Disorders of perception, Neglect, Blindsight, Prosopagnosia

Question 1

disorders of visual perception

Answer 1

• Visual illusions are not disorders of visual perception – but demonstrate to us that vision is not purely the veridical representation of an ‘external reality’.
• Our visual perception is susceptible to a whole host of strange perceptual illusions.

Question 2

• Anamorphotic display?

Answer 2

create a unique and completely different visual experience from what we’re used to with traditional displays. These displays are more immersive and captivating because they provide a shift from the norm.

Question 3

Law of meaningfulness or familiarity:

Answer 3

• Thelaw of meaningfulness(also referred to as the principle of familiarity) is a Gestalt law of perceptual organisation
  
  • which describes the way humans perceive certain combinations of lines, curves and shapes as forming a meaningful object, figure or image.

Question 4

does the brain lie?

Answer 4

• Your brain lies to you all the time!
  
  • It isconstantlytrying to fill in perceptual gaps, filter a barrage of incoming information, andmake sense of the world.
  • This happens (largely) outside of conscious awareness.
  • we do things even though they’re harmful
  • attraction is bias of brain/eyes?

Question 5

What is neglect?

Answer 5

> Neglect:“A failure to report, respond, or orient to novel or meaningful stimuli presented to the sideoppositea brain lesion, when this failurecannotbe attributed to either sensory or motor defect” (Heilman, 1979).

• stroke - 50% of people survive and improve
• Also referred to as hemi-neglect, visual neglect, visuo-spatial neglect and unilateral neglect.
• EXTREMELYheterogeneous condition!

Question 6

The neglect different subtypes:

Answer 6

• Sensory
• Motor
• Spatial - reasoning with world
• Personal
• Representational - mental
• Neglect dyslexia
• Neglect dysgraphia
• Facial neglect
• Auditory neglect
• Tactile neglect
• Extrapersonal

Question 7

Experience of neglect

Answer 7

• Patients behave as though one half of the world does not exist (they aren’t aware of the missing half).
• not visual but perceptual representation
• 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

Personal neglect:

Answer 8

• a lack of orientation or exploration of the side of the body contralateral to the injured hemisphere (Beschin & Robertson, 1997)
• doesn’t exist in their mind = proves its perceptual disorder
• human awareness is ‘all-or-nothing’
• in tune with yourself → senses danger earlier → sixth sense

Question 9

Extrapersonal neglect:

Answer 9

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

Question 10

Neglect can be assessed in a variety of ways:

Answer 10

• Cancellation tasks (e.g. Star/Line).
• Line bisection.
• Copy drawing or draw from memory.
• One-item test (aka The Personal Neglect Test)
• Explain difference between visuo-spatial, repress and personal neglect

Question 11

Personal neglect test:

Answer 11

• Requires the patient to touch their contralesional hand using their ipsilesional hand.
  
  • 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 12

What causes neglect?

Answer 12

• Strong association with right hemisphere lesions.
• Particularly the parietal lobe (most common)
• But also:
  
  • Frontal lobe
  • Sub-cortical regions (basal ganglia, thalamus)
• Different damage linked to different neglect subtypes (Mesulam, 1999)
• Lesion – circumscribed area of brain damage.
• Studies of neglect have revealed a great deal about how attention and space are processed in the brain.
• For example, neglect is far more frequent following damage to theright-hemisphere, resulting in failure to attend to the left.
• This suggests that there is likely to be ahemispheric asymmetrysuch that the right hemisphere is more specialised for attention than the left (see also Posner & Petersen, 1990).

Question 13

Representational Account

Answer 13

-Neglect is NOT just a visual field defect.
- Representational neglect has since been studied in numerous other patients using other locations and various other stimuli (e.g., Rode et al., 1998, 2004).
- 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.
- Data from the Piazza del Duomo experiment (and other similar experiments) appear fairly convincing.
- However, it remains unclear in this explanation exactly how neglect is brought about.
1. Is the representation of space itself impaired?
2. Is the representation preserved but the ability to scan it lost?

Question 14

Bisiach & Luzzatti (1978)

Answer 14

-suggest that the parietal lobes contain an elaborate representation of the world.
- Patients asked to imagine and describe the landscape from two different vantage points.Since the descriptions were not contingent on direct sensory input the findings are interpreted to imply that the patients internal representation of the world was impaired.
- Bisiach & Luzzatti (1978) asked two neglect patients to imagine being in thePiazza Del Duomo.A well known square in Milan and the patients’ native city.
- Describe the buildings and other features around the square.
- 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
- Very few things on the left were recalled.
- When asked to imagine standingat 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.
- 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.
- Knowledge of features on both sides was in their memory, but they were unable to access all of it normally from their imagery.

Question 15

Attention

Answer 15

• William James (1890) “no one knows what attention is” - or at least there is no obviously agreed definition.
• “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).
• Selection of information for conscious processing and action – the ‘spotlight’ idea.

Question 16

Attentional Accounts of Neglect

Answer 16

• Impairedorienting of attentionto neglected side. (Heilman, 1979; Riddoch & Humphreys,1983).
• Overly strongorienting of attentionto non-neglected(intact) side. (Kinsbourne, 1978; Ladavas, 1990).
• Impaireddisengagementof attention once it is oriented to the non-neglected (intact) side. (Posneret al, 1982).
• Performance of neglect patients on the Posner spatial cueing task provides further insight into the working of attention.
  
  • Congruency/incongruency of cue-target
• When a brief cue is flashed to either the left or right side, neglect patients may engage attention andmove their eyes to the side normally.
• 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.
• The fact that patients can detect targets on the left when cued to the left suggests that the deficit is related toshifting attentionrather than a problem of initial perception.
• Posner & Petersen (1990) therefore suggest that theparietal lobes are not critical for the initial orientingof the cue but are necessary todisengage

Question 17

Awareness for the neglected side

Answer 17

• There is evidence to suggest some processing of stimuli on the neglected side.
• What happens to information in the neglected side?
• Do patients show evidence that they are able to process it at all, or is it lost into oblivion?
• Although patient with neglect claimed not to detect any differences in the two houses, when asked which she would prefer to live in she chose the one without the flames.

Question 18

Marshall & Halligan (1988)

Answer 18

• The left side differed such that one of the two houses had flames coming from a left window.
• 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.
• This points to the fact that neglected information is implicitly coded to a level that supports meaningful judgments to be made.
• Suggests that theinformation is being processed at an early stage(bottom-up) but there is a problem with selective attention at ahigher stage of processing(top-down).

Question 19

What can we learn about awareness from neglect?

Answer 19

• In addition to the importance of the right-hemisphere in attention…
• Evidence from patients with neglect demonstrate that our awareness is not derived directly from incoming sensory information.
• Mental representations and attentional mechanisms are key factors in awareness.
• 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 20

What is blindsight?

Answer 20

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 21

evi for blindsight

Answer 21

• Patient TN 2 strokes damaged T.N.’s primary visual areas→as a result, he was completely blind
• Patient DB had part of his primary visual cortex (V1) removed to cure a chronic and severe migraine (Weiskrantz, 1986).
• DB reported seeing nothing when stimuli were presented to his blind visual field.
• However, if asked to point or move his eyes to the stimulus he could do so accurately, while maintaining that he saw nothing.
• 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).
• In all tasks DB reported guessing – but he clearly was not.

Question 22

describe the Visual pathways

Answer 22

• The largest contribution to human visual perception is made via the retinal-geniculate-striatal pathway.
• e. the pathway that goes from the retina to the Lateral Geniculate Nucleus (LGN) of the thalamus and on to V1…
• Gives rise to (subjective)consciousvisual perception.
• Different portions of the retina in both eyes process information coming from the left and the right visual field→ depth perception

Question 23

Non-cortical (subcortical) routes to seeing

Answer 23

• This main pathway from the retina to the cortex is not the only visual pathway in the brain.
• Around 10% of retinal ganglion cells branch away from the optic nerve before reaching the LGN.
• These cells pass to subcortical regions, making up several (around 10) different pathways.

Question 24

Subcortical Visual Pathways

Answer 24

• These subcortical pathways are evolutionarily more ancient (andunconscious).
• 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.
• One route goes via the superior colliculus…
  -Some visual processing is carried out bysubcorticalpathways(~20%), which project to the cortex via a Retina > Superior Colliculus > Pulvinar (thalamus) >secondary visual cortex pathway.

Question 25

superior colliculus pathway

Answer 25

• 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.
• These pathways are faster than the V1 route, and can therefore provide an early warning sign to potentially threatening stimuli.
• This can explain how it is possible tounconsciouslyturn to look at something without realising its importance until after orienting.
• e.g. sensation of being watched

Question 26

Dorsal vs. Ventral Cortical Pathways

Answer 26

• After reaching V1 visual information forms two distinct pathways or ‘streams’ (Ungerleider & Mishkin, 1982):
  
  • Dorsal stream - where?
  • Ventral stream - what?

Question 27

Neuropsychological Evidence for blindsight

Answer 27

• Evidence that this pathway makes an important contribution to human vision has come from the phenomenon ofblindsight.

Question 28

Cortical Blindness

Answer 28

• A condition in which a patient sustains damage to the primary visual cortex and loss of (at least part of) the visual field.
• What would happen if V1 were entirely damaged?

Question 29

Critiques of Bindsight

Answer 29

• The existence of blindsight remains controversial.
• Some researchers claim that blindsight is the result of islands of spared cortex within the supposedly damaged region (Campion et al., 1983).
• 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).
• 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).
• However, the stray light hypothesis seems to be an unlikely explanation for several reasons…(see Cowey, 2004).
• DB is still able to make perceptual decisions in the presence of strong ambient light, which reduces the amount of stray light.
• The most satisfactory explanation of blindsight (at present) is that it reflects the operation of other visual routes from the eye to the brain.

Question 30

What can we learn about visual perception from blindsight?

Answer 30

• Evidence from patients supports the proposal that we have visual systems that operate both within andoutsideour conscious awareness.
• Blindsight provides evidence for the existence of unconscious (subcortical) routes to vision – but remains controversial.

Question 31

what is Prospagnosia?

Answer 31

• Inability to recognise previously familiar faces.
• Includes famous faces, friends, family – even own face!
• Apperceptive
  
  • Cannot perceive faces.
• Associative
  
  • Cannot recognise faces they perceive.
• Unlikely to resolve whether or not the model is adequate from looking at normal people’s intact face processing. More insightful to look at face recognition impairments.
• Face processing system can be fractionated into its components and relationships by looking at the patterns of impaired and intact performance of patients with face processing problems.
• Vast majority of this work has been done by examining patients with prosopagnosia

Question 32

Faces?

Answer 32

• Apparently effortless. No formal training.
• No limit to number of faces recognised.
• Great social significance.
• Newborns prefer face-like stimuli (Farroni et al., 2005) and top-heavy faces but not necessarily real faces (Macchi-Cassia et al., 2004)
  
  • Farroni et al. (2005)-infants preferred to look at photos or cartoon faces than inverted faces or objects→ also demonstrates that faces are recognised from birth
  • Macchi-Cassia-infants’ cortical processing of faces initiallybroad and poorly tuned (more elements in the upper than in the lower half)
• To understand that others have beliefs, intentions and desires that are different from yours, you must be able to recognise social others and know who they are – they are not me
  => Prospagnosia

Question 33

Functional Neuroanatomy of Object and Face Processing

Answer 33

• Kanwisher and colleaguesargue that the fusiform gyrus is specialised for the detection and identification of faces.
• Gauthier and colleaguesargue that face recognition involves expert discrimination of visually similar objects, with fusiform gyrus being specialised at this general function.
  
  • Category specific vs function specific

Question 34

What brain areas are involved in face processing?

Answer 34

• Impaired face recognition (prosopagnosia) is usually associated with bilateral damage of the fusiform gyrus (inferior temporal cortex - BA 37).
• Unilateral RH damage to same area sufficient to produce prosopagnosia whilst sparing object recognition of equal difficulty.
• Impaired object recognition (agnosia) with relatively spared face recognition (without prosopagnosia) observed following unilateral LH inferior-temporal cortex damage.
• Are there anatomically specialised areas devoted to processing only faces? (the debate continues)
• Are faces processed differently from other complex visual stimuli (objects)? (the debate continues)

Question 35

Face Inversion Effect

Answer 35

• If faces are show upside-down,the speed and accuracy of recognition is reduced.
• Interpreted as evidence that inverted faces are processed differently from upright faces.
• Upright faces processed as a unique pattern, rather than as components.
• Same effect NOT found for general objects.
• We hardly notice the distortion when the same pictures are upside down. This phenomenon is referred to as theThatcher illusion, as its discoverer Peter Thompson used a photo of Margaret Thatcher in his original experiment.
• There are several proposed explanations – but no-one knows for sure why this effect occurs.

Question 36

Faces vs. objects: separate systems?

Answer 36

• PatientCK(see Tovée, 1998) – severe object agnosia (can’t recognise objects) but unimpaired face recognition (can recognise faces).
  
  • Evidence that two processes are separate?
  • Any other possible explanation for thisdissociation?
• What if faces are just easier to recognise because we’re more practiced?
• Would expect to find patients like CK, butNOTpatients able to recognise objects whilst unable to recognise faces.
• If objects are simply harder to recognise than faces, then it should be possible to observe patients with impaired object recognition (object agnosia) and intact face recognition.
  
  • Similarly should be able to find patients impaired at both face recognition (prosopagnosia) and object recognition (agnosia).
• However, IF IT IS THE CASE THAT OBJECT RECOGNITION IS SIMPLY MORE DIFFICULT THAN FACE RECOGNITION it should be impossible to find patients able to recognise objects but not faces (i.e. prosopagnosia without object agnosia).

Question 37

neuropsychological Evidence

Answer 37

• Patient WJ (McNeil & Warrington, 1993) severe prosopagnosia but object recognition intact.
• Double dissociation.
• Different objects are easier to recognise than diff people
• Of course – other objects can be difficult to distinguish within category.
• Whatwe need is evidence that patients with prosopagnosia can still make these types of fine, within-category discriminationsin order to eliminate this as a possible explanation for the disorder.

Question 38

What can we learn from prospagnosia?

Answer 38

• A specific disorder of the face recognition systemORa problem making fine within-category discriminations?
• Possible that patients with prosopagnosia can make between-category discriminations (e.g., face, car, house) but notwithin-category
• • 51-year-old right handed professional man became severely prosopoagnosia following a stroke.
  • Unable to identify 10/12 famous faces (and appeared to identify other 2 by deduction rather than identification).
  • Unable to judge age, sex, or facial expression of faces & impaired at face matching task.
  • Became sheep farmer (flock of 36) post-stroke.
• There does seem to be processes specialised for faces.
• 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.
• This is an area of on-going debate.

Question 39

WJ(NcNeil & Warrington, 1993)

Answer 39

• 51-year-old right handed professional man became severely prosopoagnosia following a stroke.
• Unable to identify 10/12 famous faces (and appeared to identify other 2 by deduction rather than identification).
• Unable to judge age, sex, or facial expression of faces & impaired at face matching task.
• Became sheep farmer (flock of 36) post-stroke.

Question 40

Experimental Investigation of WJ

Answer 40

• 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.
  • Able to recognise own sheep (i.e., identify sheep by ID number) & claimed to recognise them more easily than human faces.
  • Recognition memory for sheep better than that of healthy age-matched controls.
  • Cannot be attributed to sheep task being easier than human faces – controls found sheep difficult to recognise(‘they all look the same’).
  • WJ suffersface-specific problemwhilst remaining able to recognise othervisually difficult and confusable stimuli.