Download Eye movements

Eye movements: a primer
Leanne Chukoskie, Ph.D.
Two classes of eye movements
• Gaze-stabilizing
– Vestibulo-ocular reflex (VOR)
– Optokinetic Nystagmus (OKN)
• Gaze-shifting
– Saccades
– Smooth pursuit eye movements
VOR
OKN
OKN
Gaze-shifting movements for directing
retinal specialization
Gaze-shifting movements for directing
retinal specialization
• If you have a fovea, or something like it, you
want to be able to aim it.
Smooth Pursuit
Saccades
• Saccades and pursuit are voluntary (vs. OKN
and VOR which are stimulus-driven and
involuntary), BUT we are thankfully not
typically conscious of our saccadic and pursuit
eye movements. We can become aware of
them and direct them, however. We’ll come
back to this point.
3-4 saccades per second!
This ends up being the most common
movement you’ll make in your life! Let’s study
it!
• The saccade itself is fast, and its velocity is
dependent on the amplitude of the saccade–
that is, bigger saccades are faster. This is
relationship is called the ‘main sequence’.
Main sequence
We don’t see during saccades
• We are effectively blind during saccades.
• Why? Two possible reasons: 1) typical speeds
of movement during saccade lies outside the
visual modulation transfer function– we just
don’t see things moving that fast. 2) an active
process believed to be in the visual thalamus
“gates” visual processing during a saccade.
• Perisaccadic mislocalization of a target also
occurs, suggesting potential mechanisms.
Saccade Circuitry
How do we measure eye movements?
• Electrooculography
– Small voltages can be measured from around the eyes which vary with
position. Easy and cheap but subject to drive and high levels of
inaccuracy.
• Video-based methods are most popular today
– Using computer vision tools to identify the center of the pupil and also
often the corneal reflection. Use these in algorithms to estimate gaze
position but importantly involve IR-light sensors for greater
robustness.
• Scleral search coil method
– Observer wears a large contact lens with a wire embedded in it and
sits inside a chamber of magnetic field. When the coil of wire moves, it
induces a voltage
Electrooculography
Small voltages can be measured from around the
eyes which vary with position. Easy and cheap but
subject to drive and high levels of inaccuracy.
Video-based methods are most
popular today
• Using computer vision tools to identify the center of
the pupil and also often the corneal reflection. Use
these in algorithms to estimate gaze position but
importantly involve IR-light sensors for greater
robustness.
Scleral search coil method
Observer wears a large contact lens with a wire
embedded in it and sits inside a chamber of
magnetic field. When the coil of wire moves, it
induces a voltage. This is really accurate but painful
and inconvenient so it isn’t used much any more.
What do we typically measure?
• Most of the time in simple saccade tasks we
look at accuracy and latency
• For scene-based tasks, we also want to know
what the observer looked at and it is less clear
how to calculate accuracy per se.
Saccades
Need better methods
• Even though we’ve been studying eye
movements since the early 1900s, we still
struggle with how to analyze sequences of
gaze.
• Most methods involve chunking a scene and
then comparing sequences of what was
fixated
Saccade endpoint choice
• Driven by a combination of low level salience
and higher level cognitive factors. These
factors also impact the amount of time people
fixate.
Passive vs. active paradigms
• Passive: I can measure what you looked at on the
screen
• Active: I can use your gaze movements to actively
CHANGE what is presented on the screen. This
has been used to study the “preview benefit” in
reading (Rayner). We are using these “gazecontingent” methods to make games to train
faster and more accurate gaze re-direction.
What can we do with these
measurements of behavior?
• Gaze behavior changes across development.
• Disorders, such as schizophrenia, autism
spectrum disorder
• Alzheimer’s Disease– interesting potential way of
identifying early risk– look at how people scan a
series of images that were presented twice. How
individuals scan the second presentation can tell
you about what they remember (Stuart Zola)