lecture 12: disorders of vision Flashcards
what is the visual world composed of
- a seamless visual world is composed of a visual world of the right eye and the left eye, in reality these two worlds interact/ overlap in the middle
- half of the fibres coming from left cross to right and half stay on the left side and vice versa
- however it is still true that the right likes to look at the left and left likes to look at the right
nasal retina
inside (projects out)
temporal retina
outside (look/projects in)
how to figure out what kind of damage causes what kind of blindness
follow the pathways back to the retina and shade in the corresponding visual fields to find out
right monocular blindness
severed optic nerve coming out of your right eye = blindness in your right eye
–> right eye shaded in
bitemporal hemianopia
damage the crossing fibres (half vision), if the image falls within the blind visual fields you dont see it
- outside half of both eyes shaded in
left homonymous hemianopia
left side of both eyes shaded in
left homonymous hemianopia with macular sparing
left side of both eyes shaded, with a small unshaded circle in both
macular sparing
tendency to have the central vision spared
blindsight
- surgical removal of a tumour in right occipital lobe = left homonymous hemianopia
hatched region = regions of residual vision
two interesting visual problems displayed with patient DB (surgical removal of tumour in right occipital lobe)
- cant identify a static visual object but can localize it in space. says he cant even see an object
- cant identify a moving visual object but can localize it in space. the difference is that with moving objects he says he has the sensation that something is out there.
how can accomodate these two aspects of DBs vision
- localising an object that he cant see is a result of information reaching the dorsal stream
dorsal = where
ventral = what - sensing a moving object is a result of information reaching V5
V5 = tells if its a moving visual stimulus or not - not being able to identify a static or moving object is because info is bypassing V1
- 5% pathway that goes through the eyes bypasses V1 and hooks up with V5 which then projects info along the dorsal pathway
achromatopsia
= absence of colour vision
- damage to V4
- people with achromatopsia are colour blind
- they see only black, white and shades of grey
- can arise from missing cone photoreceptors. indeed most achromatopsics are colour blind for this reason
what is the main function of V4
to assign colour to what we are seeing
akinetopsia
= absence of motion vision
- damage to V5 (MT)
- Patients have difficult perceiving objects set in motion
- extremely rare
- objects dont have to be moving fast for them to lose sight of it
what is v5 responsible for
area of the brain that processes whether there is motion out there
visual agnosias list
- apperceptive agnosia
- dorsal simultagnosia
- ventral simultagnosia
- associative agnosia
visual agnosia def
neurological disorder characterised by the inability to recognise or interpret sensory info
appreceptive agnosia
- failure of object recognition due to fundamental failure of visual perception
- preserved elementary visual function such as colour perception and motion perception
- poor matching and copying
neuropathology = bilateral damage to V1
pepper mask hypothesis of apperceptive agnosia
- V1 is retinotopically mapped
- as a result, damage to V1 results in multiple blind spots known as “scotomas”
- its like seeing the world through a peppery mask, hence vision is severely impaired
dorsal simultagnosia
- failure of object recognition due to a spatial perceptual impairment
- preserved elementary visual function such as colour perception and motion perception
- can recognise objects but not more than one at a time
neuropathology = bilateral damage to parietal lobes
ventral simultagnosia
- failure of object recognition due to a complex perceptual impairment
- preserved elementary visual function such as colour perception and motion perception
- can recognise objects clearly but not more than one at a time But can see multiple objects (but not one clearly)
neuropathology = ventral stream beyond V4
associative agnosia
- failure of object recognition due to a higher order complex perceptual impairment
- preserved elementary visual function such as colour perception and motion perception
- seemingly normal copying
- however perception is not normal, copying is accurate but “slavish”
slavish = a slave to the picture, being super cautious, very clear they are struggling
