Defective discrimination: cerebral visual impairment Flashcards
what is visual dysfunctions caused by
damage to the brain - cortical or extracortical
what things may cerebral visual impairment affect
- visual fields
- VA
- other aspects of perception e.g. motion detection, colour vision, object recognition
why can cerebral visual impairment often get misdiagnosed and mismanaged by optometrists
due to contradictory findings of
- normal VA’s & visual fields (routine measurements can still seem fine)
&
- unusual visual complaints (caused by cerebral changes)
which part of the brain is involved with object recognition i
inferotemporal area/cortex (IT)
what do RF cells in the inferotemporal area respond to
specific shaped stimuli = selective
some respond to simple geometric shapes and some respond to more complex biological stimuli e.g. hands and faces
how are the cells in the inferotemporal (IT) area grouped
into columns, where cells in the same column respond to the same shapes
what do 10% of IT cells show preference for
only face shape (monkey or human), not the features of a face e.g. if the features were jumbled up then the cells won’t respond
what types of faces do IT cells respond more strongly to and what is this called
faces of specific people/monkeys i.e. selective for identity
but the same face will excite many cells, so its more likely that recognition of a face is coded by pattern of activity within population of cells
what is the meaning of agnosia
loss of ability to recognise objects, faces, sounds, shapes, or smells when specific sense is not defective & no significant memory loss
what is not necessary lost when someone has an agnosia
memory is not lost
what is not necessary lost when someone has an agnosia
memory or vision is not lost
what is a visual agnosia and explain the symptoms
selective deficits to IT cause inability to recognise object or a class of objects e.g. unable to match a shape with another that looks exactly the same correctly, but they can see the shape and they know what it is, but lost the ability to recognise the shape from their visual appearance
what is colour agnosia
lack of ability to recognise colours
what is prosopagnosia and explain the symptoms
face blindness
- patient cannot recognise familiar faces, even their own
- can see all the features
- can classify faces e.g. male/female, race etc
- but faces have no meaning
- can recognise family and friends when they speak
what to lesions to IT of the right hemisphere cause
greater impairment in face processing
as right hemisphere IT is responsible for face recognition
what is prosopagnosia often associated with and why
achromatopsia (colour loss), because IT is near v4
what is cerebral achromatopsia caused by and what does it result in
damage to v4 results in complete or partial loss of colour vision
why is it easy for an adult to describe that they have prosopagnosia or achromatopsia as opposed to a child
because adults know what their vision used to be like, but when children have this problem from a young age, they won’t know that this problem is normal or they cannot explain it
when a child is unable to recognise familiar people such as their mother until they speak, and are unable to recognise pictures of everyday objects e.g. unable to distinguish between a dog and a cat until given a verbal clue but can still pick objects up if they were dropped on the floor, and their vision is fine, what two things does this child have
prosopagnosia associated with visual agnosia
sometimes children are not believed by the parent or teacher as they can still see and pick up objects
how can a person with prosopagnosia who misread facial expressions and struggle to recognise individual faces built strategies to recognise certain people
by remembering their hair style, style of walk and hair colour, recognising their voices and scents
these can be taught to children by their teachers and parents
what are the causes of more general neurological damage which results in pure visual agnosia
- congenital (hereditary)
- stroke
- dementia
- brain damage
- degenerative neurological conditions
- brain infection
- trauma
how is word recognition a complicated visual task
because whilst reading you have to identify the symbol, and then translate it into phonological and semantic codes (i.e. sounds and meaning of word)
how is word recognition a complicated visual task
because whilst reading you have to identify the symbol, and then translate it into phonological and semantic codes (i.e. sounds and meaning of word) involves co-operations between different brain systems
what different things can word recognition be affected by
- trauma
- strokes
- developmental abnormalities
what is the condition of not recognising words called
alexia
describe one form/symptom of alexia
letter by letter reading
these patients can only read one letter at a time (as can’t judge by shape of work & control & can’t guess the next word as we go along)
describe one form/symptom of alexia
letter by letter reading
these patients can only read one letter at a time (as can’t judge by shape of work & control & can’t guess the next word as we go along)
what area of the brain is visual guidance and attention related to
parietal lobe
what does visual attention enable us to process
process selected aspects of the retinal image more fully than non-selected aspects (aware of other parts of the visual scene, but aren’t attending to them) selection is usually made automatically at a non-conscious level (depending on what our intentions are & the circumstances)
what is inattentional blindness
a form of temporary blindness where we are impaired in perceiving the appearance of unexpected objects
what is inattentional blindness
a form of temporary blindness where we are impaired in perceiving the appearance of unexpected objects
i.e. when concentrating particularly hard on one part of a scene, we may not notice fairly conspicuous things happening elsewhere in the scene
what problems can inattention blindness cause
when driving & selectively focussing on cars may not notice the pedestrian or bike
what is change blindness
when observers fail to perceive quite obvious changes in a scene over time, if the changes are separated by a transient event such as a blank frame or an eye movement
it is an inability to correctly compare scene with previous image of scene stored in memory
i.e. by transient loss of attention, makes it hard to notice change in visual scene
which part of the brain damaged as a cause of unilateral neglect
damage to parietal cortex
what happens in unilateral neglect
patients systematically fail to notice objects on the side of the world opposite to the brain injury
what is unilateral neglect most commonly caused by
a stroke e.g. in the right hemisphere, causing neglect of objects in the left visual field - even though the visual field is still intact
so still able to see entire visual field but only able to pay attention to one half of the visual scene that corresponds to their undamaged parietal lobe
what is unilateral neglect most commonly caused by
a stroke e.g. in the right hemisphere, causing neglect of objects in the left visual field - even though the visual field is still intact
so still able to see entire visual field but only able to pay attention to one half of the visual scene that corresponds to their undamaged parietal lobe
name and describe 2 types of unilateral neglect
- egocentric neglect: patients neglect everything on the left side of the mid plane of their body
- allocentric neglect: patients neglect the left side of each individual stimulus (e.g. an object) regardless of its location in space
which part of the brain is concerned with visual guidance
parietal cortex
what is the purpose of visual guidance
to avoid bumping into things or be able to catch objects
what 2 parts of the brain does visual guidance receive input from
MST (optic flow) and IT (object recognition)
what is the symptom of optic ataxia
visually guided misreaching for objects
subjects can perform reaching tasks when mechanic proprioceptive feedback is used e.g. can touch own nose (as using proprioceptors) but not when visual guidance is used e.g. touching someone else’s nose
what is the cause of Balint’s syndrome
bilateral damage to the parietal cortex e.g. due to stroke, alzheimer’s, inter cranial tumours, brain damage
list and describe the 4 main symptoms of Balint’s disease
- ocular motor apraxia: inability to change fixation )some have to shut their eye, move their head and then open their eyes again, or do rapid head movements to change fixation)
- simultagnosia: one object only perceived during fixation (can only perceive one object at a time)
- spatial disorientation: inability to localise objects correctly in space
- optic ataxia: inability to grasp a clearly perceived object (inability to use vision and guide movements)
as well as perception of the visual scene, what else is vision dependent on
awareness of what we’re seeing
as well as perception of the visual scene, what else is vision dependent on
awareness of what we’re seeing
can have vision, but not be aware of what they’re seeing
with prosopagnosia, what affect does same vs different comparison for faces have
faster for familiar than unfamiliar
although not aware of face recognition occurring
with someone who has unilateral neglect following parietal damage, when shown two houses where one house is on fire and asked which one they would like to live in, which house will they chose and why
houses look identical to the patient, but when asked they will chose the house without the fire, because although they can only attend to something in one side of their visual field, they still have perception so know the house will be burnt, but can explain why (as they have no awareness of the perception they have)
what is blindsight
certain patients with damaged/absent v1, so they have no conscious vision, they can only respond to visual tasks with greater than chance accuracy (AFC tasks, where targets are large and are presented for a long time), but this is not of use in everyday life as it cannot support intentional actions (as not aware of their vision)
how are people with blindsight still able to perform forced-choice tasks
because of the 10% of LGN axons that go to the superior colliculus for eye movements, which allows for basic tasks such as 2AFC
which stripes of v2 is magnocellular information processed
thick stripes
which two pathways is magnocellular information processed, beyond v2
- dorsal (where)
- ventral (what)
where does motion information travel through in the dorsal pathways
to v3 and then to MT (for motion perceptions), then to MST (concerned with optic flow) used for visual guidance and overall movement in the visual scene
what are 90% of MT cells
direction selective
many are also speed selective
what condition can damage to MT result in
akinetopsia (motion blindness)
which area of the brain is concerned with optic flow
MST/medial superior temporal area
which area of the brain is concerned with motion perception
MT/middle temporal
what can cause cerebral akinetopsia
- cortical ischaemia
- traumatic brain injury
- alzheimer’s disease
how much of the brain is devoted to visual processing
40%, so many people with brain damage have some type of cerebral visual impairment
why is cerebral visual impairment very common in high income countries
due to survival of premature babies with neurological damage and children with severe medical conditions
why is it difficult to diagnose children with cerebral visual impairment
because they accept their visual impairment as normal or it may be subtle
how may you detect a child with cerebral visual impairment in motion - tracking
they will compensate by moving their head to follow slow moving objects and will struggle with tracking fast moving objects with head movements
what will you advise for a child who has cerebral visual impairment in motion - impaired tracking
ask others to use slow movement and gestures and child will need to avoid video games with lots of movement
what will you advise for a child who has cerebral visual impairment in motion - impaired movement perception (speed/direction)
warn parents of crossing roads
how may you detect a child with cerebral visual impairment in motion who struggles to see a stationary scene (can only see objects when they’re moving)
they will rock back and forth to stimulate vision (as they can only see objects when they’re moving, in their visual field)
list 3 types of problems children can have when they have cerebral visual impairment of visual guidance
impaired visual guidance = problems making accurate movement through visual space
- difficulty with steps: hard to estimate depth (e.g. foot lifted too high and too early)
- problems with reaching for objects and manipulating them
- hand doesn’t match orientation and shape of object during reaching (as can’t use vision to make that judgement)
what solution can there be for a child who has cerebral visual impairment of visual guidance
practise e.g. with soft play areas
which part of the brain is damaged with cerebral visual impairment of attention mechanisms
parietal/dorsal stream damage
what problems does a child with cerebral visual impairment of attention mechanism have
- problem locating a toy on a patterned carpet much less difficultly than on a plain carpet
- problem locating object is it is surrounded by other objects
what solution is there for a child who has cerebral visual impairment of attention mechanism
make parents/teachers aware and simplify their home environment e.g. use plain carpets so can recognise toys against it
what problems does an older child with cerebral visual impairment of attention mechanisms have
- they’re only able to process a few words at a time when learning to read
- crowing effect: problems identifying letters presented in rows, but no problem with individual letters
what solution can there be for an older child who has cerebral visual impairment of attention mechanisms who struggles to process a few words at a time
enlarge text or use computer programme showing one word at a time or try a magnifier, to avoid text being crammed onto one page
