Spatial Attention

1
Spatial Attention

• Chris Rorden
• Posterior Right Hemisphere Injury
• Extinction
• Neglect
• Balints Syndrome
• Anosognosia

www.mricro.com
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Extinction

• Patients can report single item at any location.
• Only report ipsilesional item when two targets
  are presented simultaneously.

left
right
right
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Explanations for extinction

• Low level perceptual deficit
• Pickpocket example
• Attentional deficit
• Disengage deficit get locked on contralesional
  item (Posner et al)
• Attentional bias

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Extinction

• Patients extinguish task-relevant information
  (Baylis et al. (1993) Journal of Cognitive
  Neuroscience 5 453-466)

E
F
E
E
F on right E on left
E on right
Task report identity
blue on right
blue on right, purple on left
Task report color
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Intact Enumeration

• Healthy people subitize can count five
  objects as fast as one.
• Vuilleumier and Rafal (1999) show patients can
  still count

Localize
Count
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Prior entry task

• Task Report which side appeared first.
• More sensitive measure of any deficit (in ms).
• Taps perception not speed of motor response.

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Normal Performance
Right-first
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Rorden et al (1997)
Right-first
Right-first
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What is simultaneous (Baylis et al. 2002)

• Target detection
• Extinction most severe when items are actually
  simultaneous.

• Prior Entry Task
• Contralesional item must lead to appear
  simultaneous.

JB left injury KH left injury TP right
injury
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Neglect

• Clinical deficits could be motoric or perceptual.
• Experimental tasks demonstrate pure perceptual
  component.
• Do patients have motoric deficits?

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Neglect Association of deficits

• Neglect is easy to diagnose acutely
• Patients ignore contralateral stimuli
• Common neglect symptoms
• Perceptual problems
• Deficits within and between objects
• Mental imagery problems
• Motoric deficits
• Extinction
• Different patients exhibit different symptoms
  (dissociations).
• Is neglect a meaningless entity? (Halligan
  Marshall, 1992)

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Line Bisection and cancellation

• Classic measures neglect.
• These tasks are not pure motoric and perceptual
  deficits can hinder performance.
• These tasks dissociate
• Cancellation is sensitive but not specific
  (patients with perseveration can fail this)
• Bisection deficits are often independent of other
  neglect like behavior.

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Space versus object neglect

• Some patients exhibit egocentric neglect
  ignoring items on the left side of a display.
• Other Patients exhibit allocentric neglect
  ignoring the left side of items regardless of
  their position in the display.

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Visual imagery deficits

• Patients can neglect information in imagined
  space.
• Even unexperienced mental imagery Imagine you
  are in the North of France looking South what
  cities do you see?

15
Visual imagery deficits

• Example What do you see when you walk from your
  home to your pub? Versus What do you see when
  you walk home from the pub?
• Demonstrates neglect not purely perceptual.

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Motoric Deficits

• In some patients, motoric deficits appear to
  dominate perceptual deficits.
• Example Line bisection task where left movements
  adjust response rightwards.

17
Motoric deficits mirror reversal task

• In some patients, motoric deficits appear to
  dominate perceptual deficits.
• Example Line bisection task observed through
  mirror.

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Different symptoms?

• Perhaps TPJ misleading two anatomically and
  behaviourally distinct patient groups (Rorden et
  al, 2005)

• IPS patients line bisection and cancellation
  deficits
• Anterior patients only cancellation deficits

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Conclusions

• We can explain previous lesion studies
• Neglect patients
• Bisection error posterior injury, Accurate
  bisection anterior injury
• Binder (1992), Mort (2003), Rorden (2005)
• Post-hoc analysis of Mort data patients with IPS
  injury have x2.5 the line bisection bias as those
  without IPS injury. Yet, these patients are much
  better (find 40/60 items) on cancellation task
  than those without (find 16/60).
• Patients without neglect
• Bisection errors posterior injury
• Machado (1999)
• Explains fMRI/TMS studies showing IPS not TPJ
  crucial for attention.
• Problem previous fMRI studies have not observed
  STG activations.

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What is normal attention like?
21
Balints Syndrome

• Dorsal Simultanagnosia
• Rapid perception of single objects.
• Appear to get locked onto one object.

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Balints syndrome

• Optic Ataxia Misdirected movement
• Tend to reach exactly where they are fixating
  (magnetic misreaching)
• Ocular Apraxia Visual scanning deficit
• Tend to keep eyes locked straight ahead, and move
  whole head. However, can make saccades on demand.
• Dorsal Simultanagnosia Can see only one object

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Simultanagnosia

• Patients with simultanagnosia appear to only see
  one object at a time.
• Object grouping seems intact.

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Covert awareness

• Unable to report the global letter (the large P)
• Yet faster to name small letter if it matches the
  large letter.
• Suggests residual global processing.

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Patient KB
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Find the O, or find the Q

• For healthy people
• Finding a Q in Os is easy it pops out.We see
  the Q immediately.
• Finding an O in Qs is hard.We have to inspect
  each item.
• How about KB?

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Patient KB
Find Q among Os
Find O among Qs
2500
2500
2
3
2000
2000
Reaction Time (ms)
8
Reaction Time (ms)
1500
1500
0
2
30
11
1
3
1000
1000
3
2
5
500
500
4
8
12
4
8
12
Set size
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Conclusions

• KB is only aware of one object.
• Yet, parts of her brain see the whole scene.

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Double neglect?

• Is Balints syndrome a type of double neglect?
  (e.g. Farah, 1990).
• Neglect usually from right parietal damage and
  neglect left space
• Balints patients neglect both sides of space but
  can see a single object.
• Do not neglect a portion of the objects they see.
  In fact, they see nothing but objects.
• Balints syndrome most often associated with more
  superior and posterior injury than neglect.
• Therefore, some argue that neglect and Balints
  probably reflect different underlying deficits.

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Anosognosia

• Anosagnosia is the denial of illness which is
  often seen in brain-injured patients. Frequently
  associated with hemineglect.
• Anton (1899) - Reported the case of UM who was
  shown to suffer from cortical blindness but
  denied this. (termed Anton's syndrome). Patients
  pupils respond to light but the patient is unable
  to demonstrate functional sight. Deny any visual
  difficulty. Confabulate responses, guess, and
  make excuses for deficit e.g., "the room lights
  are too dim" or "I don't have my glasses with me"
• Von Monokow (1885) - Reported a 70 year old
  patient who had suffered bilateral damage to
  posterior brain areas and exhibited loss of sight
  of which the patient was not aware (patient
  attributed visual deficit to loss of ambient
  light).

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Explanations for Anosognosia

• Several possible explanations
• Psychological defence mechanism
• Absent feedback
• Confabulation
• Heilman intentional model

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Denial

• Anosognosia may reflect denial, as a defence
  mechanism (Weinstein and Kahn, 1955)
• psychologically motivated, an unconscious defence
• mechanism to attenuate the distress of a
  catastrophic event (e.g. hemiplegia).
• The location of the brain lesion determines the
  disability. This is separate from the mechanism
  of denial.
• Most Anosognosia patients have RH damage (also
  shown with Wada testing Gilmore et al., 1992)

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Sensory feedback

• Anosognosia might result from reduced or absent
  sensory feedback (Levine et al., 1991)
• Think they have moved hand but dont know they
  have not because no somatosensory feedback to
  provide mismatch
• But many patients still unaware of hemiplegia
  when given visual feedback
• Could this be unawareness of their hand
  (asomatognosia)?

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Confabulation

• Feinberg suggests confabulation, with strong
  association between
• illusory limb movements (claim they can move
  paralyzed limb),
• Neglect
• Anosognosia
• Also suggests that patients with neglect often
  confabultate what happens in neglected space.

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Feed-forward
Anosagnosia as a failure of monitoring associated
with

• Failure to set the monitor
• 2. Absence of feedback
• 3. False feedback (monitor dysfunction).

Heilman et al., 1998.