SymmetryDetecting Symmetry in a Random Background

Monica Cook

Bilateral Symmetry

Bilateral symmetry occurs in many objects and scenes.

Eiffel Tower

Brooklyn Bridge

Animals Faces

A Symmetric Target from our Experiment

Bilateral Symmetry

Symmetry could be important because:• Almost everything is symmetric• It directs or catches attention• It is important in object recognition and memory

Purpose

Hypothesis: Attention and fixation are drawn to symmetric areas.

Determine if symmetry can be detected quickly within a random background

Methods

Experiment 1: Target located along the horizontal meridian

Methods

Experiment 2: Target located at any location within the display

 

ASL 504 Remote Eye Tracker

• Participant positioned in a chin rest facing monitor

• A video camera captures the eye

• Pupil and corneal reflection used to determine eye position

• 17-point calibration target

Eye camera and infrared LED

Calibration Check

A participant looking through the 17-point calibration target after being calibrated.

Example Trial

After a video has been calibrated, it looks like this…

7 83

910 11 12

1314

15

2

45

61

Experiment 1: Results

Time to First Fixation Time to Response

10.0

5.0

0.0

Tim

e (s

ec)

Fixation on Symmetric Target

Conclusions: Experiment 1

• Symmetry sometimes detected at a glance

• Detection sometimes required a much longer search

• Fixated on the target but still hesitated to respond

• The further the target was from the center, the longer it took to be detected

Experiment 2: Results

NUMBER OF FIXATIONS BEFORE TARGET FIXATED

0

10

20

30

40

50

60

70

Above/Below Left/Right Off Axis Near Off Axis FarSYMMETRY LOCATION

NUM

BER O

F F

IXATIONS

SLKPFCDASCNSCLGYAS

Average

NU

MB

ER

OF

FIX

AT

ION

S

Experiment 2: Results

If fixation was drawn to symmetrical areas, this graph would slope progressively downward until the target was found. The blue line below is a good example.

TRIAL 19 - J UST TO R AND ABOVE FIX

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

FIXATION NUMBER

CNS

DAS

PFC

SLK

DIS

TA

NC

E IN

PIX

EL

S

FIXATION NUMBER

Experiment 2: Results

TRIAL 21 - TO R AND ABOVE FIX

0

50

100

150

200

250

300

350

400

FIXATION NUMBER

CNS

DAS

PFC

SLK

TRIAL 18 - TO R AND ABOVE FIX

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

FIXATION NUMBER

CNS

DAS

PFC

SLK

However, graphs like these do not follow this pattern.

Conclusions: Experiment 2

• The closer the target is to the center, the more quickly it is found

• Occasionally, participants gradually fixated closer to the target - inconsistent

Why?

There may still be preconscious symmetry detection –

Accidental symmetry attracts fixations

Further Research

A symmetry detection algorithm has been developed to test this.

Symmetrical axis of target

The Next Step…

• Compare fixation locations with the algorithm output for each image

• If fixations line up with lighter areas:– Indicates eye movements guided by

symmetry – Explains why noise in the background is a

problem

Thanks!

Jeff Pelz and Andy Herbert

Joe Pow and Bob Callens

Sue, Brian, Meredith, and Chris

The Other High School Interns