1. Eye movements: Lab # 1 - Catching a ball
Simple tutorial on Eye Movements:

2. What can be learnt from natural tasks?

4. Gaze exclusively on task-relevant objects (see Land chapter)
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)

5. Why do we move our eyes? - Image stabilization
- Information acquisition

6. The Eye and Retina
A reason that it helps to move the eyes is the construction of the retina. Humans and many animals have foveated
visual system - what this means is that spatial resolution varies across the eye and that the best sensitivity (ie detail) is concentrated
In a one degree of visual agle in an area called the fovea – additionally – RGC’s pool over receptors - in the fovea have 1:1 input
So there is no spatial summation

7. Cone Photoreceptors are densely packed in the central fovea
120 million rods, and are more sensitive (low light) than the cones. However, they are not sensitive to color.
The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in fovea

8. Visual Acuity matches photoreceptor density
Relative visual acuity
Receptor density
This is basically the same graph but also demonstrates that the level of acuity (Ie the sharpness of objects)
Is best and falls off drastically in the periphery
Peak acuity - can distinguish lines separated by a photoreceptor width.

9. Why do we move our eyes?
1. To bring objects of interest onto high acuity region in fovea.

10. Why eye movements are hard to measure.
A small eye rotation translates into a big change in visual angle x a
tan(a/2) = x/d
a = 2 tan-1 x/d
Visual Angle d
18mm
For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre
1 diopter = 1/focal length in meters
55 diopters = 1/.018
0.3mm = 1 deg visual angle

11. 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)
Now we should review the major types of eye movements
Mediated by different brain areas which we’ll review on thursday
Fixation: period when eye is relatively stationary between saccades.

12. Primary Cortical Sub-divisions

13. Visual Projections
– optic nerve exits the eye – hemifields decussate at the optic chiasm so left brain gets left hemifield
- right brain gets right hemifield. These projections go to several different areas - 90% of retinal projections go to LGN. (20% of lgn’s inputs are from retina) -
this is the major stop before cortex –
superior colliculus also receives direct retinal projections.
Mid-brain projections: pupil dilation, accomodation, eye movements/head movements.

14. Brain Circuitry for Saccades
1. Neural activity related to saccade
2. Microstimulation generates saccade
3. Lesions impair saccade
the frontal eye field, parietal eye field, supplementary eye field, prefrontal eye field, and area 7m
In each of these regions: (1) there is neural activity closely related to eye movements; (2) electrical microstimulation produces or modifies eye movements; (3) surgical lesions or chemical inactivation impairs eye movements; (4)
V1: striate cortex
Basal ganglia
Oculomotor nuclei

15. Function of Different Areas
monitor/plan movements
target selection
saccade decision
saccade command
inhibits SC
signals to muscles

16. Posterior Parietal Cortex
Intra-Parietal Sulcus: area
of multi-sensory convergence
reaching
grasping
PPC contains areas LIP, VIP, MST, and areas 7a and 7b. Sensory motor interface
Many types of representations in space have been found in the PPC. It is an area of high convergence where vision, audition, eye & head position, eye velocity, vestibular, and proprioceptive signals combine.
LIP: Lateral Intra-parietal Area
Target selection for saccades: cells fire before saccade to attended object

17. Brain Circuitry for Pursuit
Smooth pursuit
& Supplementary
Smooth pursuit
frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation.

18. Brain Circuitry for Pursuit
Smooth pursuit
& Supplementary
Velocity signal
Early motion analysis
Smooth pursuit
frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation.

19. 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.

20. Eye movements in cricket:
Batsman anticipate bounce point
Better batsman arrive earlier
saccade
pursuit
Show - Not confined to expert cricketers- general aspect of behav
Land & MacLeod, 2001

21. Why are eye movements predictive?
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.

22. Is prediction seen in cricket a general property of behavior, or only
seen in skilled performance like cricket or baseball?

24. Catching: Gaze Patterns
smooth pursuit X X
saccade X
Thrower
Catcher

26. Unexpected bounce leads to poor performance, particularly in the
pursuit movement after the bounce.
Implications of this?

28. After three trials, pursuit has improved a lot.
Implications of this?

30. Different pattern of eye movements when watching (earlier, no pursuit).
Implications of this?

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

32. Lab groups

33. 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, about10 times.
Then use a different ball,10 trials.
Other balls…
Compare one versus two eyes???

34. 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)