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Eye movements: Lab # 1 - Catching a ball. What can be learnt from natural tasks?. 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)

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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)


Why do we move our eyes?

- Image stabilization

- Information acquisition


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.


Visual Angle




tan(a/2) = x/d

a = 2 tan-1 x/d

Why eye movements are hard to measure.

A small eye rotation translates into a big change in visual angle


1 diopter = 1/focal length in meters

55 diopters = 1/.018

0.3mm = 1 deg visual angle


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


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.

brain circuitry for saccades
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

saccade command

inhibits SC

signals to muscles


Posterior Parietal Cortex

Intra-Parietal Sulcus: area

of multi-sensory convergence


LIP: Lateral Intra-parietal Area

Target selection for saccades: cells fire before saccade to attended object


smooth pursuit1

Brain Circuitry for Pursuit

Smooth pursuit

& Supplementary

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.


Batsman anticipate bounce point

Better batsman arrive earlier



Eye movements in cricket:

Land & MacLeod, 2001


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.


Is prediction seen in cricket a general property of behavior, or only

seen in skilled performance like cricket or baseball?



smooth pursuit



Catching: Gaze Patterns





Unexpected bounce leads to poor performance, particularly in the

pursuit movement after the bounce.

Implications of this?





Gaze Patterns Different when Watching




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???
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)