Eye movements: Lab # 1 - Catching a ball
How do we use our eyes to catch balls?
What information does the brain need?
Most experiments look at simple
movements in response to targets.
What happens in the real world?
Cricket
Eye movements in cricket:
Batsman anticipate bounce point
Better batsman arrive earlier
Land & MacLeod, 2001
saccade
pursuit
Why the need for prediction?
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.
Is prediction a general property of behavior, or only
seen in skilled performance like cricket or baseball?
Catching: Gaze Patterns
X
X
saccade
Thrower
X
smooth pursuit
Catcher
Gaze Patterns Different when Watching
saccade
X
X
X
Thrower
Catcher
Different pattern of eye movements when watching (earlier, no pursuit).
Implications of this?
Unexpected bounce leads to poor performance, particularly in the
pursuit movement after the bounce.
Implications of this?
After three trials, pursuit has improved a lot.
Implications of this?
1. 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?
• What is the difference between throwing and catching? Why?
•
2. Choice of task:
• Catching and throwing a ball.
•
3. Procedure:
• Select subject and calibrate eye tracker. Two throw the ball back and forth,
with a bounce in the trajectory at a comfortable distance. Need to measure the
distance.
• First throw in a predictable manner, about10 times.
2. Data analysis
2. Play video frame-by-frame using Video-Viewer software.
• ….
• What to look for:
– Describe eye movements sequence for each trial
• eg Trial 1: fixate near hands/saccade to bounce point/fixate/smooth pursuit for portion of
trajectory/fixate for last part of trajectory (??)
• Trial 2: fixate near hands/saccade to bounce point/fixate/smooth pursuit for portion of
trajectory/fixate for last part of trajectory (??)
• ….
• B How regular is the sequence of movements?
• C 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?
• D. How accurate are fixations near the bounce point? (Need to measure visual angle.)
– What happens with the different balls? Do the eye movements change with additional
experience? How quickly do they adjust?
•
What is the role of the pursuit movement?
Notes for analyzing data: Ball Catching
Use Video Viewer to analyze your data.
Question: Do subjects make a saccade ahead of the ball when it bounces?
Look at video record, find the saccade around the time of the bounce.
Find the time at which the saccade begins and ends.
Find the time of the bounce.
Does the saccade end before the bounce? How much? Find this value on each
trial, then find the mean and standard error.
Question: Where does the saccade land?
Mark the bounce point and saccade landing point. Find the difference in pixels,
then convert to degrees. Does the saccade land above the bounce point?
Question: When does eye begin to move after the ball leaves the thrower’s
hand?
Measure time ball leaves hand.
Measure time eye starts to move.
Find the difference. Do this for each trial, then find the mean and standard error.
Question: Do Subjects pursue the ball after the bounce?
Identify pursuit movement in the data. This will be indicated by the eye cursor
staying close to the ball, and moving at the same speed.
Question: Is there any difference between the two balls?
Is the landing point higher for the more bouncy ball? Mark the bounce point and
saccade landing point. Find the difference in pixels, then convert to degrees.
Compare with the other ball.
Question: Does it take time to adjust to the new ball?
Are there any differences in performance on the first or second trial after the
ball is switched versus after 10 trials.
Data Figures.
1. Units for the X and Y axes should make sense – eg msec for X,
degrees for Y.
2. Choose a suitable time interval so you can see the detail you
need, so no more than 1 or 2 sec for the X axis.
3. Indicated the scale with well spaced tick marks eg ticks marks
every 100 or 200 msec.
4. Keep the Y axis identical for different graphs eg when plotting
horizontal and vertical components of the eye movements on
different graphs,
Suppose you have a random variable, such as height of the UT student population.
This population has a standard deviation of S.
Take a sample of size N from the population, and calculate the sample mean, M.
Do this many times.
What is the standard deviation of M?
SEM = S/√N
Useful website for understanding statistics
http://onlinestatbook.com/rvls/index.html