Eye Movements and Eye Tracking
Why move the eyes?
• see the same thing better (stabilize image, but not too well!)
• see something else (overcome low peripheral resolution)
S. Anstis
• only small part of visual field is sampled at full resolution
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
1
Cortical Magnification in V1
Approximately:
2
2
1



log
x

y
 x'   2
   
y 
 y '   arctan x 


Or in complex form:
z  x  iy , z '  x'iy '
 z '  log z
Eric Schwartz
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
2
Types of eye movements
Major Distinctions:
• conjugate vs. disjunctive
• abrupt vs. slow
Types:
• vestibuocular reflex: counteract head rotation
• optokinetic reflex: counteract large field retinal motion
• smooth pursuit: counteract object motion (<30 deg/s)
• saccades: rapidly shift gaze direction (<600 deg/s)
• vergence movements: adjust vergence angle
• accomodation: adjust focus
• microsaccaes: counteract drift during fixation (1-2 Hz, <5’)
• microtremor: .5’, 30-100Hz
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
3
Saccades
Yarbus (1950s)
• alternations of fixations and saccades, 3 per second
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
4
Yarbus, 1967:
• examine the picture at will
• estimate the economic level
of the people
• estimate the people’s ages
• guess what the people were
doing before the arrival of
the visitor
• remember the people’s
clothing
• remember the people’s (and
objects’) position in the
room
• estimate the time since the
guest’s last visit
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
5
Saccade Metrics
approximately linear
relationship between
saccade amplitude A and
saccade duration D:
D  D0  dA
20 ms < D0 < 30 ms
2 ms/° < d < 3 ms/°
Example:
for 20° saccade display
needs to be changed
~75ms after saccade onset
from Becker, 1991
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
6
Anthropomorphic Robot Head
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
7
System overview
Gigabit Router
Desktop A
Desktop B
Desktop C
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
8
Motor System
• 9 DoF (hobby grade servo motors)
• 2 Mini SSC II interface boards
• Four bar linkage connection
• Eye/neck system is redundant
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
9
Vision System
• 2 Point Grey Research Firefly cameras (Firewire)
• 640x480 resolution up to 30 fps
• 2, 4, 6, and 8mm focal length lenses
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
10
Saccade Performance 1
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
11
Saccade Performance 2
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
12
Demo video
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
13
Why Eye tracking
Basic Neuroscience:
- measuring brain output
- understanding neural control architecture
- psychophysics: how do we use gaze during natural tasks
Applications:
- user interface design, human factors: e.g. eye tracking for
driver of a car
- advertising: do people look where I want them to look (in
my web page, my newspaper ad, etc.)
Note 1: now several conferences solely devoted to this
Note 2: can also be done in fMRI
Note 3: possible in the real world: “portable eye trackers”
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
14
Eye tracking techniques
(Review in Duchowski&Vertegaal, 2000)
Contact lenses: with mirrors or induction loops
- fast, accurate, uncomfortable (often used with bite bars)
Video based:
- temporal accuracy limited to camera frame rate (60Hz)
- less accurate (~1 deg)
- typically with infrared illumination of the eye (dark pupil vs. bright pupil)
- can be done remotely or camera can be attached to head
Electro-oculogram:
- exploits electric dipole property of eye ball
- fast but imprecise
Limbus tracking:
- predecessor of the video based tracker
- imprecise
- small operating range
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
15
Electro-oculogram
•eye is electric dipole
•electric field of moving dipole induces measurable voltages
•provides analog voltage output that can be digitized and
processed extremely fast, but technique is not accurate
electrode
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
16
Limbus Tracking
utilize difference in reflective properties of iris vs. sclera
iris
limbus
sclera
photo diode
infrared LED
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
17
Search Coil
gold standard for speed and accuracy (5-10’’), but quite
uncomfortable and head movement restrained
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
18
Video based tracking
bright pupil image
dual purkinje tracker
most widely used today, good accuracy and speed, easy-to-use
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
19
inside of head mounted display
U. of Rochester
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
20
Gaze contingent display changes
Idea:
decide what is being displayed contingent on where
observer is looking
Saccade contingent updating (SCU):
a special case of this: make display changes while a saccade
is progressing
(pioneering work by McConkie and colleagues)
• powerful technique for studying visual perception
• frequently used in e.g. change blindness studies
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
21
Combination of video and limbus tracker
inside HMD for gaze contingent display
video based pupil
tracker’s sensor
LCD displays
inside the HMD
analog limbus
tracker’s
sensor with
infrared LED,
photo diodes
(horizontal)
• video based tracker for good accuracy
• limbus tracker for low latency saccade detection
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
22
System Overview
• 4 processor high-end graphics computer as backbone
• images rendered in V8 helmet (Virtual Research)
Three sensors:
• magnetic tracking device (Polhemus Fastrak)
• limbus tracker (ASL 210), sampled at 1.25 kHz
• pupil tracker (ASL 501), sampled at 60 Hz
Sensors send data via separate serial lines
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
23
Latency Comparison
in this example:
limbus tracker has
26 ms advantage
over video tracker
on average: limbus tracker has 37±13 ms advantage (15 trials)
Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch
24
chess pieces (dis-)appear contingent on
saccade
(~25-30°)
with 50% probability
Jochen
Triesch, UC
San Diego, http://cogsci.ucsd.edu/~triesch
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