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How is vision used to catch a ball?
What can we tell from the eye movements?
Types of Eye Movement
Information Gathering
Voluntary (attention)
Stabilizing
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.
Catching: Gaze Patterns
X
X
saccade
X
smooth pursuit
Thrower
Terminology: saccadic eye movement
Catcher
Catching: Gaze Anticipation
Saccade
reaction time = 200ms
61 ms
X
-53 ms
X
X
Thrower
Catcher
Timing of departure and arrival linked to critical events
What is the significance of prediction?
Brain must learn the way ball moves etc and program
movement for an expected state of world. Not reacting simply to
current visual information. Stimulus
Response
Why is prediction necessary?
Analysis of visual signals takes a lot of time!
Photoreceptors
ganglion cells
Primary visual cortex
mid-brain
brain stem
LGN
other cortical areas
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.
How good is prediction?
Accuracy of Fixations near Bounce
20 deg
bounce point
2D elevation
Subjects fixate above the bounce point
Poor tracking when ball is unexpectedly bouncy
Better tracking 2 trials later.
Pursuit accuracy following bounce
%age of time gaze on Target
Measure proportion of time between bounce & catch that eye is close to ball
100%
tennis ball
90%
80%
70%
60%
bouncy ball
50%
40%
30%
20%
5 subjects
10%
0%
1
2
3
4
5
6
Trial Number
Does pursuit accuracy improve with repeated trials?
Does it matter which ball is used first? What can we conclude if it does?
target selection
Planning?
saccade decision
saccade command
inhibits SC
Cerebellum
Learning?
signals to muscles
decision to pursue/attention
Supplementary eye fields
planning?
detect/analyze
retinal image motion
prediction/
learning?
signals to muscles
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?
• How do subjects adjust to different balls?
• Is it difficult to adapt to a less elastic ball as well as to a more elastic ball?
• …..
• 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???
Data analysis
• Play movie 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/tracking 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.
Different gaze pattern for watching
but still anticipate bounce and catch events.
Gaze Patterns Different when Watching
saccade
X
X
X
Thrower
Catcher
Watching:Gaze Anticipation
-51 ms
X
-517 ms
X
-167 ms
X
Thrower
Catcher
Head rotation begins 200-500 msec before release
Prediction in Squash
Prediction in Squash
Predictive Saccade
Anticipation: 183 +/- 35 ms
Ball
Anticipatory saccade to predicted location 183 msec before ball.
Ball arrives at fixation point
Predictive Saccade ctd
Racquet
Error = 2.6 deg
Fixation after saccade
Duration: 250 +/- 21 ms
Ball
Since the predicted location follows the bounce, it is not based on simple
extrapolation, but a more complex prediction.
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
Difference in retinal distance between the objects in the two eyes is called
“retinal disparity” and is used to calculate relative depth.
Binocular Vision
The eye fixates the front of the obstacle, plans the foot placement, and
moves ahead before the foot is placed.
Monocular Vision
The eye fixates the front of the obstacle, and guides the foot placement
before moving ahead.
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.