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

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

2. Types of Eye Movement Information GatheringStabilizing 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.

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

5. Saccade reaction time = 200ms Catching: GazeAnticipation 61 ms X X -53 ms X Thrower Catcher Timing of departure and arrival linked to critical events

7. 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

8. 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.

9. How good is prediction? Accuracy of Fixations near Bounce 20 deg bouncepoint 2D elevation Subjects fixate above the bounce point

10. Poor tracking when ball is unexpectedly bouncy

11. Better tracking 2 trials later.

12. 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?

13. Prediction in Squash

14. Prediction in Squash

16. “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.

17. 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

18. Difference in retinal distance between the objects in the two eyes is called “retinal disparity” and is used to calculate relative depth.

19. Binocular Vision The eye fixates the front of the obstacle, plans the foot placement, and moves ahead before the foot is placed.

20. Monocular Vision The eye fixates the front of the obstacle, and guides the foot placement before moving ahead.

21. 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.

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

23. decision to pursue/attention detect/analyze retinal image motion Supplementary eye fields planning? prediction/ learning? signals to muscles

24. Even the simplest action must involve linkage between memory, vision, eye movements, and body movements. from Land et al, 1999

25. 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???

26. 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/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. • Binocular information versus monocular (looming)

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

28. saccade X X Gaze Patterns Different when Watching X Thrower Catcher

29. Watching:Gaze Anticipation -517 ms -51 ms X X -167 ms X Thrower Catcher Head rotation begins 200-500 msec before release