11) Perception, Agnosia, etc. Flashcards
What is agnosia?
= visual information is transported via retina but cannot be processed –> objects (and faces) cannot be recognised
- apperceptive agnosia = failure in perception and therefore recognition
- associative agnosia = perception occurs but still not recognised –> V1 ok but association between visual and semantic brain areas defect
- lesions often occur due to hypoxia or reanimation
What are the principles of retinotopy?
- things are represented next to each other in the brain and visual field
What are the three retinal ganglia cells?
M-cells - magnolayer
- 10% of ganglia cells
- large receptive fields
- small spatial but good temporal resolution
- high contrast sensitivity
- achromatic (colour blind)
P-cells - parvolayer
- 80% of ganglia cells
- small receptive fields
- high spatial but poor temporal resolution
- small contrast sensitivity
- chromatic (colour competent)
Gamma-cells
- 10% of retinal ganglia cells
- heterogeous properties
Why are retinal ganglia cells well myelinated?
good for motion processing, because faster signal transmission
How is the lateral geniculate nucelus (LGN) organised and what does it do?
organised in layers
- magnolayers
- parvolayers
- interlaminar layers
- forwards information to primary visual cortex/V1/striate cortex/BA17 (= only area in the brain where one can see stripes just when slicing the brain, no additional staining needed)
–> visual information is from V1 forwarded to other areas in visual cortex (BA16-19/V1-V4) (?)
What are the retino-genikulo-striate pathways?
N Suprachiasmaticus
- circadian rhythm
- dark-light perception
Colliculus superior
- rapid eye movement
–> colliculus inferior: auditory
Corpus geniculate laterale
(- still unclear what it does, but postulated to be involved in spatial processing of objects in visual field and temporal differentiation of the same object at different time points)
How does the (primary) visual cortex process information (Hubel&Wiese)?
Ice-cube model
- organised in columns
- cells in column only react to a certain characteristic in the visual field (orientation, intensity..)
–> extraction of characteristics
(- same principle ‘Eigenschaftsextraktion’/extraction of characteristics found auditory cortex but more bitalerally)
Hemifield and hemispheres: how is it represented?
hemifield
Right hemifield:
- visual information of the temporal part of the retina of the right eye
- information of the nasal of the retina of the left eye
Left hemifield :
- temporal part of rentina (left eye)
- nasal part of the retina (right eye)
Viusal field
= the total area in which objects can be seen in the side (peripheral) vision as you focus your eyes on a central point
Hemispheres
- left and right side of the brain
- information of the right hemifield is represented in the left hemisphere
- information of the left hemifield is represented in the right hemisphere
- information is represented mirrored (left-right and up-down’
- everything in the centre is more polar (over representation of fovea)
- periphery more to the front
Hierarchy in the visual cortex
from V1 to V5
- V1 most accurate in retinotopy - V4 and V5 least accurate in retinotopy
- V1 least complex (specific to responses, 1° resolution) - V4 and V5 most complex (specialised, 20° resolution)
- V1
- most accurate in retinotopy but less complex information processing
- contrast and orientation - V2
- differentiation background and figures - V3a
- disparity/incongruity/discrepancy –> mismatch when looking at a face from different angles
- depth - V4
- colour and form
- ventral: closer to temporal cortex (memory), remembering faces, objects etc –> recognition - V5
- motion, movement
- dorsal: closer to motor areas, motion/movement via dorsal stream –> system advantage (vision guided movements)
What are the dorsal and ventral stream?
dorsal stream
= where/ how/ action/ location/ egocentric
- closer to motor areas, V5
ventral stream
= what/ object/ perception/ allocentric
- closer to temporal cortex, V4
Network map and complexity
- more complex than described (dorsal vs ventral)
–> complex motion perception involves more
Retinotopy and lesions: What does it mean for patients?
lesions at different points in the system cause different visual impariments
contralateral homonymous hemianopia: lesion after optic chiasm (D), in optic radiations (G) or in upper and lower bank of the calcarine fissure (H)
What is homoymous hemianopia?
= lesion in occipital cortex (upper and lower banks of calcarine fissure), lesion after optic chiasm or optic radiations (one-side)
- both eyes –> always temporal (one eye) and nasal (the other eye) fibers
- often people are nor aware they had a stroke because they don’t see ‘a black field’ –> aren’t aware that something is missing
What is achromatopsia?
= a higher dysfunction (not in the retina), caused by a stroke/lesion
–> NOT genetic (genetic = retina)
- gyrus lingualis and gyrus fusiformus
–> 9% of cerebral visual disorders - depending on where the lesion is, it can effect the upper visual field or is related to other agnosias
- patients complain that things look dirty/sad
-nothing much can be done to get colour vision back
Neuronal implementation (retinotopy) of object and face perception
- less precise retinotopy but more complex (specialised)
- contralateral and ipsilateral
- 25-30° receptive field size
- functional specialisation: complex
Alexia without agraphia?
= visuoverbal disconnection
- hemanopia to the right
- later quadrant anopsie
- difficulties in spatial orientation, initially disturbed colour vision
- additional lesion: information is not sent to corpus callosum –> not able to read
–> BUT rest intact: can write - some regeneration possible: patients can learn to read again
- hypertension, nikotin, diabetes, adipositas
What is prosopagnosia?
= face blindness, faces cannot be recognised properly (only highly contrasted and stylised faces can be recognised)
- temporal side: gyrus fusiformus and gyrus lingualis
- large latency to discover something is wrong
- if stylised like a face –> recognised as one (object and face recognition impaired)
What is akinetopsia?
= motion blindness, motion is not perceived in a fluent wys but as stagnant pictures
- bilateral lesion in medial temporal gyrus –> V5 rarely affected
- gyrus temporalis medialis
- often combined with lower quadrant field defects, optic ataxia and neglect
- no fluent processing, objects jump by frame, no continuity of rising liquids
MT = retinotopic representation of visual motion
dMST = object motion
IMST: retinal slip by self or eye motion
- good remission
What is neglect?
= most common disorder of parietal lesion, lack of attention (in one hemisphere) to sensory stimuli or motor function, syndrome of contralaterl suppression of sensory stimuli
- mostly right hemisphere in inferior parietal lobe
- often combined with: hemianopia, neglect in further modality, anosognosia (=being cognitively unaware of disability)
Test for neglect?
- copying
- patient needs to copy picture –> usually only draw half of it - Poppelreuther Durchstreichtest
- patient needs to cross all horizontal lines –> usually only cross out half of the lines - Line bisection test
- patient needs to draw a vertical line to part lines in the middle (lines are of different length and skewed to one side) –> usually cannot mark the middle
- hemianopia patients usually overcompensate
- difficult to compensate for neglect patients but with awareness and practice can become better
Pathophysiology of negelct
Usually after right parietal lesions
- but also possible after:
–> lesions in cingulum
–> lesions in PFC
–> lesions in thalamus
–> lesions in striatum
Reason
- disruption of the reticulo-thalamo-cortico-limbic loop
Course of neglect
- sponatenous restitution in 75% within 6 months
- extend correlates with size and location but not with gender or age
- affects other neurological functions
