What is vision used to catch a ball

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What is vision used to catch a ball? What can we
tell from the eye movements?
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Types of Eye Movement
Information Gathering Stabilizing Voluntary
(attention) Reflexive Saccades vestibular
ocular reflex (vor) new location, high velocity,
ballistic body movements Smooth
pursuit optokinetic nystagmus (okn) object
moves, velocity, slow whole field image
motion Vergence change point of fixation in
depth slow, disjunctive (eyes rotate in opposite
directions) (all others are conjunctive)
Fixation period when eye is relatively
stationary between saccades.
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• Catching Gaze Patterns

X
Thrower
Catcher
Terminology saccadic eye movement
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• Catching Gaze Anticipation

Saccade reaction time 200ms
61 ms
X
X
-53 ms
X
Catcher
Thrower
Timing of departure and arrival linked to
critical events
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What is the significance of prediction?Brain
must learn the way ball moves etc and
programmovement for an expected state of world.
Not reacting simply to current visual
information. Stimulus Response
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Why is prediction necessary?
Analysis of visual signals takes a lot of time!
Photoreceptors ganglion cells
LGN Primary visual cortex other cortical
areas mid-brain brain stem muscles
Round trip from eye to brain to muscles takes a
minumum of 200 msec. Cricket ball only takes
about 600 msec. Prediction gets around the
problem of sensory delays.
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How good is prediction?
Accuracy of Fixations near Bounce
20 deg
bounce point
2D elevation
Subjects fixate above the bounce point
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Poor tracking when ball is unexpectedly bouncy
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Better tracking 2 trials later.
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Pursuit accuracy following bounce
Measure proportion of time between bounce catch
that eye is close to ball
tennis ball
bouncy ball
5 subjects
Does pursuit accuracy improve with repeated
trials? Does it matter which ball is used first?
What can we conclude if it does?
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Prediction in Squash
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Prediction in Squash
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Reduced gain means eye lags behind ball.
Gain 1 means perfect tracking
These speeds are much higher than expected. Too
high for a reactive system. That is, prediction
is necessary. 100deg/sec 10 deg in 100 msec.
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Binocular Vision
Stereoscopic information image in the two eyes
is different. This information is used to
perceive the depth relations in the scene.

• When is stereoscopic information useful?
• - reaching and grasping
• walking over obstacles
• catching??

Development of stereoscopic vision - amblyopia/
astigmatism - critical period
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Difference in retinal distance between the
objects in the two eyes is called retinal
disparity and is used to calculate relative
depth.
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Binocular Vision
The eye fixates the front of the obstacle, plans
the foot placement, and moves ahead before the
foot is placed.
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Monocular Vision
The eye fixates the front of the obstacle, and
guides the foot placement before moving ahead.
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Other information that may be useful for catching.
Motion parallax change in relative position of
objects at different depths when the head
moves. Looming image of ball increases in size
as ball gets closer. Rate of change of size can
be used to calculate time-to-contact Pursuit
movement keeping the eye on the ball.
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target selection
Planning?
saccade decision
saccade command
inhibits SC
Cerebellum Learning?
signals to muscles
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decision to pursue/attention
detect/analyze retinal image motion
Supplementary eye fields planning?
prediction/ learning?
signals to muscles
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Even the simplest action must involve linkage
between memory, vision, eye movements, and body
movements. from Land et al, 1999
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• What are the questions?
• Is the behavior observed by Land in cricket also
  true for a simple task like catching a ball?
• What eye movements are made in this case?
• Do subjects anticipate the bounce point? By how
  much?
• Do Subjects look at floor or above the bounce
  point?
• What happens after bounce?
• How do subjects adjust to different balls?
• ..
• Is there a difference between throwing and
  catching? Why?
• What eye movements are made when observing others
  throw and catch?
• Similarity between individuals?
• Procedure
• Select subject and calibrate eye tracker. Three
  people stand at equal distances apart and throw
  the ball back and forth, with a bounce in the
  trajectory. First throw in a predictable manner,
  about 10 times.
• Then use a different ball, 10 trials.
• Compare one versus two eyes???

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• Data analysis
• Get dvd from Travis . Play it frame-by-frame on a
  Mac computer using the RIT program.
• What to look for
• Describe eye movements sequence for each trial
• eg Trial 1 fixate near hands/saccade to bounce
  point/fixate/track portion of trajectory/fixate
  for last part of trajectory (??)
• Trial 2 fixate near hands/saccade to bounce
  point/fixate/track portion of trajectory/fixate
  for last part of trajectory (??)
• .
• What is the timing of the saccades/fixations/trac
  king relative to movement of the ball. How much
  do subjects anticipate the bounce point, if at
  all?
• How accurate is pursuit? Calculate percent time
  eye is on the ball in the period between bounce
  and catch.
• Compare different conditions.
• What happens with the different balls? Do the eye
  movements change with additional experience? How
  quickly do they adjust?
• Other Aspects
• How similar are different individuals? Where
  would we expect similarities/ differences?
• What is the role of the pursuit movement? Is
  pursuit is used to guide hands. Maybe position of
  eye in head.
• Binocular information versus monocular (looming)

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Different gaze pattern for watching but still
anticipate bounce and catch events.
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• Gaze Patterns Different when Watching

X
Thrower
Catcher
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• WatchingGaze Anticipation

-517 ms
-51 ms
X
X
-167 ms
X
Catcher
Thrower
Head rotation begins 200-500 msec before release