Holcombe lectures Flashcards
(99 cards)
1
Q
What is reductionism?
A
• Reductionism- the practice of analysing and describing a complex phenomenon in terms of its constituents, especially when this is said to provide a sufficient explanation
2
Q
What is the main feature of a good explanation?
A
o Explanation often requires an account of how the parts work together (leads to satisfaction of explanation)
3
Q
What level of explanation did Barlow think should be used to explain how the brain produces behaviour and experience?
A
• Explaining how the brain produces behaviour and experience
o Barlow’s neuron doctrine- how components give rise to perceptual experience
Appropriate level of explanation for this would be neuronal coordination (neural networks)
4
Q
What is emergence? What is its impact on computational neuroscience?
A
• Emergence- to relate to something like an unexplained or unexplainable appearance of an entity or property and/or something which is not “reducible” to well-defined interactions of other entities
o The relation between parts is important
When and what the parts are made/made of is not very important
o Behaviour of the system emerges from the relation between the parts
o Computational neuroscience is based on the idea that behaviour and experience are emergent
5
Q
What are the four main characteristics of computational modelling?
A
• Computational modelling
o Assumes behaviour and experience are emergent
o Is not entirely satisfied until we can build something that accomplishes the target behaviour
o Seeks simple explanations of behaviour and experience (as simple as possible while satisfying requirement above)
o Copies aspects of the brain as building blocks for model
6
Q
What basic building block does computational modelling use to model the brain? Give example, reason and limitation
A
Typically uses a simplified neuron as its basic building block
• Simplified models typically require prior knowledge of the components
• Have full understanding of monosynaptic stretch reflex and a successful model
• Must understand many interactions for better explanation
7
Q
What is the aim of modelling and simulation?
A
o Build something which mimics the thing that is trying to be understood
Can confirm explanation and allow for further exploration of level of understanding
• If a model can be built that models the subject, then the theory has a chance of being right
8
Q
What is the sign of a good model?
A
o Doesn’t duplicate- strips away irrelevant detail
Only represent relationships between the parts- not as concerned as to what the parts are
9
Q
What models were used to explain the brain over time? Why?
A
The mind is like a wax tablet (Plato, 1st century)
• Impressionability
• Wax could be too hard, soft, or full of impurities
Clock (17th century)
• Spurred the scientific revolution
• The mind is like an automaton
Clockwork automation
• Descartes: nerves act like hydraulic pipes
Computer (20th century)
• Biased our understanding of the brain
• Computer metaphor:
o Computers are programmed with a series of steps, and different functions done by different modules of code that interact and execute in a series of steps
• Different parts of the brain do different things- led to box and arrow theories
21st century-Copying the brain itself
• Need to reduce the complexity of the brain
• Cognitive neuroscience should succeed by finding level of abstraction by finding diagrams that capture the important aspects of what is interacting such that behaviour emerges
o However, although ion channels may be more complex than needed, brain areas are too simplistic
10
Q
What is an algorithm?
A
• Algorithm- a specific way to achieve something
o There’s more than on way to do things
11
Q
What is the aim of psychologists?
A
• As psychologists, want to understand human abilities
o Computers and the brain may do things in different ways
12
Q
At what level can behaviour be explained?
A
• Explaining behaviours requires considering some aspects of neurons
o Ignore many small details so that a simplified simulation of neurons can be built
13
Q
What is the connectionist model approach? How can it be used to explain behaviour?
A
• Connectionist model approach- consist of a number of different nodes that interact via weighted connections that can be adjusted through by the system through different ways, the most common being backpropagation of error
o Simplification: can represent the ability of neurons to make the next one fire, ignoring the details of the neurotransmitters or the membrane potentials
14
Q
Why is hard to make robots behave like humans?
A
Robots vs humans-
• Easy to design a robot for a specific task, but extremely hard to design one for general tasks
• Robots have difficulty doing things that humans can do with ease
• Robots don’t process information like humans
15
Q
What are the capabilities of single neurons?
A
• Singular neuron capabilities-
o Simple enough to understand fully- reflex, Pavlovian learning
16
Q
Why are neuronal interactions are important?
A
o Important brain functions for information processing-
Excitation or inhibition at synapse
17
Q
How many neurons are there in the cortex?
A
86 billion neurons
18
Q
How many connections does the average neuron have with other neurons?
A
10,000
19
Q
What is the threshold activation rule and what does it affect?
A
Each connection is affected by a threshold activation rule
• Threshold activation rule determines whether neuron fires or not
20
Q
Is the computer an accurate representation of how the brain works? Why/why not?
A
• Compare to computer
o Naïve computer-style box-and-arrow psychological theory-. Understand how network functioning differs
o The computer is a misleading metaphor-> computers become full but brains do not
• Computers vs brain-
o Computers-
Stores attributes of objects separately from each other
No interference (advantage) but no generalisation (disadvantage)
o Brain
Stores more than one item using the same set of units
• Because the network is small, interference rapidly sets in
o When learn something new, changes some of the connections we were using for old memories
Interference (disadvantage) but there is generalisation possible (advantage)
A lot of remembering is reconstructive- it’s not like retrieving a video file
Hard to know whether one is good at storing and retrieving the details from the episode vs how good you are at reconstructing and predicting the details from previous experiences
No separation between computation and memory
21
Q
How is the connectionist model represented diagrammatically (especially in Simbrain)?
A
• Connectionist model representations
o Neurons are represented by circles
o The number inside is the neuron’s activation, perhaps its firing rate or voltage potential
o Synapses are modelled by weights or connections
o The synaptic terminal is represented by the semicircle- its colour indicates the connection strength
Red- excitatory connection
Blue-inhibitory connection
o Synaptic strength= connection strength= weight
22
Q
What is the linear activation rule and how does it work?What is a disadvantage of ti?
A
o Linear activation rule- simply pass on the overall votes/stimulation
Adding excitation and subtracting inhibition
But linear is not enough for binary decisions
Synapses can have different weights- weighted average
23
Q
What is the threshold activation rule and how does it work? What is an advantage of it?
A
o Threshold activation rule-
If stimulation is more than threshold, activate
Adequate for modelling of binary decisions
24
Q
What do neurons representing the same location do to each other?
A
Mutually excite each other
25
What do neurons representing different locations do to each other?
Inhibit each other
26
What is the impact of lateral inhibition on a layer?
Lateral inhibition causes units on all layers representing a single object to eventually all become active
Neurons that do recognition also do attention- via lateral inhibition
27
What is posner cuing?
Posner cuing: cue pre-activates the location, helping subsequent target to win the competition sooner
28
What kind of connectionist network explains human rapid categorization performance?
A feedforward connectionist network explains human rapid categorization performance
29
How does a parallel distributed processing network work?
• Parallel distributed processing network-
o Categories emerge from overlap among all instances
o Strengthened pathways result in more rapid responses, whilst weaker pathways results in slower responses
30
How does the human connectionist network compare to a computer in terms of:
- Location
- Content
- Interference
- Generalisation
- Capacity limit
Connectionist network:
>Individual concept distributed widely
>Content-addressable (advantage)
-----Address memory with content and then same neurons that represent content will have connections to other subjects related to content----> search is guided and directed by connections
>Prone to interference
>Generalises naturally (advantage)
------Wisdom
----------Expert can more rapidly activate core that they are experts in-> can lead to predictions
>Soft capacity limit (interference as memories accumulate) (advantage)
---Every time acquire new memory, subtly degrade old memories
--------If new memory activates nodes of old memories, some aspects of old memories may be strengthened
```
Computer:
>Memories localised
>Time-consuming search
>No interference (advantage)
>No built-in generalisation
>Once full, it’s full
```
31
Compare the computer and the brain in terms of:
- Basic computation unit
- Communication between units
- Speed of messages
- Number of units
- Time for a single computation
- Storage
- Robustness
- Energy needed
- Tasks
```
Computer:
>Transistor
>Electrical wires
>About 300000km per second
>55 million transistors (P4)
>3 Ghz: 109
>Memory, manipulation separate
>Catastrophic failure from minor injury
>45 watts
>Chess:
--Almost entirely serial
-------Only use of one unit at a time
--Deep Blue evaluated between 100 million and 200 million chess positions per second
--During matches with Kasparov, it averaged 126 million positions per second
```
Brain:
>Neuron
>Synapses, transmitters, modulators, hormones
>7-430 km per hour
>More than 80 billion neurons
>About 200 Hz
>Memory, manipulation integrated and distributed memory
>Continually adaptive and graceful degradation
Information is distributed-> loss of a unit is less catastrophic
>20 watts
>Chess:
---Massively parallel
--------Can use neurons at the same time
---Chess skill: pattern recognition
32
Why is the brain never full?
• Individual memory distributed across thousands or millions of synapses
• Each new memory may degrade many of the old memories
• But also build on old memories, strengthening elements in common while degrading others (interference)
o Forgetting is more about interference
• Never hit a limit, but constantly degrading/losing old memory while making new
• Make new synapses
• Make very few neurons (too few to help significantly)- this is not the explanation of how we make memories
33
What is Moravec's paradox?
• Some things that we do, we can understand well whilst we cant understand other processes we use
o Many things that are easy to us are hard for computers e.g. walking
o Many things that are easy for computers are hard for us e.g. arithmetic
• The discovery by artificial intelligence and robotics researches that while high-level reasoning requires very little computation, sensorimotor skills require enormous computational resources
34
What is Polanyi's paradox?
• Polanyi’s paradox
o Many tasks people understand tacitly and accomplish effortlessly but neither computer programmers nor anyone else can enunciate the explicit rules or procedures
o The tasks that have proved hardest to automate are those demanding flexibility, judgment and common sense- skills that we understand only tacitly
Do not understand these consciously
o Polanyi’s paradox also suggests why high-level reasoning is straightforward to computerize and certain sensorimotor skills are not. High-level reasoning uses a set of formal logical tools that were developed specifically to address problems: for example, counting, mathematics, logical deduction and encoding quantitative relationships.
o In contrast, sensorimotor skills, physical flexibility, common sense, judgment, intuition, creativity, and spoken language are capabilities that the human species evolved, rather than developed. Formalizing these skills requires reverse-engineering a set of activities that we normally accomplish using only tacit understanding
35
What paradoxes can explain why we've been slow to make robots that can perceive the world and walk around?
• We’ve been slow to make robots that can perceive the world and walk around
o Moravec’s paradox-
Logical reasoning doesn’t require much computation; perception and locomotion does
o Polanyi’s paradox
We know more than we can tell
We don’t know consciously how we do perception and locomotion
o Perception and locomotion not easily done by conventional serial computation computer architecture
36
Who was Rodney Brooks and what was his opinion on the current progress of AI? What direction do we need to go in for AI according to him?
• Rodney Brooks-
o Early AI researchers made a big mistake: they thought intelligence was stuff they found hard to do, when it is the things that we find easy to do
No cognition. Just sensing and action. That is all I would build and completely leave out what traditionally was thought of as the intelligence of artificial intelligence
37
Compare the computer and the brain in terms of maths and sensory-motor capabilities in terms of:
- Arithmetic speed
- Storage of books
- Novel capabilities
- Motor abilities
- Sensory capabilities
Computer
Can do millions of arithmetic operations in a second
Can easily store and instantly access thousands of books
Can’t even write trashy novels
Can barely walk down stairs
Had bad visual recognition, until imitated connectionist networks (still easily fooled)
```
Brain
Can’t even do ten arithmetic operations a second
Has trouble memorizing even one book
Can write good novels
Can walk, hop and jump
Can recognise objects and people
```
38
What is dyspraxia?
• Need to connect sensory signals to movement
• Dyspraxia- disorder of movement not due to:
o Weakness
o Inability to move/paralysis
o Abnormal muscle tone or posture
o Intellectual deterioration
o Other direct motor deficits
• Dyspraxia is a perceptual problem and it is the reduced ability to coordinate, perform, plan or carry out specific movements even when there is no paralysis
39
What are the symptoms of people with dyspraxia?
o Perseveration- Some people with dyspraxia repeat the same movement over and over again
o Movements are either over exaggerated or appear slightly odd
40
Where are long-term memories of spatial location stored?
```
• Representing space-
o Long-term memories of spatial location are stored in medial temporal lobe/hippocampus
o Maps in brain-
Representation of where things are
Always relative to something
```
41
What are the two main coordinate frames?
• Coordinate frames-
o Allocentric- where things are relative to an external reference
o Egocentric- where things are relative to you
42
What is an allocentric coordinate frame?
o Allocentric- where things are relative to an external reference
How we store spatial layouts long-term
Allocentric coordinates state its location with respect to some other object
Allocentric coordinates can also be thought of as object centered
Areas in the what stream tend to encode locations in allocentric (object centered) coordinates
43
What is an egocentric coordinate frame?
o Egocentric- where things are relative to you
What we need to act on things
Egocentric coordinates- state where the object is relative to you
Areas in the where stream tend to encode locations in egocentric coordinates
44
What is controlled by top-control action?
Top-control action- which direction to:
• Look
• Walk
• Reach/point
45
What coordinates are used to look?
o Looking- retinal coordinates
46
What is retinotopy?
Retinotopy- different locations on the primary visual cortex correspond to different locations on the retina in an orderly fashion
• Retinotopic map
• Angle of object is specified by which neurons stimulated by object that hits the retina
47
What coordinates are used to walk and how are these calculated?
o Walking- body coordinates
Calculate body centric direction by getting eye and head direction
If know how neck is turned (proprioceptive sensors), can translate this to body direction
Add head-centric direction and head direction relative to body to get body-centric direction
48
What coordinates does pointing used and how is it calculated (and with what)?
o Pointing- hand coordinates
If want to aim at something, should put arm next to body and eyeline
• Otherwise, brain is quite bad at this
A sense of where the finger is relative to your body is thought to be computed in association areas within the dorsal stream along the intra-parietal sulcus (IPS)
• This position is represented in a nested reference frame because changing the shoulder angle changes the positions of all the more distal limbs
49
What are neurons in the intra-parietal sulcus involved in? Specify
• Neurons in the IPS are involved in visual attention and control of saccadic eye movements (LIP and VIP), visual control of reaching and pointing (VIP and MIP) visual control of grasping and manipulating hand movements (AIP) and perception of depth from stereopsis (CIP)
50
What is the intraparietal sulcus?
• Intraparietal sulcus- the end of the dorsal stream of the visual association cortex; involved in the perception of location, visual attention, and control of eye and hand movements
51
Along what 2 streams does somatosensory input flow, where does the signal originate from and what is the purpose of these 2 streams?
• The somatosensory input flows along 2 streams-
o The where stream codes for the egocentric position and movement of the limbs and body parts
This originates primarily from the receptors, those in the muscles and joints that signal proprioception and kinesthesia
The where stream originates primarily from area 3a but also 3b, 1 and 2 and flows to the various areas along the intra-parietal sulcus
o The what stream uses touch information to determine object shape using allocentric coordinates
Its signal originates from the receptors located in the skin which are first mapped in area 3b
From there the signal flows to areas 1 and 2 and then to the secondary somatosensory cortex (S2) located in the lateral parietal, mostly in the Sylvian fissure
52
What are the different coordinate frames of our bodies?
```
o Need different coordinate frames for different actions
Eye-centred (V1, V2, V4)
• Retinotopic map
Body-centred (somatosensory cortices)
Head/ear centred
Hand-centred
```
53
Is there a full map in the brain for hand-relative and torso-relative coordinates? What are the implications of this?
• Full map not found in brain for hand-relative and torso-relative coordinates
o Location calculated on-the-fly each time object is attended/ coordinate calculation made on the fly
54
Are body maps static?
• Updating maps-
| o Locations of things in each map change, often rapidly
55
Describe the visual pathway (in detail in terms of anatomy)
```
o Visual pathway full:
Retina
• Magnocellular (large, transient colour-blind)+ parvocellular (small, sustained, colour-sensitive)+ koniocellular
Thalamus
• LGN
Cortex
• V1-> V2
• Dorsal (where/how)
o V2-> V3-> MT -> MST-> parietal areas-> PF (cerebellum)-> brainstem (MN)-> retina relative position determined
• Ventral (what)
o V2-> V4-> FFA
```
56
What is gaze angle (eye position) needed for when calculating eye angle relative to the head?
• Gaze angle (eye position) needed for:
o Calculating direction for pointing, walking etc.
o Resolving whether, when retinal motion detected, did eyes move or objects move
• Eye movement command gets sent to parietal cortex to:
o Update representations of where things are
o Compensate for retinal motion, so don’t perceive the world to move when your eyes move
57
How is eye angle relative to the head calculated? Give two options and why one is more likely than the other
• Direct sensing of eye position (via proprioceptive sensation)
o This is wrong-no sensors telling us where our eyes are
• Remembering where you told your eyes to move (aka efference copy)-> means brain is correctly interpreting that the motion of the world is due to eyes moving, not the world moving
o This is the right explanation
o Proved by finger-in-the-eye demo
58
What is the issue with the finger in the eye demo?
This demo has a problem though
| • When push on eye, there are compensatory muscular reflex reactions that cause muscle to observe in opposite directions
59
Describe Brindley and Merton (1960)s experiment on the calculation of gaze angle
Brindley and Merton (1960)
• The lateral or medial rectus muscle was seized through the conjunctiva with fine-toothed forceps and moved
o Ss perceived objects to move opposite way
• Both eyes were held with forceps while he attempted to deviate the eyes actively
o Ss perceived objects to move opposite way even if eyes did not move at all-> the brain thought that the eyes were still moving
60
Describe Brindley and Merton (1970)s experiment on the calculation of gaze angle
Brindley and Merton (1970s)
• Would move eyeball with suction cup and string
• Results were consistent with previous results
• Stopped research due to lack of participants
61
Describe the coordinate systems needed, and their actions to look and reach at things
• More than one coordinate system needed
o Direction of things relative to eye (retinotopic map)
Used to look at things
Angles added to it to create torso-relative and hand-relative coordinates, probably in the parietal lobe
o Direction of things relative to hand, used for reaching
Need to know orientation of eyes relative to head, head relative to neck, etc. all the way down to hands
o Need to know your eyes’ position
You keep a copy of command telling eyes to move
Command also used to compensate for eye motion, so don’t think the world moved
• Full map has not been found in brain for hand-relative and torso-relative coordinated
o Location calculated on the fly each time object is attended
62
What are the functions of the parietal lobe?
• Parietal lobe is involved in spatial and somatosensory perception, and it receives visual, auditory, somatosensory and vestibular information to perform these tasks
• Using sensory signals for:
o Calculating position of body parts relative to objects
o Representing space for consciousness
o Connects sensory signals to motor movement-> does calculations for action plans
o Involved in focusing our attention to the spatial location of objects in space and to a particular object
o Feelings of body ownership
63
What is most likely to produce neglect syndrome- a right or a left parietal lesion? Why?
• Interestingly a left parietal does not result in neglect of things on the right- this may be because the right parietal directs attention to the left and right side of objects, while the left parietal directs attention only to the right
o Thus, after a lesion in the left parietal, the right parietal still attends to the right as well as the left side of objects
64
Describe how neglect syndrome influences spatial memory
o Memory locations intact, but half neglected when used
Map located elsewhere
Gets unpacked by parietal cortex
65
Describe how neglect syndrome influences using memories for spatial imagery (including how spatial memories are transformed for spatial imagery, and why a lesion to the parietal lobe may be problematic for this)
o Using memories in spatial imagery
Egocentric transformation
Imagery uses info from allocentric memory-> parietal and visual areas transforms allocentric frames to egocentric frames
• Rather than activating long-term memory, parietal cortex puts allocentric spatial memory into egocentric spatial memory
66
Describe Bisiach and Luzzatti's (1978) experiment in neglect patients, and what this demonstrated
• Bisiach and Luzzatti (1978)
o Asked patients with damage to right parietal lobe to imagine themselves in Milano cathedral and reporting what they would see from the steps
o Reported what they would see on their right, but neglected their left
o Demonstrated that parietal neglect syndrome was not just a syndrome of neglecting sensory signals that were coming in, but also left half of spatial long-term memory once transformed into an egocentric frame
o To check that the building was not the problem, made them perform the same experiment from the other end, so that building that was on the left was on the right
Demonstrated same effect- could not detect left building but could detect right
o Suggests that memory map is intact, but that imagery uses the same mechanisms as that which causes neglect of real stimuli
o This, neglect is found for items in visual memory during remembrance of a scene as well as for items in the external sensory world
o The key point in the Bisiach and Luzzatti experiment is that patient’s neglect could not be attributed to lacking memories bur rather indicates that attention to parts of the recalled images was biased
67
Describe Marshall and Halligan's 1988 experiment on neglect patients
o Marshall and Halligan 1988-
Showed two pictures of houses-asked which house they’d rather live in (one was on fire, but smoke coming to the side of the patient’s neglect)
• Even if patients did not consciously describe the smoke coming out of the house, they’d always pick the house not on fire
o As if smoke is getting processed by ventral stream enough to signal danger
However, works under the assumption that the smoke would be recognised as smoke- may simply have been recognised as black smudge and patients picked based on that
68
Describe Berti, Frassinetti and Umita's 1994 experiment on neglect patients and what is showed
o Berti, Frassinetti and Umita 1994
Stroop effect-
• If the colour written down is written in a different colour than the written colour, people are slower in saying the colour of the word
• In this experiment, they put word identity (a colour word) on left visual field (or on side of neglect) and a coloured square on the right visual field: patients were asked to determine the colour of the block as fast as they could
o In order for the Stroop effect to occur in hemispatial neglect patients, the word would have to be processed in the ventral stream-> would slow down the production of the right response
• Patients showed a Stroop effect- were slower in reporting incongruent colours an congruent colours
• Demonstrates that even if patients had no conscious awareness of the words, the words were getting processed by the ventral stream and hence the ventral stream were seemingly intact
69
How is visual processing affected with people with unilateral neglect syndrome/hemispatial neglect?
• Visual processing without parietal function
o Processing of sensory signals in neglected half of field
o Summary of visual processing in hemispatial neglect
Imagery still works, but half neglected
Words still read, even when neglected
But locations not available to consciousness
Object recognition is still pretty good because of the ventral stream
70
Do people with hemispatial neglect process unperceived stimuli? Provide evidence
Compared fMRI signals for a shape represented alone in the normal right visual field to when shape was presented with an extinguished face in the left visual field (that is, different stimuli but same percept)
• People couldn’t report faces in left visual field when shape was in right visual field
Found that the unseen faces activated right visual cortex and inferior temporal areas with face-specific responses but the activation was weak compared with seen stimuli
71
What maps/representation translations are needed for object recognition?
Object recognition compensates for object movement, and eye movements
• In order to do object recognition, need something that compensates for egocentric/spatial orientation that is in
In order for object recognition to occur within the ventral stream, it needs to use long-term memory (with allocentric frame)
For object recognition, sometimes need to translate retinotopic sensory signals into a stimulus-centred or object-centred representation
72
How does the brain orient an object during object recognition? What is an object which the brain has particular trouble with in terms of object recognition?
Some parts of the brain have trouble with the correct orientation of upside-down faces
• Facial expression recognition does not compensate well for orientation
• Sometimes, brain does not transform faces well into canonical orientation
73
What are the three coordinate frames for object recognition?
Coordinate frames for object recognition
• Viewer-centred USN
• Stimulus-centred USN
• Object-centred USN
74
What is the impact of viewer-centred unilateral spatial neglect during object recognition and how is it impacted by hemispatial neglect?
• Viewer-centred USN: for directing gaze, need to know coordinates relative to visual field
o Hemispatial neglect patients miss the neglect side half of the word/sentence
75
What is stimulus-centred unilateral spatial neglect during object recognition and how is it impacted by hemispatial neglect?
• Stimulus-centred USN: for processing orientation of each shape (e.g. arrows)
o Hemispatial neglect patients miss the neglect side half of each word in the sentence
76
What is object-centred unilateral spatial neglect during object recognition and how is it impacted by hemispatial neglect? Explain why this might occur
• Object-centred USN: for word recognition and defined by canonical object orientation
o Hemispatial neglect patients miss the neglect side half of each word IN THE RIGHT ORIENTATION even if it was in the wrong orientation
o Transform the word into its canonical orientation BEFORE reading it
On this account, although orientation-invariant object-centred representations may be used by the object recognition system to identify the object and locate its vertical axis, spatial attention is nevertheless allocated with respect to an egocentric reference frame.
o Mirror-reversed and vertical stimuli show that a patient with right neglect failed to read the canonical right side of words regardless of where the right side fell in egocentric space. The same pattern of errors is also observed in written spelling, oral spelling, and backward oral spelling. These and other data are suggestive of damage to the word-centred (i.e., object-centred) grapheme representation
o Object-based neglect is neglect that occurs with respect to the intrinsic principal axes of objects, and thus putatively arises at the level of an internal image of the canonical object, i.e., the structural description.
77
What is a stroke and what is its most common cause?
• Stroke (cerebrovascular injury)
o If you live to 85, about 1 in 5 chance of having one
o The most common cause of a stroke is a thrombosis- when a blood vessel supplying vital nutrients to the brain becomes blocked with a blood clot
Stroke in one side results body weakness in opposite side of the body
78
What are syndromes that can be due to strokes?
o Hemiparesis
o Hemianopia
o Hemispatial neglect
• Bilateral parietal injury
79
What is hemiparesis, how many stroke survivors does it affect and what can be done for it?
o Hemiparesis
Weakness of one entire side of the body
Affects 80% of stroke survivors
Brain basis is straightforward
• Pathway from motor areas of the brain towards the spinal cord
Physical therapy helps strengthen the pathways affected
Hemiparesis of right side- stroke affecting left primary motor cortex and movement pathways
80
What is hemianopia and how can it occur?
o Hemianopia-
Injury to left occipital lobe- hemianopia of right visual field
Blindness of one visual field
81
What is hemispatial neglect and how can it occur?
Results when the brain is damaged in only one hemisphere
• Unilateral spatial neglect may result from damage to the right parietal, temporal and/or frontal cortices, as well as subcortical structures. This kind of damage leads to reduced attention to and processing of the left-hand side of scenes and objects
Often caused by stroke in one parietal lobe
82
What are the symptoms/characteristics of hemispatial neglect?
A prominent feature of neglect is extinction, the failure to perceive or act on stimuli contralateral to the lesion (contralesional stimuli) when presented simultaneously with a stimulus ipsilateral to the lesion (ipsilesional stimulus)
Neglect affects external personal hemispace and objects as well as internal memory for objects arrayed in space
Depending on the severity of the damage, its location and how much time has passed since the damage occurred, patients may have reduced arousal and processing speed, as well as an attention bias in the direction of the lesion
Neglect involves deficits in attending to and acting in the direction that is opposite the side of the unilateral brain damage
Attention/awareness of the side opposite to the stroke is impaired
83
How many patients with unilateral visual neglect also show imaginal neglect and why?
• However, only a minority of patients with visual neglect, however, also show imaginal neglect, perhaps because imaged details have less attention-capturing power than real ones
84
Is left or right unilateral neglect more common? Why?
• Most patients who have neglect have it on the left side because the left hemisphere seems better at covering both visual fields
o Have right hemisphere damage
85
What occurs in most unilateral neglect patients when they are asked to draw with a blindfold on? What does this suggest?
Neglect may be due to attentional mechanisms- asymmetry of drawings are less evident if patients are blindfolded when they draw
86
Is there a chance of recovery for patients with hemispatial neglect?
Some patients recover
87
Why is hemispatial neglect described as a bias, rather than a loss of the ability to focus on attention contralesionally?
Attention/awareness of the side opposite to the stroke is impaired
• If pointed towards the left however, is able to attend to it
• Attention for one side impaired
• Biases against the contralesional sides of space and objects can be overcome if the patient’s attention is directed to the neglected locations of items. This is one reason the condition is described as a bias, rather than a loss of the ability to focus on attention contralesionally
88
What are tests to test for hemispatial neglect? How do patients with hemispatial neglect perform on these tests?
Tests to test for neglect:
• Line cancellation test- show people a small line segment and ask people to make a mark in the middle of the line
o Person with hemispatial neglect will put the dividing mark away from the side that they neglect
• Copying objects or scenes
o Will neglect to draw half of the image
89
Are people with hemispatial neglect conscious that they have it?
People with hemispatial neglect are often unaware of their condition (anosognosia)
90
On what spatial coordinates can hemispatial neglect be based on?
Neglect can be based on spatial coordinates either with respect to the patient (egocentric reference frame) or with respect to an object in space (allocentric reference frame)
91
Describe the different contributions of parietal and frontal damage to hemispatial neglect
Models of neglect which postulate a dysfunction of large-scale right-hemisphere networks are supported.
• Parietal components of the network may determine the perceptual salience of extrapersonal objects
• Frontal components may be implicated in the production of an appropriate response to behaviourally relevant stimuli, in the online retention of spatial information, or in the focusing of attention on salient items through reciprocal connections to more posterior regions.
92
What is Balint's syndrome?
o Balint’s syndrome- severe disturbances of visual attention and awareness, caused by bilateral damage to regions of the posterior parietal and occipital cortex
93
What are the three characteristics of bilateral parietal injury/Balint's syndrome? Describe
o Three characteristics-
Simultanagnosia- the difficulty perceiving the visual field as a whole scene
Ocular apraxia- deficit in making saccades to scan the visual field, resulting in the inability to guide eye movements voluntarily
Optic ataxia-a problem in making visually guided hand mmovemments
94
Define simultanagnosia in Balint's syndrome and how an object can be pulled to focus for people with this syndrome?
o Simultanagnosia
Only one object perceived at a time (cannot perceive two separate objects at once)
• Object identification is good for only one object
Wiggling (exogenous attention cue) results in perception the object
• Motion pulls attention to object-> enhances activity in visual cortical areas and hence enhances perception of that object
95
What is selective attention and what is this prioritization determined by?
o Selective attention is the ability to prioritize and attend to some things while ignoring others. This prioritization is determined by:
Top-down/goal-driven control:
• Driver by an individual’s current behavioural goals and shaped by learned priorities based on personal experience and evolutionary adaptations
Bottom-up or reflexive control:
• Reaction is stimulus driven
96
What are the anatomical attention systems in the brain?
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Attention systems-
• Superior colliculus- control of attention
• Portions of the frontal cortex
• Posterior parietal cortex
• Possterior superior temporal cortex
• Medial brain structures
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97
What visual stream is important for egocentric representations?
o Egocentric- dorsal stream (where/how)
98
What visual stream is important for allocentric representations?
o Allocentric- ventral stream (what)
| Used for spatial memory
99
Can allocentric long-term memory be transformed into egocentric representation? Describe
o Allocentric long-term memory can be transformed into egocentric representation if imagining yourself in a situation
Egocentric transformation
