Eye movements: Lab # 1 - Catching a ball

What can be learnt from natural tasks?

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.

18mm

0.3mm = 1 deg visual angle

x a

tan(a/2) = x/da = 2 tan-1 x/d

Visual Angle

d

1 diopter = 1/focal length in meters

55 diopters = 1/.018

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

Types of Eye Movement

Information Gathering StabilizingVoluntary (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

Vergencechange point of fixation in depthslow, disjunctive (eyes rotate in opposite directions)(all others are conjunctive)

Fixation: period when eye is relatively stationary between saccades.

Primary Cortical Sub-divisions

Visual Projections

Brain Circuitry for Saccades

Oculomotor nuclei

V1: striate cortex

Basal ganglia

1. Neural activity related to saccade2. Microstimulation generates saccade3. Lesions impair saccade

target selection

signals to muscles

inhibits SC

saccade decision

saccade command

monitor/plan movements

Function of Different Areas

LIP: Lateral Intra-parietal AreaTarget selection for saccades: cells fire before saccade to attended object

Posterior Parietal Cortex

reaching

grasping

Intra-Parietal Sulcus: areaof multi-sensory convergence

Smooth pursuit& Supplementary

Brain Circuitry for Pursuit

Smooth pursuit& Supplementary

Brain Circuitry for 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 singlemovements in response to flashes of light.

Eye movements in cricket:

Batsman anticipate bounce point

Better batsman arrive earlier

Land & MacLeod, 2001

pursuitsaccade

Photoreceptors ganglion cells LGN

Primary visual cortex other cortical areas

mid-brain brain stem muscles

Why are eye movements predictive?

Analysis of visual signals takes a lot of time!

Round trip from eye to brain to muscles takes a minumumof 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 onlyseen in skilled performance like cricket or baseball?

Catching: Gaze Patterns

CatcherThrower

saccade X

X

smooth pursuit

X

Unexpected bounce leads to poor performance, particularly in thepursuit 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?

CatcherThrower

saccade

X

X

Gaze Patterns Different when Watching

X

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, 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)