Eye Movements: Anatomy, Measurement

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Eye MovementsAnatomy, Measurement Research
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Goals for Today

• An introduction to practical work in
  psychologyfocusing on how we can characterize
  cognitive processing components by using eye
  movements as a quantitative measure
• Basic understanding of
• structure and organization of the CNS associated
  with eye movements
• how cognitive processing (i.e., attention)
  influences eye position and the reverse
• practical procedures for recording eye movements

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Try This !

• - Try reading without moving your eyes
• If you can read this you are moving your eyes
• - Try shaking your head side to side and read the
  sentence below
• If you can read this you are not shaking your
  head hard enough

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Function of Eye Movements

• Resolution falls away as you go out from the
  foveaneed to move eyes
• We make about 3 saccadic eye movements a second
  and over 150,000 a day
• Stabilization of gaze relative to the external
  world
• Two general classes of movements
• - Voluntary
• - Involuntary

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5 Types of Eye Movements

• Saccades brings the target to the fovea
• Vergence looking from far to near
• Pursuit moving objects are kept still on retina
• Vestibular Ocular Reflex (VOR) when we move our
  head eyes stay locked on target
• Optokinetic Reflex (OKR) when image slips on a
  large portion of the retina

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Eye Anatomy
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6 Extraocular Muscles
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Generation of a Saccade
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What Initiates a Saccade?

• Activity at the centre of the Superior Colliculus
  must be removed
• This activity keeps eyes fixating on current
  location
• Thus activity at A and at the same time removed
  activity at centre

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What Stops a Saccade?

• Feedback?
• Too slow
• Called ballistic movements
• Uses an internal sense of eye position to guide
  and stop saccades
• Internal estimate generated by the PPH

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• Exogenous movement to a novel stimulus in the
  periphery (e.g. flashing/moving)
• Short pathways for rapid responses
• directly through the superior colliculus
• indirectly through the visual cortex

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• Visual search requires attention
• Area LIP (lateral inter-parietal) helps direct
  attention
• Pathway is longer, so there is a longer response
  time

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• Saccade to a remembered target
• Prefrontal association area holds target
  locations in working memory
• LIP selects the target of interest and the
  Frontal Eye Fields (FEF) generates a saccade

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Eye Tracking in Cognitive Research

• Valuable tool because
• Good measure of visual attention focus (on-line
  processing)
• Fine temporal resolution
• Methods can be non-intrusive
• Easy to learn

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Eye Movements in Reading

• Eye moves about every 250 ms
• Average distance is about 8 letter spaces
• Backwards movements about 10-15 of the time
• Much between and within variability
• Perceptual span 3-4 letters left of fix and about
  15 letters to right
• Identification span is only about 7-8 letters

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Eye Movements in Dyslexia

• General problems moving eyes?
• More regressions in English less in German

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Face Scanning in Autism

• Why problems scanning?
• Cognitive disorder?

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Attention in Autism

• Evidence of abnormal visuo-spatial attention
  processing ability (i.e. face processing)
• Visual discrimination ability is enhanced in
  autism (ORiordan and colleagues at Cambridge)
• Debate regarding neurological origin of
  abnormalities
• Low Level (engagement or disengagement)
• Higher Level (voluntary control)

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Low Level Attention in Autism

• Loss of lower level attentional engagement Gap
  Task (Van der Geest, et al.,2001)
• Cerebellar dysfunction visually guided task
  (Takarae, et al., 2004)

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High Level Attention in Autism

• Inability to suppress behavioural responses to
  compelling stimuli
• Anti-Saccade Task
• No diff in bottom up tasks but diff with top-down
  tasks (Minshew, Luna, and Sweeney,1999)

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Tracking Practicalities

• Hardware
• tracker, monitor, computers, seating
• Software
• recording, stimuli presentation
• Calibration
• luck
• Environment
• lighting, field of view, distractions

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Techniques for Measurement

• Electrooculography
• Pros
• cheap
• relatively accurate
• Cons
• artifacts
• affected by metabolic change and dark adaptation

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Techniques for Measurement

• Scleral Search Coils
• Pros
• torsional movements recorded
• very high temporal and spatial resolution
  allowing even the smallest types of eye movements
  (eg micrsaccades) to be studied
• Cons
• very invasiveouch!

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Techniques for Measurement

• Infra-Red Oculography
• Pros
• rage of lighting conditions
• high spatial (0.1 deg) and temporal resolution (1
  ms)
• portable
• Cons
• problems with blinks

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How Does IR Work?

• In a dim lit room viewing a lighted stimuli,
  there is a reflection of the image on the cornea
  (the First Purkinje Image)
• Corneal surfaces is spherical so possible to
  determine the centre when illuminated by two
  known light sources
• The pupil is the main landmark
• Measure the Purkinje reflection relative to the
  pupil

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Parameters Measured
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Amplitude

• The size of the saccade
• Gain ratio of actual saccade divided by desired
  saccade
• lt1 too small hypometric
• gt1 too big hypermetric
• For saccades up to 60 deg, duration linearly
  depends on amplitude

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Amplitude vs. Duration
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Peak Velocity

• Highest velocity reached during a saccade
• Profiles are usually symmetrical for small and
  medium sized saccades

Duration

• Time taken to complete the saccade
• Most saccades usually take a few tens of
  milliseconds
• Saccades can be modified before info reaches
  cortex (80ms)

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Latency

• Time taken from appearance of a target to the
  beginning of a saccade in response
• Medium amplitude saccades (5-10deg) usually take
  around 200ms to start
• Range can be as low as 100ms up to 350ms
• Latency is greatly related to attention

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Microsaccades

• Kind of fixational eye movement
• Small, jerk-like, involuntary
• Role in perception still debated
• Thought to help maintain visibility or fixation
  correction
• Stabilized images can disappear from perception
  in a few seconds or less

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Fixation

• During fixation the eye is not still
• Micro saccades and slow drift
• The figure shows how these movements move the
  retina's cones across the image
• If the image is artificially stabilized on the
  retina, it disappears

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Main Sequence

• Saccades are extremely stereotyped with
  relatively fixed relationships between the
  amplitude, duration and peak velocity
• Can use quantitative measurements of saccades to
  assess the oculomotor system and cognitive
  processing

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Examples
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Clinical Eye Movement Disorders
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Clinical Slow Saccades

• Drugs
• Fatigue
• Basal Ganglia Syndromes (Huntington's, Wilsons
  Disease)
• Cerebellar Syndromes (Olivopontocerebellar
  atrophy, Josephs disease)
• Peripheral Oculomotor Weakness (Miller Fisher,
  Thyroid Disorders)
• White Matter Diseases (Adrenoleudokystophy,
  Internuclear opthalmophegia)
• Miscellaneous Disorders (Niemann-Pick, Wernickes
  opthalmoplegia)

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Clinical Fast Saccades

• Calibration Errors
• Opsoclonus (or ocular flutter)
• Restriction Syndromes
• Saccade programmed centrally
• Muscular restriction
• Small saccade is made with the velocity
  appropriate to a bigger saccade

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Clinical Asymmetrical Saccadic Velocity

• Ocular restriction syndromes
• Cranial nerve, muscle palsies
• Nuclear Lesions
• Normal and slowed adduction from internuclear
  ophthalmoplegia (INO)often in multiple sclerosis

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Tracking Examples
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• There has been, and continues to be, much
  scientific effort devoted to understanding how we
  see. However, it is a fact of our experience that
  we do not simply see vision is linked to action
  and behaviour. To understand how the brain brings
  about this synthesis, it is necessary to know how
  the visual system interacts with other sensory
  and motor systems.

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