for depth
• later studies showed that experience with
  crawling is necessary to recognize depth

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Other studies of development of visual perception
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Fantz, 1961 visual discrimination

• do infants prefer one visual stimulus over
  another?
• preference operationalized as amount of time
  spent looking at one stimulus as opposed to
  another

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B
A
C
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Results

• 1-month old distinguish gray card from
  black-and-white striped card even when stripes
  1/8 inch wide
• 6 months distinguish when stripes 1/64 inch wide
• 2 weeks-5 1/2 months prefer real faces to
  nonsense stimuli

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Auditory Perception

• 4 months synchronize visual and auditory rhythm
  in film sountrack of toys bouncing at different
  rates. Spent more time watching appropriate
  visual stimulus

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Auditory Perception (conít)

• 3 days old will change sucking pattern on
  electronic pacifier to hear motherís voice
• no more than 12 hours of contact with mother at
  the time
• even when extensive exposure to fatherís voice,
  did not distinguish his from other males
• prenatal exposure to motherís voice?

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• 18 hours old cry along with tape-recording of
  other infants crying stop crying when hears own
  voice

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CONCLUSIONS

• infants probably born with certain basic
  perceptual abilities
• other perceptual abilities--particularly fine
  discriminations-- develop with experience

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Physiology of Experience
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Cortical vs. Subcortical processing
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Schneider, 1967

• removed superior colliculus (in midbrain) on both
  sides of hamstersí brains

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• could solve visual discrimination problem by
  choosing correct door (horizontal vs. vertical
  stripes)
• could not retrieve seed held near face

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Subcortical processing

• superior colliculus
• locating objects in space
• orienting eyes and head to object
• where an object is, but not what it is

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Schneider, 1967 conít

• removed visual cortex
• could retreive seeds near face
• couldnít solve discrimination problem

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Cortical processing

• what the object is, but not where it is