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Spatial Attention

• Chris Rorden• Posterior Right Hemisphere Injury

• Extinction• Neglect• Balint’s 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

Patient’s extinguish task-relevant information (Baylis et al. (1993) Journal of Cognitive Neuroscience 5: 453-466)

E F E E

“F on rightE on left”

“E on right”

“blue on right”

“blue on right, purple on left”

Task: report identity

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

Loca

lize

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

Left-first Right-first

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Rorden et al (1997)

Left-first Right-firstRight-first

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What is simultaneous (Baylis et al. 2002)

Prior Entry Task– Contralesional item must lead

to appear simultaneous.

Target detection– Extinction most severe when

items are actually 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?

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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.

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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?

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Balints Syndrome

Dorsal Simultanagnosia– Rapid perception of

single objects.– Appear to get ‘locked’

onto one object.

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Balint’s 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 O among Qs

500

1000

1500

2000

2500

4 8 12

Reaction Time (ms) 3

2

8

11 30

3

Find Q among Os

500

1000

1500

2000

2500

4 8 12Set size

Reaction Time (ms)

01 2

32 5

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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 Balint’s syndrome a type of double neglect? (e.g. Farah, 1990).– Neglect usually from right parietal damage and

neglect left space– Balint’s 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. Balint’s syndrome most often associated with

more superior and posterior injury than neglect.– Therefore, some argue that neglect and Balint’s

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 explanations1. Psychological defence mechanism

2. Absent feedback

3. Confabulation

4. 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 don’t 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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Motor 'intentional' activation

system

Upper motor neuron

Lower motor neuron

Effector

Comparator or monitor

Heilman's intentional model

1. Failure to set the monitor

2. Absence of feedback3. False feedback (monitor

dysfunction).

Anosagnosia as a failure of monitoring associated with:

Heilman et al., 1998.

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