How is vision used to catch a ball? What can we tell from the eye movements?

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) Eye movements revealing about these questions Fixation: period when eye is relatively stationary between saccades.

Catching: Gaze Patterns smooth pursuit X X saccade X Thrower Catcher Terminology: saccadic eye movement

Catching: Gaze Anticipation Saccade reaction time = 200ms Time? X X Time? X Thrower Catcher When is the saccade made? Is it predictive?

Mean, standard deviation, standard error of the mean.

What is the significance of prediction 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 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.

How good is prediction? 20 deg bounce point 2D elevation Accuracy of Fixations near Bounce 20 deg bounce point 2D elevation Do Subjects fixate the bounce point or above it? Why?

Calculating visual angle x a tan(a/2) = x/d a = 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 For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre

What happens when ball is unexpectedly bouncy?

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

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?

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

target selection saccade decision saccade command inhibits SC Planning? saccade decision saccade command inhibits SC Cerebellum Learning? signals to muscles

decision to pursue/attention detect/analyze retinal image motion Supplementary eye fields planning? 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?

When? Where? 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 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.

Different gaze pattern for watching but still anticipate bounce and catch events.

Gaze Patterns Different when Watching saccade X X X Sometimes watcher, sometimes catcher - vision used differently Thrower Catcher

Watching:Gaze Anticipation -517 ms -51 ms X 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.