W3) 7) Disorders of perception, Neglect, Blindsight, Prosopagnosia

Question 1

Answer 1

Visual illusions…
Once you see it, you can’t unsee it. Cognitive impenetrability - even in 25 yrs you can see it even if you swear you’ve never met the person that showed you…

Conscious thought doesn’t effect perception.

Question 2

Answer 2

Change blindness - bars change levels.
Like blindspots = image on retina… brain changes things.
50% of brain processes visual information. It makes sense… Crucially, as hunters, we need vision. Tragectory, move out of the way, spot things etc.

This means with things like memory can change… Brain alters things.

Question 3

What is the uncomfortable truth?

Answer 3

Your brain lies to you all the time!​

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It is constantly trying to fill in perceptual gaps, filter a barrage of incoming information, and make sense of the world.​

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This happens (largely) outside of conscious awareness.​

Comfabulation - choosing an answer that sounds plausable, even if it’s wrong. What if the brain does this with everything?

Question 4

What is neglect?

Answer 4

Neglect: “A failure to report, respond, or orient to novel or meaningful stimuli presented to the side opposite a brain lesion, when this failure cannot be attributed to either sensory or motor defect” (Heilman, 1979).​

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Also referred to as hemi-neglect, visual neglect, visuo-spatial neglect and unilateral neglect.​

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EXTREMELY heterogeneous condition!​

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Stroke = lack of oxygen, blood, brain uses glucose. Brain region dies. After the stroke, plasticity… learns how to do things.

Question 5

What are the type sof neglect?

Answer 5

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Sensory​

Motor​ = contralateral difficulty in using other side.

Spatial​ = navigating.

Personal​= see everything in the world, but neglect half of your personal space. Forget left side of body.

Representation = Eg. can’t see left side of house, but know there is a left side…

Question 6

What are the other types of neglect?

Answer 6

Neglect dyslexia​

Neglect dysgraphia​

Facial neglect​

Auditory neglect​

Tactile neglect​

Extrapersonal​

Question 7

What is an everyday example of neglect?

Answer 7

Patients behave as though one half of the world does not exist (they aren’t aware of the missing half).​

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In everyday life patients with neglect may fail to:​

draw portions of a picture​

shave / apply make-up to only half their face​

dress only one side of their body​

eat food on only one side of their plate​

read part of a word or sentence​

Question 8

What is personal and extrapersonal neglect?

Answer 8

Personal neglect: a lack of orientation or exploration of the side of the body contralateral to the injured hemisphere (Beschin & Robertson, 1997)​
Eg. Someone paralised down one side of the body says they can tie their shoe laces = very clever right hand. They are unaware… It is perceptual.

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Extrapersonal neglect: a failure to detect visual and auditory stimuli on the contralesional side (Peru & Pinna, 1997).​

Question 9

How can you assess neglect?

Answer 9

Neglect can be assessed in a variety of ways. Examples:​

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Cancellation tasks (e.g. Star/Line).​ Have a group of letters, circle the ‘a’ and they would only do half of it.

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Line bisection.​

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Copy drawing or draw from memory.​ Eg. Show half of house.

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One-item test (aka The Personal Neglect Test)​

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Question 10

How do you test for personal neglect?

Answer 10

Requires the patient to touch their contralesional hand using their ipsilesional hand.​

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0 = the patient promptly reaches for the target.​

1 = the target is reached with hesitation and search.​

2 = the search is interrupted before the target is reached.​

3 = no movement towards the target is performed.​

Question 11

What is the neuroanatomy of neglect?

Answer 11

Parietal = omission of motor. Navigation.

Question 12

What can you understand from the insight from neglect patients?

Answer 12

Studies of neglect have revealed a great deal about how attention and space are processed in the brain.​

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For example, neglect is far more frequent following damage to the right-hemisphere, resulting in failure to attend to the left.​

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This suggests that there is likely to be a hemispheric asymmetry such that the right hemisphere is more specialised for attention than the left (see also Posner & Petersen, 1990).​

Question 13

In a representational account, what does it show?

Answer 13

Neglect is NOT just a visual field defect.​

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Bisiach & Luzzatti (1978) suggest that the parietal lobes contain an elaborate representation of the world.​

Question 14

What did asking patients to draw this building in Venice show?

Answer 14

Asked to imagine what the it would look like facing one way, then in another direction.

They found that they drew the other half of the building. This shows that there is no problem in both representation of the world.

Question 15

What is the Bisiach & Luzzatti study in 1978?

Answer 15

Bisiach & Luzzatti (1978) asked two neglect patients to imagine being in the Piazza Del Duomo. A well known square in Milan and the patients’ native city.​

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Describe the buildings and other features around the square. ​

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When asked to imagine standing on the steps of the cathedral at one end of the Piazza, nearly all of the features mentioned were ones that would have been to their right from that viewpoint​

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Very few things on the left were recalled. ​

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When asked to imagine standing at the opposite end of the square (facing the cathedral) most of the features mentioned were ones on the previously neglected, which was now to their right. ​

Question 16

What was the interpretation of the Bisiach & Luzzatti study?

Answer 16

The patients were forming a mental image of the Piazza, as viewed from the specified location, and attempting to read off the features around it from their imagery. ​

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Knowledge of features on both sides was in their memory, but they were unable to access all of it normally from their imagery. ​

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Representational neglect has since been studied in numerous other patients using other locations and various other stimuli (e.g., Rode et al., 1998, 2004). ​

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Question 17

What is another representational accounts?

Answer 17

Parietal cortex on each side of the brain contains an elaborate spatial representation of the external world. Ergo, damage to parietal on one side of brain causes loss of half the spatial representation of the world.​

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Data from the Piazza del Duomo experiment (and other similar experiments) appear fairly convincing.​

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However, it remains unclear in this explanation exactly how neglect is brought about.​

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Is the representation of space itself impaired?​

Is the representation preserved but the ability to scan it lost?​

Question 18

What is attention?

Answer 18

William James (1890) “no one knows what attention is” - or at least there is no obviously agreed definition.​

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“The taking possession by the mind, in clear and vivid form of one out of what seem several simultaneously possible objects or trains of thought” (James, 1890).​

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Selection of information for conscious processing and action – the ‘spotlight’ idea.​

Question 19

Could neglect be an attention problem as opposed to visual perception?

Answer 19

Impaired orienting of attention to neglected side. (Heilman, 1979; Riddoch & Humphreys,1983).​

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Overly strong orienting of attention to non-neglected (intact) side. (Kinsbourne, 1978; Ladavas, 1990). ​

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Impaired disengagement of attention once it is oriented to the non-neglected (intact) side. (Posner et al, 1982).​

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Question 20

What does insight from neglect patients do?

Answer 20

When a brief cue is flashed to either the left or right side, neglect patients may engage attention and move their eyes to the side normally.​

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However, if the cue orients them to the right and then the target appears on the left (the neglected side), they may fail to detect this.​

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The fact that patients can detect targets on the left when cued to the left suggests that the deficit is related to shifting attention rather than a problem of initial perception.​

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Posner & Petersen (1990) therefore suggest that the parietal lobes are not critical for the initial orienting of the cue but are necessary to disengage attention.​

Question 21

Which house do you want to live in?

Answer 21

6th sense?
There is a level of awareness.

Question 22

What is the Marshall & Halligan’s study?

Answer 22

The left side differed such that one of the two houses had flames coming from a left window.​

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Although the patient claimed not to be able to perceive the difference between them, he/she stated a preference to live in the house on without the flames.​

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This points to the fact that neglected information is implicitly coded to a level that supports meaningful judgments to be made.​

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Suggests that the information is being processed at an early stage (bottom-up) but there is a problem with selective attention at a higher stage of processing (top-down).​

Question 23

What can we learn about awareness from neglect?​

Answer 23

In addition to the importance of the right-hemisphere in attention…​

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Evidence from patients with neglect demonstrate that our awareness is not derived directly from incoming sensory information. ​

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Mental representations and attentional mechanisms are key factors in awareness.​ Eg. thinking in different languages = different solutions.

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Different levels of awareness/information processing occur – such that it is possible to be influenced by something we are not consciously aware of having seen.​

Question 24

What is the visual pathway?

Answer 24

The largest contribution to human visual perception is made via the retinal-geniculate-striatal pathway.​

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I.e. the pathway that goes from the retina to the Lateral Geniculate Nucleus (LGN) of the thalamus and on to V1…​

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Gives rise to (subjective) conscious visual perception.​

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Unaware you can see, but you can see. Unconscious, but uses vison to navigate the world.

Question 25

What are the non-cortical routes to seeing?

Answer 25

This main pathway from the retina to the cortex is not the only visual pathway in the brain.​

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Around 10% of retinal ganglion cells branch away from the optic nerve before reaching the LGN.​

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These cells pass to subcortical regions, making up several (around 10) different pathways.​

Consciousness is costly - takes 200ms to be conscious. Throw something at us, takes 80ms.

Question 26

What are the subcortical visual pathways?

Answer 26

These subcortical pathways are evolutionarily more ancient (and unconscious).​

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Evolution appears to have replaced these old routes with new (better/conscious) ones, but has retained them and added new routes that enable finer levels of processing.​

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One route goes via the superior colliculus…​

Question 27

What does the superior calliculus does?

Answer 27

The superior colliculus (SC) is involved in the control of automatic reflexes and orienting movements of the head and eyes – especially when new stimuli appear in the visual field.​

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These pathways are faster than the V1 route, and can therefore provide an early warning sign to potentially threatening stimuli.​

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This can explain how it is possible to unconsciously turn to look at something without realising its importance until after orienting.​

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Question 28

What does the dorsal and ventral streams do?

Answer 28

After reaching V1 visual information forms two distinct pathways or ‘streams’ (Ungerleider & Mishkin, 1982):​

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Dorsal stream;​

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Ventral stream.​

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Question 29

What is cortical blindness?

Answer 29

A condition in which a patient sustains damage to the primary visual cortex and loss of (at least part of) the visual field.​

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What would happen if V1 were entirely damaged?​

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Question 30

What is blindsight?

Answer 30

Blindsight: a symptom in which the patient reports not being able to consciously sees stimuli in a particular region but can nevertheless perform visual discriminations (e.g. long, short) accurately.​

Question 31

What did Patient DB do?

Answer 31

**Patient DB had part of his primary visual cortex (V1) removed to cure a chronic and severe migraine (Weiskrantz, 1986).​

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DB reported seeing nothing when stimuli were presented to his blind visual field.​

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However, if asked to point or move his eyes to the stimulus he could do so accurately, while maintaining that he saw nothing.​

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He was also able to perform a number of other discriminations: orientation (horizontal/vertical/diagonal), motion detection (static/moving), and contrast discrimination (gray on black vs. gray on white).​

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In all tasks DB reported guessing – but he clearly was not.​**

Question 32

What are the critiques of blindsight?

Answer 32

The existence of blindsight remains controversial.​

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Some researchers claim that blindsight is the result of islands of spared cortex within the supposedly damaged region (Campion et al., 1983).​

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However, many patients have undergone structural and functional MRI. These have established that no cortical activity remains in the region corresponding to the ‘blind’ field (Cowey, 2004; Storeig et al., 1998).​

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This evidence suggests that the spared striate cortex explanation is weak.​

Another explanation is that stray light from the stimulus is scattered onto intact parts of the visual field and is detected by intact parts of area V1 (Campion et al., 1983).​

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However, the stray light hypothesis seems to be an unlikely explanation for several reasons…(see Cowey, 2004).​

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DB is still able to make perceptual decisions in the presence of strong ambient light, which reduces the amount of stray light.​

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The most satisfactory explanation of blindsight (at present) is that it reflects the operation of other visual routes from the eye to the brain.​

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Question 33

What can we learn about visual perception from blindsight?​

Answer 33

Evidence from patients supports the proposal that we have visual systems that operate both within and outside our conscious awareness.​

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Blindsight provides evidence for the existence of unconscious (subcortical) routes to vision – but remains controversial.​

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Question 34

Prosopagnosia = Why are faces important?

Answer 34

Faces are subject of intense research.​

Apparently effortless. No formal training.​

No limit to number of faces recognised.​

Great social significance.​

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Question 35

Why might we recognise faces?

Answer 35

Newborns prefer face-like stimuli (Farroni et al., 2005) and top-heavy faces but not necessarily real faces (Macchi-Cassia et al., 2004)

Question 36

What is prosopagnosia?

Answer 36

Inability to recognise previously familiar faces.​

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Includes famous faces, friends, family – even own face!​

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Apperceptive.​

Cannot perceive faces.​

Associative.​

Cannot recognise faces they perceive.​

Question 37

What is the functional neuroanatomy of object and face processing?

Answer 37

Kanwisher and colleagues argue that the fusiform gyrus is specialised for the detection and identification of faces.​

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Gauthier and colleagues argue that face recognition involves expert discrimination of visually similar objects, with fusiform gyrus being specialised at this general function.​

Question 38

What brain areas are involved in face processing?

Answer 38

Impaired face recognition (prosopagnosia) is usually associated with bilateral damage of the fusiform gyrus (inferior temporal cortex - BA 37).​

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Unilateral RH damage to same area sufficient to produce prosopagnosia whilst sparing object recognition of equal difficulty.​

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Impaired object recognition (agnosia) with relatively spared face recognition (without prosopagnosia) observed following unilateral LH inferior-temporal cortex damage.​

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Question 39

Are faces special?

Answer 39

Are there anatomically specialised areas devoted to processing only faces? (the debate continues)​

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Are faces processed differently from other complex visual stimuli (objects)? (the debate continues)​

Question 40

How does face inversion effect perception?

Answer 40

If faces are show upside-down, the speed and accuracy of recognition is reduced.​

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Interpreted as evidence that inverted faces are processed differently from upright faces.​

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Upright faces processed as a unique pattern, rather than as components.​

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Same effect NOT found for general objects.​

Question 41

Are faces and objects separate systems?

Answer 41

Patient CK (see Tovée, 1998) – severe object agnosia (can’t recognise objects) but unimpaired face recognition (can recognise faces).​

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Evidence that two processes are separate?​

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Any other possible explanation for this dissociation?​

Question 42

What if it is just easier to recognise faces because we had more practise?

Answer 42

What if faces are just easier to recognise because we’re more practiced?​

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Would expect to find patients like CK, but NOT patients able to recognise objects whilst unable to recognise faces.​

Question 43

What is double dissociation?

Answer 43

Patient WJ (McNeil & Warrington, 1993) severe prosopagnosia but object recognition intact.​

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Double dissociation.​

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Question 44

So what is prosopagnosia?

Answer 44

A specific disorder of the face recognition system OR a problem making fine within-category discriminations?​

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Possible that patients with prosopagnosia can make between-category discriminations (e.g., face, car, house) but not within-category discriminations.​

Question 45

What is the WJ case?

Answer 45

51-year-old right handed professional man became severely prosopoagnosia following a stroke.​

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Unable to identify 10/12 famous faces (and appeared to identify other 2 by deduction rather than identification).​

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Unable to judge age, sex, or facial expression of faces & impaired at face matching task.​

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Became sheep farmer (flock of 36) post-stroke.​

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Question 46

What was the prediction based on WJ?

Answer 46

Prediction:​

If prosopagnosia is a (human) face specific disorder, patients should be able to perform within-category discriminations for other visually difficult categories, but not human faces.​

Question 47

What did they find when WJ had to recognise the faces of sheep?

Answer 47

Able to recognise own sheep (i.e., identify sheep by ID number) & claimed to recognise them more easily than human faces.​

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Recognition memory for sheep better than that of healthy age-matched controls.​

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Cannot be attributed to sheep task being easier than human faces – controls found sheep difficult to recognise (‘they all look the same’).​

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WJ suffers face-specific problem whilst remaining able to recognise other visually difficult and confusable stimuli.​

Question 48

Does this prove that faces are special?

Answer 48

There does seem to be processes specialised for faces.​

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BUT – this could be a component of a general object processing system that is divided into specialised domains (e.g. tools, animals, faces, buildings, words), rather than a specialised system.​

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This is an area of on-going debate.​