visual disorders Flashcards
amblyopia
something that leads to an imbalance or mis-integration of the input to two eyes
why does amblyopia occur?
something has gone wrong at an early age which stops the two eyes working together
how do we cause monocular deprivation in animal subjects?
suturing one eye shut
what happens when one eye was sutured shut in a monkey soon after birth?
results in a wider ocular dominance columns from the non deprived eye
why is the timing of monocular domination critical?
the adjustment of ocular dominance columns reduces the older the animal, and the less time the eye is closed for
-large effect for age 2 and 5.5 closed 18 months
-smaller effect 10 weeks, only 4 months
-no effect 14 years for 14 months
Hubel and Wiesel Ocular dominance scale
quantifies electrophysical responses of V1 cells to stimuli applied to the left or right eye
following monocular deprivation, a vast majority of cells are driven…
exclusively by the non deprived eye
what drives segregation of retinal outputs? why does representation change with monocular deprivation?
co-ordinates waves of left or right retinal output
less firing in the deprived eye
amblyopia critical period cats
3-4 months
amblyopia critical period macaque
highly susceptible for the first 6 weeks
by 10 weeks more moderate, needs longer periods of deprivation
no effect if carried out after 1 year
amblyopia critical period man
up to age 5-10
most pronounced during first year
what are signals from the deprived eye suppressed by?
actively supressed by local interneurons
what can be done to induce low level responses from the deprived eye in V1 cells?
a blockade of local GABA signalling using Bicuculline (GABAa antagonist)
occlusive stroke
due to closure of a blood vessel
blood clot, embolism
atherosclerosis
what percentage of strokes are occlusive?
> 80%
haemorrhagic stroke
due to rupture of a blood vessel
hypertension, aneurysm
what is a scotoma
a blind or partially blind spot in the visual field
what can cause scotoma?
stroke effecting V1
absolute scotoma
no vision
relative scotoma
some remaining vision
hemianopic scotoma
one half of the visual field gone
facultative/ suppression scotoma
in alternating foveation in squint conditions
what artery supplies the V1 on each side?
the posterior cerebral artery
occlusion of posterior cerebral artery can cause?
widespread loss of vision
e.g. across one full hemisphere
homonymous hemianopia
loss of vision on the same side in both eyes
homonymous hemianopia can be a result of?
often damage to the V1
can also be damage to other areas such as LGN, optic radiations
requires differential diagnosis/ scan
fixation of eyes homonymous hemianopia
eyes fixate on everything in the scene but are only consciously aware of half of this area
what is blindsight a result of?
extensive damage to the striated cortex
what is blind sight a type of?
scotoma
how does blindsight manifest?
patient ‘unaware’ of blind hemifield
when asked to locate a target they will protest they are blind
what happens when a blindsight patient is asked to guess about a target located in their ‘blind’ hemifield?
they will do better than change
can identify orientation and some colour
mechanisms of blindsight guesses
portions of V1 remain intact and active?
residual vision in higher cortices (routed through the superior colliculus/ pulvinar)
Cowey et al. task one
striate cortex removed unilaterally in 4 macaques
animal fixates
fixation light goes out and 1 of 4 peripheral target comes on
animal touches the target
got a higher percentage of correct responses than chance
cowey at al. task 2
striate cortex removed in 3 macaques
normal trials: target appears in one of the 5 locations in the good field – touch target
blank trials: no target appears – touch the outlined rectangle on the upper left
probe trials: a target appears in the blind field – touch this target
during probed trials monkeys responded as if no target had appeared: blindsight
motion agnosia in Zihls patient
- a patient with intact visual fields lost all perception of motion and could not distinguish between stationary and moving objects
loss of movement in all three dimensions
Due to bilateral damage of the human homologue of V5(MT)
why is motion agnosia so rare
only occurs when damage to the V5 is bilateral
neglect
error of the interpretation and interaction with objects in space due to parietal damage
most extreme form of neglect
patient looses one half of the world
what half of the world do neglect patients see? what is this different to?
they can only view the hemispace contralateral to their parietal lesion
unlike hemianopes
what does neglect effect other than vision?
study example
memory of a space
patients asked to remember the names of the shops in the Piazza in Milan
-remembering the view facing south, they identified only the shops on their right
-remembering the view facing north, they name only the shops on their right
what frame is neglect seen in
supra retinal reference frame
what side of the parietal cortex is more often damaged with neglect
right side, loss of vision in left
what may neglect be due to
competition or inability to disengage attention
prosopagnosia
an object agnosia, unable to recognise objects
agnosia
a loss of knowledge
what causes prosopagnosia
stroke damage to the inferior temporal cortex
prosopagnosia and facial recognition
- can identify a face as a face, or parts of faces, or even emotions on faces
- cannot recognise individual people, even close family members (potentially even own face)
- identities are not lost, only the connections between particular faces and identities
