Neuronal mechanisms of attention - hemispatial neglect Flashcards Preview

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1

stroke

When someone has a stroke a blood clot breaks off and travels up the artery until it gets caught and blocks the blood flow to one side of the brain.

Neurons die off in that area that lead to functional impairment

2

lesion

area of brain damage

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contralesional stim

Things occurring on the opposite side to the lesion

4

ipsilesional stim

Things occuring on the same side as the lesion

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cancellation tests

Because of the crossed nature of the human sensory system, a right hemisphere lesion affects the left side of space and vice versa.

Thus, patients with right hemisphere lesions have weakness on the left of the body and also have problems noticing things on the left side of space – ignore things on the left side

Influences contralesional stimuli

This is a standard test for hemispatial neglect, requiring the patient (who has a right hemisphere lesion) to cross out all of the lines on the page.

As you can see, the patient only crosses out lines on the right, ignoring all those on the left.

6

what happens when the attention system breaks - hemispatial neglect

Another test simply requires patients to copy what they see. Here, you can see they ignore all the numbers on the left and the left side of the house.

7

neuroanatomy of neglect

Temporoparietal junction (TPJ) is the most common area but neglect can occur after damage to all of these regions.

8

how do we know the patent is not just blind on one side?

As the patient recovers, neglect often resolves into extinction.

In extinction, the patient can perceive individual touches on the left and the right side.

However, when presented with a simultaneous touch on both sides, the patient only perceives the one on the right.

It is the same with visual stimuli – patients can see things on either side individually but will generally miss things on the contralesional side (the side opposite the lesion) when presented together with things on the ipsilesional side (the same side as the lesion).

primary tactile processing in tact

Neglect/extinction is not specific to any sense

Patients with extinction often can detect contralesional stimuli, just not when they occur simultaneously with ipsilesional stimuli

Primary visual cortex is often intact and shows activation to (ignored) contralesional stimuli

9

Rees et al. (2000)

fMRI evidence demonstrates that patients with neglect/extinction have preserved visual processing.

This patient has a lesion in the right hemisphere.

As you can see, when a single face is presented in their contralesional visual field, they show normal activation in visual cortex.

When the face on the left is presented simultaneously with an object on the right, the patient reports no awareness of the face.

However, there is still activation in visual cortex (albeit reduced). Therefore, the visual cortex is still processing the object despite the lack of (conscious) awareness.

modulation of primary visual cortex processing

10

so what is 'broken' in neglect?

Consensus is that neglect/extinction is a disorder of attention

Inability to consciously detect or respond to stimuli in the contralesional side of space

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neglects has helped shed light on the mechanisms underlying attention

1. Attention can operate at a late stage of processing – contralesional stimuli are able to influence behaviour despite a complete lack of awareness of such stimuli
- early = filter out low level properties of stim

2. Attention selects internal representations as well as external stimuli
- e.g. imagination/visual memory
- allocate attention to diff internal representations
- overlap with WM

3. Attention operates in an object-centred frame of reference
- attention as spotlight

4. Attention is a competitive process

12

Mattingley, Davis and Driver (1997)

In this study Mattingley and colleagues wanted to test whether attention operates at a late stage in processing, i.e. after low level visual processing

They presented subjects with Kanizsa figures, in which removing a segment of each circle produces the illusion of an object in the centre.

These figures rely on substantial low level visual processing – edges, brightness combine to form a single illusory surface.

They found that extinction of left sided circles was substantially reduced when an illusory surface was formed.

Suggests attention operates after the low level visual processing has occurred – even after stimuli in the environment have been interpreted as objects.

lots of processing needed to see square

aware of left side stim when paired into full shape

see notes and slides

13

further evidence for unconscious processing of contralesional stim: capture of attention by fear related stim - Vuilleumier and Schwartz (2001)

They tested patients with extinction on a task where they were asked simply to identify what they could see.

Patients were presented with two pictures at a time, which could either be fearful (e.g. spiders) or neutral (e.g. a ring).

They found that extinction was highest in all patients when the stimulus on the left was neutral but that extinction was reduced when the stimulus on the left was fearful.

This suggests that despite the patient not being aware of information on the left side of space, if the information is sufficiently meaningful, or important, the stimulus can ‘break through’ the attentional filter

Parallels with the cocktail party effect from the previous lecture.

emotional response

see notes

14

evidence that unattended items are processed up to the semantic level in extinction - Vuilleumier and Rafael (2000)

Here subjects were shown two words, either the same (e.g one one) or different (e.g. one two)

The first thing to note is that extinction was higher when the words were the same demonstrating that the meaning of the word is processed preattentively.

However a potential confound is that the words also look visually the same – same features etc – so could be just visual features that are processed.

So next they showed subjects words that looked different but with either the same semantic meaning (e.g. one 1) or a different meaning (e.g. one 2).

Found that neglect was still lower for words that had the same meaning, even if they looked different, than words that had different meaning

This shows that it is not the low level visual features of the words that drives the difference, but the semantic meaning of the words.

see notes

15

presence of targets drives perseveration - Manly et al. (2002)

Perseveration behavior reduced when targets were removed from the left (unattended) side – suggests influence of unseen targets on behavior to seen targets.

re-cancel stars already cancelled

see notes

fewer stars presented = less perseveration

influence behav to stars can see - processed to level that know missing something

confound - just removing clutter?

So in the next experiment they again removed targets from the left but this time replaced each target with a distractor.

Thus, the number of items (clutter) remains the same but the number of targets reduces.

see notes

They found that perseveration still reduced dramatically with the number of targets.

Again suggests that the unseen targets on the left can influence responding.

This demonstrates a really interesting aspect of attention, which is that although things are not perceived, they can still influence our behaviour.

Parallels with influence of unattended stimuli in dichotic listening task.

16

how much info is processed 'preattentively' (prior to attention and awareness?)

Task-relevance of stimuli (response related information) - processed to level know about

Integration of features into whole objects/shapes

Emotional significance of stimuli

Semantic information

17

imagine yourself in the Piazza del Doumo - Bisiach and Luzatti (1978)

asked Italian patients with neglect to recall a famous landmark in Florence and asked them what they could see in their minds eye.

The patients mostly reported seeing the buildings, cafes, shops on the side of space ipsilateral to their lesion (their ‘good’ side).

It was as if they had lost the representation of things on the left side of space, even those that only existed in their memory or imagination, not simply in the external world.

The researchers also asked the patients to imagine they were standing at the opposite end of the square and to recall what they could.

This time, they recalled all of the previously neglected information and again couldn’t report all of the contralesional information (the information they had previously recalled correctly!)

This finding suggests that attention operates also on ‘internal’ representations (e.g. memory, imagination) as well as ‘external’ representations (perception)

see notes

18

additional ev that attention operates on internal representations: spatial WM deficit in neglect - Wojciulik et al. (2001)

Patients cancelled either Os or distinct objects with either visible or invisible marks

Patients made more perseverations for Os than objects when marks were invisible

even attention operates on internal representations, possible neglect patients will show a deficit not only in attention but also in spatial working memory.

These authors gave patients a cancellation task and asked them to cancel either the Os or the objects with either visible or invisible marks.

What you tend to find normally on this task is that patients revisit previously cancelled targets (perseveration) i.e. they actually cross out the same targets over and over again.

They reasoned that if patients have a spatial working memory deficit this is likely to be more frequent when they can’t see the marks they’ve made (because they can’t remember which ones they’ve marked)

If the deficit is purely spatial then it should only occur for items that are defined purely by their spatial location, not for different objects.

This is exactly what they found – higher perseveration for invisible marks made to Os

Also found patients cancelled more targets overall if visible marks were used.

Results suggest patients with neglect have trouble with spatial working memory (internal attention) as well as external attention.

see notes

19

ev from hemispatial neglect for object-based attention

In the picture of the two flowers, the patient copies the right half of each flower, rather than only the flower on the right.

Demonstrates that attention operates in an object, rather than spatial, frame of reference.

20

ev from extinction for object-based attention - Behrmann and Tipper (1994)

In this study patients were presented with a barbell stimulus and a target (small white square) could appear in either end of the barbell.

In the static condition the barbell appeared and then the target appeared. In the moving condition, the barbell appeared and subsequently rotated and then the target appeared.

The study showed that in the static condition, patients with right hemisphere lesions were slower to detect targets on the left, as you would expect.

However, in the rotating condition, the direction of their neglect suddenly reversed – targets that appeared on the right side of the barbell (which was previously on the left side of space) were now detected slower.

This shows that attention is more focused on objects than spatial locations. Attention operates in an object-based frame of reference.

see notes

21

ev that neglect involves deficit disengaging from ip stim - Posner et al. (1984)

see notes

Reflexive attention has also been studied using the Posner cueing paradigm, but in this version subjects are cued exogenously.

This means that a flash of light or something attention grabbing occurs in a peripheral location, and then a target appears either in the location of the flash (called a cued trial) or in the other location (uncued trial).

see notes

As you can see, RTs are much higher for the uncued contra condition indicating a problem disengaging attention from cues on the ipsilesional side.

attentional comp

attentional imbalance rather than deficit - hyper attention to good field

Suggests patients have trouble disengaging from stimuli on the ipsilesional side to focus on contralesional stimuli

Spatial attention is a competitive process

22

Balint's syndrome

rare disorder where patients have had strokes damaging both sides of their brain encompassing both parietal lobes.

In this case, the attentional disorder is not spatial but patients lose the ability to attend simultaneously to two objects – again demonstrates the competitive nature of attention

23

neglect helped resolve central debates about how attention operates

Attention operates on both internal and external representations (Piazza del Duomo experiment)

Role of competitive processes in attention (extinction, disengage deficit)

Attention can operate at a late stage (experiments showing substantial processing of unattended stimuli)

Attention operates in an object-based frame of reference (e.g. rotating barbell experiment)