How is vision used to catch a ball?

What can we tell from the eye movements?

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.

Catching: Gaze Patterns

CatcherThrower

saccade X

X

smooth pursuit

X

Terminology: saccadic eye movement

Catching: Gaze Anticipation

CatcherThrower

X

X XTime?

Time?

When is the saccade made? Is it predictive?

Saccade reaction time = 200ms

Mean, standard deviation, standard error of the mean.

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

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.

20 deg

Accuracy of Fixations near Bounce

2D elevationDo Subjects fixate the bounce point or above it? Why?

bounce point

How good is prediction?

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

What happens when ball is unexpectedly bouncy?

How do we characterize effects of change in elasticity? What are implications of this?.

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

Does change in elasticity affect height above bounce where Ss look?

target selection

signals to muscles

inhibits SC

saccade decision

saccade command

CerebellumLearning?

Planning?

signals to muscles

detect/analyzeretinal image motion

decision to pursue/attention

prediction/learning?

Supplementary eye fieldsplanning?

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?

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?

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

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, andmoves 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.

Different gaze pattern for watchingbut still anticipate bounce and catch events.

CatcherThrower

saccade

X

X

Gaze Patterns Different when Watching

X

CatcherThrower

X

X

Watching:Gaze Anticipation

X-51 ms-167 ms

-517 ms

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.

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.