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How 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 StabilizingVoluntary (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
Vergencechange point of fixation in depthslow, 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
CatcherThrower
saccade X
X
smooth pursuit
X
Terminology: saccadic eye movement
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Catching: Gaze Anticipation
CatcherThrower
X
X XTime?
Time?
When is the saccade made? Is it predictive?
Saccade reaction time = 200ms
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Mean, standard deviation, standard error of the mean.
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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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Photoreceptors ganglion cells LGN
Primary visual cortex other cortical areas
mid-brain brain stem muscles
Why is prediction necessary?
Analysis of visual signals takes a lot of time!
Round trip from eye to brain to muscles takes a minumumof 200 msec. Cricket ball only takes about 600 msec.Prediction gets around the problem of sensory delays.
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20 deg
Accuracy of Fixations near Bounce
2D elevationDo Subjects fixate the bounce point or above it? Why?
bounce point
How good is prediction?
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x a
tan(a/2) = x/da = 2 tan-1 x/d
Visual Angle
d
A measure of size that is independent of distance.In our expt: 10 pixels = 1 degree
Calculating visual angle
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What happens when ball is unexpectedly bouncy?
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How do we characterize effects of change in elasticity? What are implications of this?.
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Pursuit accuracy following bounce
tennis ball
bouncy ball
Does pursuit accuracy improve with repeated trials?Does it matter which ball is used first? What can we conclude if it does?
5 subjects
Measure proportion of time between bounce & catch that eye is close to ball
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Does change in elasticity affect height above bounce where Ss look?
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target selection
signals to muscles
inhibits SC
saccade decision
saccade command
CerebellumLearning?
Planning?
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signals to muscles
detect/analyzeretinal image motion
decision to pursue/attention
prediction/learning?
Supplementary eye fieldsplanning?
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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? • 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?
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Data analysis
• Play movie frame-by-frame on a Mac computer using the RIT program. • General description:
– Describe eye movements sequence for a typical trial• eg Trial 1: 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, if at all?
• Do Ss look at bounce point or above it?– 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.
When?Where?
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Binocular Vision
Stereoscopic information: image in the two eyes is different.This information is used to perceive the depth relations inthe 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, andmoves 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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Different gaze pattern for watchingbut still anticipate bounce and catch events.
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CatcherThrower
saccade
X
X
Gaze Patterns Different when Watching
X
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CatcherThrower
X
X
Watching:Gaze Anticipation
X-51 ms-167 ms
-517 ms
Head rotation begins 200-500 msec before release
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Prediction in Squash
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Prediction in Squash
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Predictive Saccade
Anticipation: 183 +/- 35 ms
Ball
Anticipatory saccade to predicted location 183 msec before ball.
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Fixation after saccadeDuration: 250 +/- 21 ms
Ball
Racquet
Predictive Saccade ctdBall arrives at fixation point
Error = 2.6 deg
Since the predicted location follows the bounce, it is not based on simple extrapolation, but a more complex prediction.