visuospatial deficits and visual agnosia Flashcards
what are the two categories of visual deficits
visuospatial deficits associated with damage to the right posterior parietal/ dorsal visual stream
language dysfunction associated with left parietal damage
what are the different kinds of visuospatial disorders that can arise from right posterior damage
visual disorientation - with visual mislocalisation and gaze apraxia
constructional apraxia - inability to copy the spatial pattern in which things are arranged through drawing or physical manipulation
spatial WM - impairment in remembering where you have searched
optic ataxia - inability to accurately point to or reach for objects under visual guidance
visual extinction - impaired awareness of stimuli in the contralesional hemifield when two stimui are simultaneously presented
neglect syndrome - impaired ability to attend to stimuli in the contralesional side of space - more severe and long-lasting with right inferior parietal lesions
topographical disorientation (egocentric) - impairment in identifying where something is in space with respect to the body
what is Balint’s syndrome
Balint (1917)
Balint identified patients could not judge where they were and presented with bilateral inattention, gaze apraxia, simultanagnosia and optic ataxia
but the visual field was intact
postmortem examination identified large bilateral strokes in the parietal lobe
Balint emphasised this as a disorder of inattention and visually guided misreaching
Holmes (1918)
patients with bilateral parietal lesions as the result of gunshot wounds to the head but intact visual cortex
could not touch objects in front of them, or do menial tasks such as count coins, had difficulty seeing more than one object at a time and navigation characterised by a tendency to bump into objects
patients presented with oculomotor apraxia, misreaching, neglect and topographical disorinetation
Holmes emphasised this as a disorder of spatial perception
what are the different mechanisms underlying visual disorientation
egocentric perceptual mislocalisation
impaired attention
spatial working memory
visuomotor control
illustrated by task of counting the number of dots on a page (Russell et al., 2010)
patients with parietal damage are impaired at this
count fewer dots than there are - failure to attend to dots
patients can also count more dots than are present - failure to keep in mind which dots have been seen before
even this menial task recruits complex processes of spatial perceptual localisation, attention, memory and eye movement control
what are egocentric perceptual mislocalisation accounts of parietal damage
the location of an object on the retina does not give a direct readout with respect to the body as this depends on the respective orientation of the head and trunk
spatial remapping of locations across eye, head and body movements so there is spatial constancy - egocentric localisation
mapping the target position with respect to body orientation to gravity requires the convergence of different types of sensory input
rential locus, eye position, head position with respect to gravity and the body
multimodal information is needed to reconstruct where an object is with respect to one’s body
representational neglect
deficits in the internal representation or mapping of space rather than solely on attentional biases
impaired construction or utilisaiton of mental representations of space
patients have difficulty forming accurate or coherent representations of the spatial environment, this results in distorted or fragmented maps that drive deficits in perceiving, attending to and navigating within the neglect side of space
what is the difference between egocentric and allocentric localisation
egocentric: viewer-centred deficit
allocentric: view-point independent representations - the relationship between places
what is the temporo-parietal network
Epstein et al., 2017
meta-analysis of 64 fMRI navigation judgement studies in healthy participants
identified the lateral parietal, temporoparietal junction, medial parietal, retrosplenial cortex and medial temporal regions (PPA and HPC)
how are egocentric and allocentric transformations achieved in the brain
Vann et al., 2009 ‘what does the retrosplenial cortex do?’
allocentric coordinate frame computations require transformation from egocentric representation
may be underpinned by lateral parietal-hippocampal inputs via medial parietal regions: the precuneus (medial parietal), posterior cingulate, retrosplenial cortex
effective episodic memory, navigation and future thinking all require the ability to integrate and manipulate different frameworks of information ego –> allo
the retrosplenial cortex is uniquely placed to enable translation within these domains for scene translation
parietal cortex for body-oriented information
anterior thalamic - head direction
perirhinal - object-based information
PFC - scene manipulation
pHPC - scene-based information
HPC for event within a scene / scene construction
occipital cortex for visual information
what is visual agnosia
impairment in recognising objects (different from anomia) due to damage in the temporal lobe affecting the ventral pathway (not an issue with retina or V1)
associative, apperceptive agnosia posopognosia, cerebral achromatopsia and akinetopsia
what is lissauer’s two stage framework for understanindg disorders of object processing
two routes to disrupting object processing
apperception (need to form a stable perceptual representation of the object)
association (need to access stored knowledge of the object)
what is the patient evidence for agnosia
patient dF - apperceptive agnosia
visual form agnosia
unable to identify shapes but could recognise objects by colour
could not identify edges, line orientation or figure-ground segmentation
poor direct copying but able to shape hand to correctly grasp different objects
able to post a letter through slots in different orientations (intact visuomotor processing) but if asked to match the orientation of the box impaired
lesioned lateral occipital complex
highlights vision for action as well as perception and spatial localisation
patient HJA - integrative agnosia (Riddoch & Humphreys, 1987)
good identification of elementary shapes
accurate copying of drawings and objects
good semantic memory
deficit in integrating single features of a stimulus coherent
unable to parse briefly presented overlapping figures
how have neuroimaging studies informed processing in the visual ventral stream
Grill-Spector & Weiner, 2014
fMRI studies reveal object, face and place regions in the human ventral visual stream
what is riddoch & Humphreys hierarchical framework of object recognition and how this breaks down in agnosia
image –> local computation of contours (V1) –> integration of contours into shapes (intermediate visual areas) –> computing object identity (inferotemporal cortex) –> semantics (temporal pole)
presentation of object
parallel processing of motion, colour, form and depth
edge grouping and colinearity - apperceptive
feature binding into shapes and multiple shape segmentation - integrative agnosia
view normalisation - deficit in viewpoint invariance
strauctural description - associative agnosia
semantic system - semantic dementia
name representation - anomia
what is marr’s computational approach
hierarchical view of the visual pipeline
vision as a process of reconstructing 3D scene from 2D information
visual system has representations of object-centred 3D geometric structures
models are viewpoint invariant
selecting models and recovering their parameters from image data is key task
intensity image –> primal sketch (edges) –> 2.5D sketch (local surface) –> 3D model representation (object centred)
