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Eye movements: Lab # 1 - Catching a ball
What can be learnt from natural tasks?
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1. Gaze exclusively on task-relevant objects (see Land
chapter)
2. Eyes deal with one object at a time, corresponding to
the duration of the manipulation.(Land: object-related
actions)
3. Tight linkage between location of gaze and information
needed at that moment. (Just-in-time strategy)
Why do we move our eyes?
- Image stabilization
- Information acquisition
The Eye and Retina
Cone Photoreceptors are densely packed in the central fovea
Visual Acuity matches photoreceptor density
Relative visual acuity
Receptor density
Why do we move our eyes?
1. To bring objects of interest onto high acuity region in fovea.
Why eye movements are hard to measure.
A small eye rotation translates into a big change in visual angle
Visual Angle
x
18mm
a
d
tan(a/2) = x/d
a = 2 tan 1 x/d
1 diopter = 1/focal length in meters
0.3mm = 1 deg visual angle
55 diopters = 1/.018
Types of Eye Movement
Information Gathering
Voluntary (attention)
Saccades
Stabilizing
Reflexive
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.
Cortical specialization
Visual Projections
Brain Circuitry for Saccades
1. Neural activity
related to saccade
2. Microstimulation
generates saccade
3. Lesions impair
saccade
V1: striate cortex
Basal ganglia
Oculomotor nuclei
Function of Different Areas
monitor/plan
movements
target selection
saccade decision
inhibits SC
saccade command
signals to muscles
Posterior Parietal Cortex
reaching
Intra-Parietal Sulcus: area
of multi-sensory convergence
grasping
LIP: Lateral Intra-parietal Area
Target selection for saccades: cells fire before saccade to attended object
Brain Circuitry for Pursuit
& Supplementary
Smooth pursuit
Brain Circuitry for Pursuit
& Supplementary
Smooth pursuit
Velocity signal
Early motion analysis
How do we use our eyes to catch balls?
What information the the brain need?
Neurophysiological experiments look at single
movements in response to flashes of light.
Eye movements in cricket:
Batsman anticipate bounce point
Better batsman arrive earlier
Land & MacLeod, 2001
saccade
pursuit
Why are eye movements predictive?
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 seen in cricket 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
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?
Different pattern of eye movements when watching (earlier, no pursuit).
Implications of this?
Gaze Patterns Different when Watching
saccade
X
X
X
Thrower
Catcher
Lab groups
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? Does it correlate with
performance?
• Do Subjects look at floor or above the bounce point?
• What happens after bounce?
• How do subjects adjust to different balls?
• …..
• Similarity between individuals?
• When do the hands start to move?
•
2. Choice of task:
• Catching and throwing a ball.
•
3. 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. Need
to measure this distance.
• First throw in a predictable manner, about 8 times.
• Then use a different ball, 8 trials.
• Other balls…
• Compare one versus two eyes???
2. Data analysis
• Label your tape. Play it frame-by-frame on the VCR in the lab.
• ….
• 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 (??)
• ….
• 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.)
– Compare different conditions.
– What happens with the different balls? Do the eye movements change with additional
experience? How quickly do they adjust?
•
Other Aspects:
–
–
–
Compare timing of eye and head movements?
When do hands start to move, relative to release of ball?
How similar are different individuals? Where would we expect similarities/ differences?
•
What is the role of the pursuit movement? If pursuit is made only on final bounce, implies pursuit is used to
guide hands. Maybe position of eye in head.
•
Binocular information versus monocular (looming)