Disorders of Perception, Neglect, Blindsight and Prosopagnosia Flashcards
What is the physiology of the eye?
• Light enters the eye via the cornea
• If then passes through the pupil (a hole in the centre of the iris)
- When light is bright the iris causes the pupil to constrict to limit the amount of light entering the eye
• It then passes through the interior lens
• It’s then focussed on the retina (a network of neurones/cells)
• Layers of cells in the retina - ganglion, amacrine, bipolar, horizontal and photoreceptors
• Photoreceptors turn light into neural impulses to send to the brain. Two types:
- RODS (dim light vision, motion)
- CONES (bright light and colour vision)
About perception and the brain…
• Sensation is not the same as perception
• Retina
• Rods and cones
• Optic disk (blind spot)
• Optic chiasma (right field of each eye to right hemisphere and left field of each eye to left hemisphere)
• Lateral geniculate nucleus (LGN)
- Carries different information in the magnocellular and parvocellular layers from rods and cones
We have a blind spot because of the entry point of the optic nerve.
- Visual cortex, V1 (striate cortex) then V2, V3, V4, MT
- The visual system separates the visual features (form, motion, colour etc) of the visual scene but combines them to form a coherent
- Ventral - what pathway
- Dorsal - where pathway - specialised for object recognition or thought of by ohters (Milner and Goodale, 1995 as the how pathway - for visually guided action)
What is the Gestalt approach to perception?
- Assumes that the simplest and most stable interpretations of visual stimulus are favoured (Principle of Pragnanz)
- Gestaltists looked at the configuration of visual scenes. Favour hollistics processing (whole is greater than sum of its parts). It is the relations between parts, and their arrangement that determines perception. Eg spatial rules - bottom up
- Proposed principles that govern how the perceptual input is organised into chunks to provide the ‘best guess’ as to what it is that we are looking at. The perceptual system is considered to have ‘preferences’ for interpreting certain arrangements.
What is form perception?
- Principles of form perception
- Principles are innate so we can’t choose how we see things
- The figure-ground concept - some things stand out (figure)and others fade into background (ground)
- Figure has distinct from and the ground doesn’t
- Figure seen as being in front of the ground
What are some principles of visual perception?
- principle of proximity
- principle of similarity
- principle of good continuity (smoothness)
- principle of closure
What is a summary of Gestalt’s theory of visual perception?
- Proximity - objects close together are perceived as forming a group
- Similarity - group ojects on the basis of similarity
- Continuity - perceive smoothly flowing forms rather than disrupted ones
- Closure - complete objects that are not complete
- BUT - the theory is purely descriptive
- How does this phenomenon occur?
- These principles can be demonstrated very well with 2D designs - but how applicable are they to the real world?
What did Bruner say about visual perception?
Bruner (1957) “all perceptual experience is necessarily the end product of a categorisation process”
What happens to form out visual experience?
the detection of sense ‘information’»_space;> formation of perceptual representation»_space;> interpretation of the perceptual representation
What is perception?
- We experience the world through our sense which receive information from the environment
- Sensory receptors stimulated by physical energy (eg for vision the energy is light reflected off object)
- The sense organs receive these sensory inputs and transmit sensory info to the brain
- How is the physical energy received by the sense organs converted into perceptions?
What is constructivist perception?
Constructivist perception - top-down processing
- Helmholtz (1909); Rock (1983); Gregory (1997) - We construct the perceived stimulus using sensory info as the base but it's our thinking processes that make sense of the info ie we need prior experience to interpret the input
What is direct perception?
Direct perception - bottom-up processing
- Gibson - ecological (1979) and Marr - computational (1982) - The info received by our sensory receptors is all we need in order to perceive (don't need prior knowledge to mediate between sensory input and perception) - We are bon with all we need to make sense of what we see
What are assumptions of constructivist theories?
- Bruner, 1957; Neisser, 1967; and Gregory, 1972, 1980 tend to make the following assumptions:
- Perception is an active and constructive process; ‘something more than the direct registration of sensations…other events intervene between stimulation and experience’. (Gordon, 1989)
- The visual image is ambiguous and needs interpretation
- Perception is not directly given by the stimulus input, but occurs as the end-product of the interactive influences of the presented stimulus and internal hypotheses, expectations and knowledge, as well as motivational and emotional factors (top-down).
- Perception is an active and constructive process; ‘something more than the direct registration of sensations… other events intervene between stimulation and experience (Gordon, 1989)
- Perception is not directly given by the stimulus input, but occurs as the end-product of the interactive influences of the presented stimulus and internal hypotheses, expectations and knowledge, as well as motivational and emotional factors
- Perception is influenced by expectations that are sometimes incorrect, and so it is prone to error. Hence visual illusions.
- Perception is influenced by hypotheses and expectations that are sometimes incorrect, and so it is prone to error
- When information from memory is being used to guide expectations about the information that is being received from the environment, this is referred to as top-down processing
- Gregory talks of ‘perceptual hypotheses’ being generated
What is direct perception?
- Sensations received by the visual system are highly organised and rich in information that we do not need to interpret them to make them meaningful perceptions
- We can perceive them directly
- This means that there are no intermediate stages between light reaching the retina and our response to it
- No need to make use of internal hypotheses or thought processes - perception is automatic. Because of the emphasis on environmental stimuli, Gibson’s theory is also ecological perception
- Characterised best by Gibson’s (1979) ecological approach
- A ‘pattern of light’ reaching the eye is an optic array; this structured light contains all the visual information from the environment striking the eye
- This optic array contains unambiguous information about the layout of objects in space, in the form of texture gradients, optic flow patterns etc
- Perception involves ‘picking up’ the rick information provided by the optic array directly via ‘resonance’, with little or no information processing
- For instance, when thinking of a pilot approaching a landing strip, there is a point in their visual field that appears to remain stationary, with all other points flowing away from it. This gives direct information regarding the point towards which the pilot is currently heading, without any model or internal representation of the environment
- What about meaning? Gibson introduced the notion of direct perception of the affordances of an object, eg a chair “affords” sitting on
- This has proved a useful idea in the context of object design, but is very incomplete as a theory of meaning in general. The idea of ‘resonance’ is also rather difficult to pin down.
What are the successes of Gibson’s perspective?
- More about ‘seeing’ rather than ‘seeing as’
- A proper acknowledgement of the environmental and ecological grounding of perception. What is perception for, if not to allow interaction with the environment?
- The transparency of some illusions with regard to action
- A highlighting of the richness of information present in the visual signal
- An anticipation of theories relating to the so-called ‘dorsal stream’ of brain processing
What are the weaknesses of Gibson’s approach?
- By not specifying exactly what constitutes resonance and invariance detection, Gibson avoids the exact problem that information processing accounts sought to address. He underestimates the amount of processing that must be performed even to detect the invariances that he proposes
- The denial of the role of stored knowledge (memories) in perception is highly questionable eg seeing that a tree has been cut down
- Even the topics Gibson discusses (eg motion towards) are more complex than he assumes
What did Palmer show about visual perception?
Palmer (1975) showed that a preceding picture of a kitchen would facilitate the recognition of a rapidly presented appropriate object(eg a loaf) but would actually slow identification of an inappropriate object (eg a mailbox), both relative to a no-context condition
What are the context effects in object recognition?
- One of the ways in which to look at the interaction of bottom-up and top-down processing is to consider the effects of context
- Experimental data support an influence of context on perception
- Palmer (1975)
- Ps viewed a scene for 2 seconds. After 1.3 seconds delay they were shown a picture of an object for either 20, 40, 60 or 120ms. Their task was to name the object. (line drawings)
- There were 4 types of relationships between the scene and the object:
What did Palmer find in 1975?
- Related context - eg a kitchen followed by a picture of bread
- Misleading context - eg a kitchen followed by a picture of a mailbox (same shape as bread)
- Unrelated context - eg a kitchen followed by a picture of a muscal instrument
- No context (baseline performance measure)
- Measure (DV) was probability of item being correctly identified
- Performance improved with increased viewing exposure (more bottom-up information)
- Performance was best for related condition - better than no context but performance on misleading and unrelated was worse than no context at all (effect on top down processing)
Is visual perception top down or bottom up?
- Top-down or bottom-up
- Most visual experience probably involves combination of bottom-up and top-down processing Neisser (1976)
- Broadbent (1977) and Navon (1981)
- Bottom-up sensory data ‘suggests’ whilst top down information ‘inquires’ or ‘interrogates’ the perceptual representation - guides the final visual experience by selecting that which matches experience/expectations/beliefs
- Under good visual condition bottom-up processing is more prevalent
- Under poor visual conditions, brief exposure top-down processing become more important
How has constructivist approach sought to explain visual illusions?
For example, Gregory’s explanation of the Mueller-Lyer illusion: that the figure on the left looks like the inside corner of the room with the corner therefore far away; that the figure on the right looks like an outside corner, with the corner near. If they look the same size but they are at different distances from the observer then the left one must really be longer than the right one (misapplied size constancy).
What is the Ebbinghaus illusion?
Note: Gregory’s explanation is far from unchallenged. Nevertheless it is a good example of constructivist thinking
Note also: that many perceptual illusions fail to affect actions.
For example, in a three dimensional version of the Ebbinghaus illusion, hand grip aperture was the same for picking up the central object in each case, even thought the objects are perceived as being different sizes
What are some problems with the constructivist approach?
- Illusions are the exception rather than the rule
- Many experimental stimuli are unnatural eg odd designs, brief presentations
- Many illusions remain unaccounted for
About illusions…
- Gregory (1972) - visual information derived form the retinal image is ambiguous
- When perceptual cues conflict, the visual system has to chose the best option based on retinal size, distance cues and expectations
- It formulates a perceptual hypothesis which in the case of illusions is incorrect
- Illusions may be able to help us understand visual perception
- How does the bottom-up approach deal with visual illusions?
- There is no single definitive theory of illusions but they remind us of the different factors which can enter into visual perception eg physiology, depth cues and expectations
What is perceptual constancy?
Even though the retinal image for objects may be different, we still perceive the object in a certain way. There is constancy in what we see even though the retinal image is different, due to different viewpoints or distance. Perceive round table, but retinal image is elliptical.
Perceptual Constancy or Invariance refers to the phenomenon whereby our perception of an object does not change even though the sensory information/image has changed - recall the distinction between perception and sensory experience.
- Size - Colour - Shape
What is size constancy (visual perception)?
• We show size constancy because we take account of the distance when judging the object’s size
• Retinal image changes with movement and distance but the size we perceive does not change
• Imagine someone walking towards you. We perceive them getting closer but we do not perceive someone growing in size
• The retinal image of an object varies with distance of the object from the retina
• Hold finger quite close to eyes then move away to arm’s length - your perception is NOT that is becomes smaller
• When our perception of an object remains the same but our sensation of the object changes
- Eg as you walk towards a person they become larger but you know they’re really the same size - why?
1. We know this from experience which affects our interpretation of the person - ie top-down processing
2. The retinal image and the perceived distance cancel each other out
• Therefore no size constancy when perceived distance of an object is very different form its actual distance
- Eg the Ames room - perceive impossible relative sizes but this is because the shape of the room is distorted
What is colour constancy (visual perception)?
- Colour is a psychological construction
- Hugh, brightness, saturation
- Knowledge of the object’s colour
- Lighting
- Reflective nature of the object
- Surrounding colours
- Shadows
About depth in visual perception…
To know where objects are we need to be able to:
- Segregate objects from each other and from background - Determine distance (depth), and movement. This is done through depth cues
What are depth cues in visual perception?
Monocular (one eye)
- Perspective, things converge in distnace - Texture, gets finer as things get further away - Occluding edges - Motion parallax
Binocular (two eyes)
- Stereopsis - Retinal disparity - Binocular convergence
About monocular cues (visual perception)…
Relative and familiar size
- In a visual scene of different sized objects, smaller ones are usually seen as more distant but we can be fooled
A number of illusions are based on the relationship between size and depth - eg Ponzi illusion
About occlusion and shadows in visual perception…
Near objects will occlude more distant objects. If you cant see all of one object because of another, it is due to the fully visible object being nearer to you than the occluded object.
Other monocular clues to depth such as shadows and shading.
About texture in visual perception…
Another invariant is the texture gradient. The appearance of the surface of objects will change with distance (think about how grass looks close to you and distant from you). As we assume that the texture itself hasn’t changed we take the change in appearance to indicate distance (roads, grass, etc).
What are other monocular cues to depth in visual perception?
• Relative size
- In a visual scene of different sized objects, smaller ones are usually seen as more distant
• Relative brightness
- Brighter objects normally appear closer
• Aerial perspective
- Objects at a greater distance have a different colour (light filtered from atmosphere) blue tint
• Height in the horizontal place
- Distant objects seem higher (closer to the horizon)
What is the motion parallax?
Motion parallax
• Motion also provides important information about the position of items in the world
• Motion parallax describes the phenomenon whereby if the observer is moving - nearby items will appear to move faster than items in the distance (think of looking out of a window of a train or bus)
• Also - more distant objects will appear to ‘move’ more slowly
About binocular cues…
- Stereopsis refers to the way that we are able to perceive distance as a result of having two eyes, each in a different position
- The difference in position means that they receive a slightly different image (retinal disparity).
What is multisensory integration?
- At many times the perceptual system has to deal with integrating and potentially competing sensory information across modalities
- What you hear depends on whether your eyes are opened or closed
- Multisensory integration raises the question of how unisensory and multisensory cognitive systems and brain regions operate.
What is the McGurk effect?
The McGurk effect is a perceptual phenomenon that demonstrates an interaction between hearing and vision in speech perception. The illusion occurs when the auditory component of one sound is paired with the visual component of another sound, leading to the perception of a third sound.[1] The visual information a person gets from seeing a person speak changes the way they hear the sound.[2][3] If a person is getting poor quality auditory information but good quality visual information, they may be more likely to experience the McGurk effect.[4] Integration abilities for audio and visual information may also influence whether a person will experience the effect. People who are better at sensory integration have been shown to be more susceptible to the effect.[2] Many people are affected differently by the McGurk effect based on many factors, including brain damage and other disorders.
What is Marr’s levels of representation?
- The grey-level description: gives the intensity of light at each point in the image (cf the retina)
- The raw primal sketch: a description of the main changes in light intensity in the image, generated by ‘smoothing’ the intensity changes in the input. This shows edges, blobs, textures etc…
- The full primal sketch: indicated which aspects of the raw sketch ‘go together’ to form larger structures in the image eg the outline shapes of objects (cf Gestalt)
- The 2 1/2 -D sketch: this adds information about depth and orientation of visible surfaces, by using texture, motion, etc. This details how surfaces and edges in the scene relate to each other
(This and all previous stages are represented from the viewpoint of the observer - they are ‘view depended’. This is an important point.)
- 3-D model: takes the 2 1/2 -D surfaces etc. and groups them into viewpoint-independent descriptions of basic perceptual elements.
- Marr and Nishihara (1978) suggested using cylinders as basic elements in these descriptions, where each cylinder’s major axis is appropriately aligned.
What is Biederman’s recognition-by-components model?
- Biederman’s (1987, 1990) model is a development of Marr and Nishihara’s (1978) theory
- Biederman proposed a more general set of basic elements of ‘primitives’ which he called ‘geons’ (from geometric ions). He proposed about 36 geons including wedges, blocks, cylinders and spheres
- Images are broken down into geon components. The breaks tend to be made at sharply concave parts of something like the 2 1/2-D sketc
What are Biederman’s non-accidental properties?
- Points in a straight line in sketch strongly suggest that those points on object also lie in a straight line
- Points or segments in a curve strongly imply a curve on the object
- Symmetrical parts of the sketch are highly likely to be symmetrical parts of the 3D object
- Parallel parts of the sketch strongly imply that the corresponding parts of the object are parallel
- Two lines in the sketch ending at the same point strongly suggests two edges of the object ending at the corresponding point.
- These five properties of parts of a sketch can be used to determine which geons can be used to determine which geons can be found where, the geons being invariant across all non-accidental viewpoints
- These properties are also detectable when only parts of given edges are visible
- Providing joining (particularly concavities) are visible, then missing parts of an image can be reconstructed.
What are the structural relationships found by Biederman?
Once the geons have been determined, Biederman specifies a number of structural relationships between them. Some major relationships are:
- Relative size - Verticality: above, below, to the side of - Centering - Relative surface size at join
What is experimental evidence done by Biederman?
Biederman, Ju and Clapper (1985) showed that participants could recognise rapidly presented complex objects even when more than half of their features were missing.
Biederman (1987) showed that it was much easier to identify degraded drawing that still showed the concave parts, supporting his view that these are important points of reference.
Biederman and Ju (1988) also found that line drawings were just as effective cues to recognition as were full colour photographs, though Sancki, Bowyer, Heath and Sarkar (1998) showed that this was only the case when ‘idealised’ line drawings were used ie those with only and all the relevant edges
Bogels, Biederman, Bar and Lorincz (2001) also found some neurons in inferior temporal cortex that seemed sensitive to the presence of particular geons, but insensitive to changes in size.
What is some criticisms of Biederman’s models?
- It is really dedicated towards the recognition of different classes of object (eg cup vs bicycle) rather than the discrimination of objects within a class (eg my cup vs your cup)
- It does not account for contextual effects (the example given earlier with the letter H in EAT and THE), though this might be incorporated at a fairly late stage of processing
- It clearly favours a viewpoint-invariant (independent) description as the target for perceptual analysis. However…
What is viewpoint-specific representation?
Tarr and colleagues (eg Gauthier and Tarr, 2002; Tarr, 1995; Tarr and Bülthoff, 1995) have found some evidence that object recognition depends on storing a number of specific views of an object, with recognition time for a given viewpoint depending on the distance between that new viewpoint and the closest learned viewpoint (E&K, p.88)
It may well be that both viewpoint-invariant and view specific representations exist, with the former being used for broad class distinctions and the latter being used for fine within-category distinctions.
What is viewpoint dependence?
- An interesting contribution has been made by Vanrie et al. (2002) who suggested that viewpoint dependency will also be likely to be seen when dorsal-stream processing (processing for action) is involved
- This issue has led to a fierce argument in recent years between the Biederman and Tarr camps
About face recognition
- Faces plat a special role in the field of object recognition, and as such there is a good deal of work dedicated to a description of the human face processing system
- A lot of work (some reviewed below) suggests that faces are processed somewhat separately form other objects, reflecting the great importance they have for members of a social species like ours
- These is also evidence that different aspects of the face eg expression and identify, are processed separately. E & K chapter 3 review some relevant literature and point to further reading.
How are faces processed?
Congfigurally (ie as a whole) or elementally (as a series of parts)? Most evidence suggests the former:
Young, Hellawell and Hay (1987) - identification of halves of split-faces better if the two halves are not aligned.
What did Tanaka and Farah do to investigate facial recognition (1993)?
Tanaka and Farah (1993) taught people the names of some normal faces and some scrambled faces (with features moved around)
When an intact face had been learned, the choice of the correct whole face was better than the choice of the correct part. When a scrambled face had been learned, the choice of the correct part was actually better than the choice of the correct scrambled face. For the scrambled face, it is learnt as a series of parts and therefore part identification is better.
What are invention effects on facial recognition?
- There is evidence, however, that configural processing is only use for faces that are presented the right way up:
- Bartlett and Searcy (1993; and S & B, 1996) - grotesque expression unaffected by inversion, but distortions induced by feature movement are less noticeable when the face is inverted.
- Thompson (1980) - the Thatcher Illusion
Holistic processing is used more with faces than other objects. That is why it is harder to identify faces that have been inverted.
Are inversion effects only found with faces?
- No. Diamond and Carey (1986) found that expert dog breeders were disproportionately affected by inversion of pictures in dogs in a dog-recognition task.
- This suggests that configural (and viewpoint dependent) processing is used in making fine-grained within-category distinctions of several different types. This is consistent with the observations of Tarr and colleagues (see earlier).
- Other data suggest that faces are not processed as viewpoint-invariant structural encodings.
How are faces stored?
- Faces are stored as images rather than structural descriptions
- Photographic negative images of faces are very hard to identify (Bruce and Langton, 1994)
- Line drawings with no shading are also difficult to identify, (Bruce et al., 1992; Davis et al. 1978), though as soon as basic threshold shading is added, the image becomes much more recognisable. The same pattern is not found for other objects.
- Remember that in Marr/Biederman-type models, texture and shading plays little role in object identification. These results suggest that faces are processed in a more image-like manner.
What is the fusiform ‘face’ area?
- Kanwisher and Yovel (2006) reviewed evidence (eg from fMRI scans) showing that intact faces were processed in a particular different brain area, consequently called the ‘fusiform face area’ (FFA)
- There are other areas of the brain associated with face processing eg the occipital face area
- However, Grill-Spector et al. (2006) also found variation in the FFA, with some parts sensitive to animals, cars and sculptures.
Is FFA sensitive to expert categories?
- But: gunther et al (1999) have found that fusiform ‘face’ area activates when participants are trained for several hours on recognition of members of a fine-grained category (eg Greebles).
- Gauthier et al (2000) found that FFA activated more to cars for car experts than bird experts, and vice versa for birds (though it’s possible that they paid differential attention).
- McKone et al (2007) reviewed various studies but didn’t properly support the expertise hypothesis (see E&K, p.106, for details)
Generally about face recognition…
- But: gunther et al (1999) have found that fusiform ‘face’ area activates when participants are trained for several hours on recognition of members of a fine-grained category (eg Greebles).
- Gauthier et al (2000) found that FFA activated more to cars for car experts than bird experts, and vice versa for birds (though it’s possible that they paid differential attention).
- McKone et al (2007) reviewed various studies but didn’t properly support the expertise hypothesis (see E&K, p.106, for details)
Generally about object recognition…
- Most models of object recognition are of the constructivist type (eg Marr, Biederman)
- Recognition proceeds via a series of hierarchically arranged stages, form grey-level description to viewpoint-specific image based representation
- This might be important of expert discrimination within fine-grained categories, and for recognition-for-action (Gibson)
- Faces appear to constitute such a fine-grained category
- Faces are unlikely to be accessed as structural descriptions
- Nonetheless, this image-based style of processing is unlikely to be restricted to faces alone
- Face processing may involve the fusiform face area (FFA)
What is neglect?
- 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 defects (Heilman, 1979).
- Also referred to as hemi-neglect, visual neglect, visuo-spatial neglect and unilateral neglect.
- EXTREMELY heterogeneous condition!
What are different types of neglect?
- The neglect literature is littered with different subtypes:
- Sensory
- Motor
- Spatial
- Personal
- Representational
- Neglect dyslexia
- Neglect dysgraphia
- Facial neglect
- Auditory neglect
- Tactile neglect
- Personal
- Extrapersonal
What are everyday examples of neglect?
- Patients behave as though one half of the world does not exist (they aren’t aware of the missing half).
- 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
What is personal neglect?
A lack of orientation or exploration of the side of the body contralateral to the injured hemisphere (Beschin and Robertson 1997)
What is extrapersonal neglect?
A failure to detect visual and auditory stimuli on the contralesional side (Peru and Pinna, 1997)
How can we assess neglect?
- Cancellation tasks.
- Line bisection.
- Copy drawing or draw from memory.
- One-touch test (aka The Personal Neglect Test)
What is the personal neglect test?
- 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.
What is the neuroanatomy of neglect?
- 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)
What insight do we have form neglect patients?
- 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 the right-hemisphere, resulting in failure to attend to the left.
- 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).
- 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.
- 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 to shifting attention rather than a problem of initial perception.
- 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.
Representation account of neglect?
- Neglect is NOT just a visual field defect
- Bisiach and Luzzatti (1978) suggest that the parietal loves 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.
• 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?
What did Bisiach and Luzzatti do on neglect in 1978?
- 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.
- 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 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.
- 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.
- Representational neglect has since been studied in numerous other patients using other locations and various other stimuli (e.g., Rode et al., 1998, 2004).
What are attentional accounts of neglect?
- Impaired orienting of attention to neglected side. (Heilman, 1979; Riddoch & Humphreys,1983).
- Overly strong orienting of attention to non-neglected (intact) side. (Kinsbourne, 1978; Ladavas, 1990).
- Impaired disengagement of attention once it is oriented to the non-neglected (intact) side. (Posner et al, 1982).
What did Marshall and Halligan do on neglect? (1988)
- 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 the right (i.e. 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 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).
What can we learn about awareness from neglect?
- 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.
About visual pathways…
- The largest contribution to human visual perception is made via the retinal-geniculate-striatal pathway.
- I.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) conscious visual perception.
What are the non-cortical (subcortical) routes to seeing?
- However, 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.
What are subcortical visual pathways?
- These subcortical pathways are evolutionarily more ancient (and unconscious).
- 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…
- 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 to unconsciously turn to look at something without realising its importance until after orienting.
Evidence that this pathway makes an important contribution to human vision has come form the phenomenon of blindsight.
What is cortical blindness?
• 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?
From Ward (2010)
What is blindsight?
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.
What is patient DB (blindsight)?
- 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.
Critiques of blindsight
- 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.
- This theory cannot explain how DB can still make decisions about the spatial dimensions of objects.
- The most satisfactory explanation of blindsight (at present) is that it reflects the operation of other visual routes from the eye to the brain.
What can we learn about visual perception form blindsight?
- Evidence from patients supports the proposal that we have visual systems that operate both within and outside our conscious awareness.
- Blindsight provides evidence for the existence of unconscious (subcortical) routes to vision – but remains controversial.
Why are faces important?
- Faces are subject of intense research.
- Apparently effortless. No formal training.
- No limit to number of faces recognised.
- Great social significance.
What is prosopagnosia?
- 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.
- A specific disorder of the face recognition system OR a problem making fine within-category discriminations?
- Possible that patients with prosopagnosia can make between-category discriminations (e.g., face, car, house) but not within-category discriminations.
What brain areas are involved in face processing?
- Impaired face recognition (prosopagnosia) Usually associated with bilateral damage fusiform gyrus, located in inferior aspect of 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 inferiotemporal cortex damage.
What is the functional neuroanatomy of object and face processing?
- Kanwisher and colleagues argue that the fusiform gyrus is specialised for the detection and identification of faces.
- Gauthier and colleagues argue that face recognition involves expert discrimination of visually similar objects, with fusiform gyrus being specialised at this general function.
What is the face inversion effect?
- 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.
What is object agnosia?
Cant recognise objects
What was the WJ study of prosopagnosia?
- 51-year-old right handed professional man became severely prosopagnosic 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.
Prediction:
If prosopagnosia is a face specific disorder, patients should be able to perform within-category discriminations for other visually difficult categories, but not human faces.
Results:
• 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 faces – controls found sheep difficult to recognise (‘they all look the same’).
• WJ suffers face-specific problem whilst remaining able to recognise other visually difficult and confusable stimuli.
