vision Flashcards
famous neurological case studies
- Patient Tan for language
- Patient P.G. for Vision (see later)
- Patient H.M. for Memory (covered during the memory lectures)
case reports
- Very important and critical task
- Often performed by a neurologist
- Neurologists will provide a full clinical assessment:
- Neuropsychological nature: psychomotor speed, attention, memory, executive and visuospatial functions
- Motor symptoms severity
- Functional status
- Understand comorbidity
visual agnosia
a condition in which a person can see but cannot recognize or interpret visual information, due to a disorder in the parietal lobes
Neuropsychological Patient Profile: IES
75 year old male with bilateral posterior lesions following a CVA (Cerebrovascular accident).
Scans revealed bilateral PCA stroke involving the ventral occipital lobes including the fusiform and lingual gyrus on the left and fusiform gyrus on the right. There was also damage to the left hippocampus and primary visual cortex.
Clinical diagnosis of visual object agnosia and prosopagnosia without alexia.
achromatopsia
(colour blindness) - No language comprehension or production problems, No loss of semantic knowledge -Upper right quadrant visual field cut (visual field loss).
blindsight
- Impairment to conscious visual experience of stimulus in part of visual field
- Hemianopias
- loss of vision to the left or right side of fixation
- Due to damage in the visual cortex (not the eye itself)
- Blindsight = some patients can respond to stimuli in ‘blind’ part of visual field under certain conditions, even though consciously they do not see anything!
blindsight (cortical blindness)
Persaud and Cowey (2008)
• Patient GY presented with an object in upper or lower part of his “blind” visual field. He had to point to the “opposite” location of object
• They found:
• When in blind area – he responded above chance to the real location of the object
• While being able to do the task fine when in “good” visual field
Agnosia
“Agnosia refers to the inability to recognize people or objects even when basic sensory modalities are intact.”
• Visual agnosia where the person has difficultly recognizing objects, faces and/or words
• Auditory Agnosia which involves the inability to recognize sounds
• Somatosensory Agnosia where the person has difficulty perceiving objects through tactile stimulation
visual agnosia
- A deficit in processing that is restricted to the visual input modality
- Patients may show impaired object recognition with or without Impaired face (proposopagnosia) and/or reading (alexia).
- Lissauer (1890) distinguished between ‘apperceptive’, ‘associative’ and ‘integrative’ visual agnosia
apperceptive agnosia
is a failure in recognition that is due to a failure of perception.
associative agnosia
is a type of agnosia where perception occurs but recognition still does not occur
intergrative
is a disorder in which the patient has symptoms of both apperceptive agnosia and associative agnosia, although their primary visual abilities are intact
Visual Agnosia patients can still do some startling things…
Milner et al (1991)
Patient DF could not identify shapes – Did this effect her movements?
Post card experiment, 2 tasks:
Matching: turn card to match the orientation of the slot
Posting: reach out and “post” the card into the slot
goodale et al 1991
When asked to distinguish blocks perceptually – DF couldn’t do it
When asked to pick the blocks up – she changed the aperture of grip to match the size of the block automatically without problems
But when asked to estimate block size using thumb and forefinger – DF couldn’t do it
dorsal system
involved in object localisation (where system)
Acts in real time
Guides actions
Enables smooth and effective movement
ventral system
involved in object identification (what system)
An abstract representation of the world
Can be stored for future reference
This enables us to organise the information in the world
It also enables us to plan future actions
evidence from lesions with monkeys
Lesions of the posterior parietal cortex = deficit on a spatially demanding “landmark” task but no affect on object discrimination (on basis of visual features e.g., patterns)
Lesions of inferior temporal cortex produced deficits discriminating objects on the basis of their visual features but did not affect their performance on a the landmark task.
where or how?
Milner and Goodale (1995)
DF has a problem in identification (What) but she can adjust her grip when picking up objects when this is automatic
DF shown a block, asked to close eyes for 2s or 30s, then reach for it. After 30s, DFs grip is no longer adjusted accordingly. Goodale et al (1994)
Thus “action” is a more appropriate term than “localisation” for the dorsal stream
Is not “What Vs Where” but “What Vs How”
other evidence for what vs how
Damage early to Posterior parietal lobe
Patient VK (Jackobson, Archibald, Carey & Goodale, 1991 )
Can see (perceive) objects but can’t interact with them
Kind of the opposite of visual agnosia
optic ataxia
Jakobson, Archibald, Carey & Goodale (1991)
Optic Ataxia – difficulty grasping objects
Good at planning the movement. So the initial movement is fine
Problem is with the final adjustments
visual illusions
Illusions can be categorised in two classes:
• Bottom-Up (physiological) Illusions - low-level physiological mechanisms
• Top-Down (cognitive) Illusions - constructivist
Café wall illusion - Gregory & Heard (1979)
‘Border locking’ “Mortar” can be seen in some circumstances
But not others
Illusion – the perceptual system tries to resolve which object the mortar belongs to when white tiles meets black
border locking
edge detection in the context of simultaneous spatial and colour registration in the human visual system.
If you look at the boundary between two dark tiles, the mortar line is plainly evident. At the boundary between two light tiles it can also be seen clearly.
At the boundary between a light and dark tile, however, your visual acuity simply isn’t sharp enough to resolve the mortar line as a separate object.
photoreceptors and lateral inhibition
Photoreceptors (light cells) are activated when a light shines on it.
They then inhibit the firing of a adjacent cells.
Why? To maximise contrast and help in detection of contrast changes
the herman grid
Why do we see the dark patches?
a. Intersection cells – more inhibition than cells elsewhere, so activate less – making this area seem darker.
b. Foveate – Cells in fovea have smaller receptive field – so illusion disappears
illusions of contrast
- Our perceptual system is interested in properties of a surface (e.g., colour) not levels of illumination (which physically influence how a surface is perceived).
- So the perceptual system tries to factor out levels of illumination.
- Black surround = low luminance – so perceptual system adjusts accordingly
- Is this “bottom up”?
- If the surround patch is black luminance, then the average brightness in the area is low, so the system adjusts accordingly making it seem like the grey
- patch is lighter than it is.
top down illusions
- Dragon illusion (version of the hollow face illusion)
- It appears that the dragon moves round watching us, but it is static!
- Occurs because our brain makes inferences about the features that do not correspond to reality!
misapplied size consistency
Some evidence that people with less familiarity with corners show the effect less (Deregowski, 1972).
size consistency
Because we know that objects far away are not really smaller, we perceive them to be bigger than their retinal size
misapplied size consistency
We thus assume that the further object must be bigger than its retinal image and perceive it as bigger
ambiguous figures
Ambiguous
2 possible interpretations
Our perceptual system jumps between the two
illusory figures
The Kanizsa Triangle
We perceive a white triangle that is brighter than the background but that is not really there!
Illusory contours may be partly accounted for by low level contrast effects, partly by more cognitive processes inferring the existence of occluding objects.
gestalt explanation
Gestalt - “essence or shape of an entity’s complete form“
reification
(the perception of an object as having more spatial information than is present)
Constructive or generative aspect of perception, by which the experienced percept contains more explicit spatial information than the sensory stimulus on which it is based
contrast illusions
- What is really amazing is that, despite visual information being the same, our visual system modifies this information to provide us with an interpretation that in reality would be correct.
- Our visual system is designed to interpret information in a way that is consistent with reality.
- Vision isn’t about seeing light it’s about seeing things
