Visual Development

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Visual Development
Lindsay Lewis HDP Talk 01.29.04
Todays notes http//psy.ucsd.edu/lblewis/HDP
notes.ppt
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Outline

• Overview of neural development of visual system
  (retina -gt brain)
• Psychophysics and methodology (adults -gt babies)
• Infants (1 - 6 months of age)
• pattern vision
• luminance and color vision
• motion perception
• depth perception

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Overview of visual development
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Why study vision development?

• What can babies see?
• its interesting!
• What is the nature of the relationship between
  perception and neural changes?
• can teach us how perception is mediated by the
  visual system
• Clinical applications
• identify and treat vision problems early

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The visual system, in a nutshell
Dorsal pathway
Ventral pathway
LGN
V1
SC
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Infant photoreceptors

• Immature morphology infant photoreceptors are
  short and fat, and do not capture light well
• So overall sensitivity is poor

fetus
newborn
4 years
Hendrickson Yuodelis, 1984
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Infant photoreceptors

• Immature (coarse) spacing photoreceptors take
  up a lot of room (since they are fat!), do not
  resolve well
• So overall resolution (acuity) is poor

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Foveal development

• When you look straight at an object, you image
  it on your fovea - best and sharpest vision
• Infant fovea doesnt have many photoreceptors
• During development, photoreceptors migrate from
  peripheral retina to fovea
• Adult fovea packed with photoreceptors

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Foveal development
newborn

• In infants, lots of other neurons are in the way
  of photoreceptors
• During development, this neural junk is pushed
  aside (thins out)

4 years
Yuodelis Hendrickson, 1984
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The visual system, in a nutshell
Dorsal pathway
Ventral pathway
LGN
V1
SC
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LGN and SC

• LGN cortical structure, part of the thalamus,
  relay station to visual cortex
• main purpose to refine/amplify signals coming
  from eyes
• SC subcortical structure (evolutionarily older
  part of the brain), part of midbrain/brainstem
• main purpose is visual orienting reflexes,
  initiation/guidance of eye movements
• In adults, cortical pathway is responsible for
  most of vision
• In infants, subcortical pathway is largely
  responsible for vision

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The visual system, in a nutshell
Dorsal pathway
Ventral pathway
LGN
V1
SC
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Different pathways
for motion vs. color
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Magnocellular
Luminance
Koniocellular
Blue/Yellow
Red/Green
Parvocellular
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Dorsal pathway (magno) vs. ventral pathway (parvo)

• Dorsal pathway (AKA where or how, magno
  input)
• very sensitive to contrast (low contrast OK)
• very sensitive to motion
• not color-selective (but sensitive to color
  differences!)
• Ventral pathway (AKA what, primarily parvo
  input)
• less sensitive to contrast (must be high
  contrast)
• not sensitive to motion
• very color-selective

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"color-sensitive" vs. "color-selective"

• Dorsal pathway (AKA where or how, magno
  input) can signal that there is a color
  difference, or color border, but cannot signal
  color identity
• Ventral pathway (AKA what, primarily parvo
  input) can signal color difference as well as
  color identity

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Dorsal pathway (magno) vs. ventral pathway (parvo)

• Dorsal pathway (where or how, magno) seems
  to mature earlier than ventral pathway (what,
  parvo)
• infants can see motion long before they can see
  color or fine detail

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The visual system, in a nutshell
Dorsal pathway
Ventral pathway
LGN
V1
SC
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Myelination in infancy
Myelination of nerve fibers matures very slowly
ADULT
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Neural connections in infancy
Neural connections are transient, then pruned
ADULT
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Neural connections in infancy

• Synaptic pruning
• 40 of visual cortex synapses are eliminated
• remaining circuits grow progressively more
  efficient
• coincides with gradual refinement of many visual
  skills
• in other words, there is a tradeoff between of
  neural connections and efficiency/refinement of
  vision
• also, the period of synaptic pruning is
  considered a critical period
• during this time, visual abilities remain
  plastic (subject to modification by experience)
• visual skills are most plastic during the rapid
  increase of neural connections during the 1st 2
  years of life

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More about the effect of early visual experience

• A cool thing about plasticity the more we
  encounter something, the more our visual system
  becomes more sensitive to it!
• the oblique effect most people are better at
  seeing horizontal and vertical orientations than
  oblique (diagonal) orientations
• is this because we are raised in carpentered
  environments with mostly horizontal and vertical
  orientations?
• people raised in less carpentered environments
  do not have this effect
• animal studies have also shown that the
  preferred orientation of cortical cells is highly
  dependent on environment

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Blakemore Cooper
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Preferred orientations of kitten cortical neurons
Horizontally-reared
Vertically-reared
Blakemore Cooper
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More about the effect of early visual experience

• A con of plasticity abnormal visual experience
• congenital cataracts, misalignments
• during critical period, each eye competes for
  brain cells
• if there is abnormal input from one eye, the
  good eye will get more brain cells at expense
  of bad eye
• This is why its so important to correct these
  visual abnormalities early

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Psychophysical Methods
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Methods

• First, some background in psychophysics..
• grating stimulus striped pattern
• spatial frequency (SF)
• contrast

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Spatial frequency (low -gt high) (coarse-gt
fine)
Gratings
Contrast (low -gt high)
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More on gratings

• spatial frequency measured in cycles/degree
• 1 cycle two stripes (one dark and one light)
• 1 degree 1 degree of visual angle
• contrast between stripes
• black white 100 contrast
• uniform gray 0 contrast
• (30-10)/(3010) 50 contrast

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Luminance Gratings
Chromatic Red/Green (Isoluminant) Gratings
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Adults two-alternative forced choice (2-AFC)

• How do we find out if a person can see a
  stimulus?
• Could simply ask them whether or not they see it
  (yes or no)
• but then response can be influenced by factors
  like attention and motivation
• A way around this problem is the 2-AFC technique
• 1 interval left or right?
• 2 interval 1st or 2nd interval?

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Infants forced-choice preferential looking (FPL)

• How do we get babies to tell us what they see?
• psychophysics
• forced-choice preferential looking (FPL)
• spatial frequency, luminance or color contrast
• directional eye movements (DEM)
• motion

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Babies tend to look at patterns
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Infants forced-choice preferential looking (FPL)
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Infants directional eye movements (DEM)
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Determining threshold
1
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9
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• if cant see it, will be correct 50 of time (at
  chance)
• threshold contrast needed to get 75 correct
• sensitivity 1/threshold
• so high sensitivity low threshold

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Infant pattern vision
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Contrast sensitivities for babies vs. adults

• Vary spatial frequency (SF), see what contrast
  needed for threshold
• Babies are most sensitive at lower SFs, (course
  stripes) and are overall less sensitive than
  adults
• Acuity/resolution highest SF (finest detail)
  you can just see

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Acuity develops gradually
high acuity
low acuity
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Infant luminance and color vision
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How well do infants see contrast?

• Contrast can be defined in terms of two
  dimensions
• luminance contrast (black/white)
• chromatic contrast (red/green)
• How well can babies see luminance and chromatic
  contrast?
• Is it true that infants cant see color well?

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Luminance and chromatic contrast thresholds
low sensitivity

• Infants have higher thresholds than adults
  (poorer sensitivity) for BOTH luminance and
  chromatic contrast

high sensitivity
low sensitivity
high sensitivity
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Luminance and chromatic contrast thresholds
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Luminance and chromatic contrast thresholds

• People used to think that infants cant see
  colors because they were using PASTELS, which are
  low contrast!
• Thats why baby toys are now usually high
  contrast (blacks, whites, reds, instead of pink
  and light blue)
• By 3 months of age, infants can make nearly all
  the same color distinctions as adults, and by 4-5
  months of age, parvo and magno pathways are
  equally developed

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Example data thresholds drop with age
4-month-old
3-month-old
t
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100
100
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Nadir
r
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90
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Contrast ()
Contrast ()
Luminance
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Infant motion perception
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Temporal contrast sensitivity function
high sensitivity

• Show infants and adults gratings moving at
  different speeds
• Infants and adults are both most sensitive to
  22 deg/sec (similar speed tuning!)
• Motion processing matures very early-gt good for
  survival!

(3 months)
low sensitivity
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Speed preferences of MT neurons and
temporal contrast sensitivity function

• in both infants and adults, MT neuron speed
  preferences mirror psychophysical sensitivity!

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Infant depth perception
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Two kinds of depth perception

• Monocular one eye
• e.g. interposition, relative size, perspective,
  shading, etc.
• Binocular two eyes
• retinal disparity each eye has slightly
  different vantage point
• brain compares different views of each eye to
  extract depth
• Although monocular cues help provide info about
  the relative locations of things, binocular depth
  perception is responsible for true 3D
  stereoscopic vision

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Stereo FPL task
Held et al., 1980
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Stereo vision develops suddenly

• Results from 3 normal babies
• This is why children with crossed eyes are
  particularly vulnerable to problems with stereo
  vision (very short critical period!)

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Take-home points

• Studying visual development teaches us about how
  perception is related to biology, and has
  clinical applications
• Since babies cant tell us what they see, we
  have to use techniques that let us infer their
  visual abilities
• Different visual abilities develop at different
  rates (acuity gradually, depth suddenly) and
  appear at different points during development
  (motion early, color late)
• The general rule is that in order for visual
  abilities to properly develop, there must be a
  joint effort between maturation of the visual
  system, and experience

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Thank you!