Transcript lecture3
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.