Visual Pathways & Visuo-perceptual Disorders Flashcards
From the retina to the primary visual cortex
Eyes –> Thalamus (Lateral Geniculate Nucleus) –> occipital striate cortex (V1)
The optic radiations
- From the thalamus to the visual cortex
- Inverted image: retinotopic map
- Axons with info from the Upper quadrant of the visual field make a loop: Meyer’s loop
Visual map
• Each neuron has a specific receptive field
• Map of the visual field on the visual cortex
→ Retinotopic organisation
• Each neuron or group of neurons respond to very specific stimulation
• Ex: line orientation, movement…
Occipital cortex
1) Primary visual areas : striate cortex
2) Secondary visual areas: extrasriate areas
The ‘where’ pathway
• The dorsal pathways
• From occipital to the parietal lobe
• Location of objects
• Movement direction, velocity of objects in space
• Spatial orientation
• Guide actions directed at objects
• Integration of body- environement spatial
relationships
• Not necessarily conscious
• V5 (MT): perception of movement, direction
The ‘what’ pathway
- Ventral pathway: occipito-temporal
- Follows the course of the inferior longitudinal fasciculus
* Recognition of objects
* Colors
* Read text
Fusiform face area (FFA)
• Perception and recognition of faces
• Area for visual expertise?
–> Some studies also reported FFA activation in experts in cars or birds
Visual Word Form Area (VWFA)
- In literate adults: a region specialized for letter strings
- Those ventral area become specialised for specific visual categories with the development of expertise
Neurons in V1 only respond to
- small receptive fields (they don’t care for the whole shape)
- They code for a specific point of light on the retina
Posterior IT Primary cells respond to
orientation, shape, colour
Scattered throughout the IT: Texture cells respond to
specific patterns
Anterior IT
- Elaborate cells: respond to shape of the contour, shape + texture, shape +color, texture + color…
- Anterior IT (TE) neurons have larger receptive fields than Posterior IT (TEO) neurons
Grandmother cell
a term that refers to a neuron that would respond only to a specific, complex, and meaningful stimulus
Mishkin’s experiments with monkeys
You can train monkey by giving them a reward if they select the right stimulus (e.g.: the +)
-to test visual discrimination, associations etc…
Lesions in L and R inferior temporal areas
• Can’t learn visual discrimination:
• But can learn auditory and tactile discrimination
• They can learn positional discrimination
Parietal lesions
• They can do visual discrimination
• They can’t do positional discrimination
• Learn the relationship between objects
Hemianopia
blindness in one hemifield
Bitemporal hemianopia
• Section (or tumor) the optic chiasm
• Each eye sends to the brain information from the controlateral visual field
→Loose periferal vision
Homonymous hemianopia
• Section of the Optic tract or Optic radiations
→Loose completely one visual field
Visual cortex lesions
- Ablation of V1 = blindness
- No initial cortical processing of visual information necessary for perception
- Lesions in the right striate cortex: not able to see what’s in the left visual field
- Can still be aware that there are things
Blindsight
- Unconscious residual vision following lesions or ablation of the primary visual areas
- Still able to detect and identify visual stimuli, in the total absence of perceptual awareness following lesions to V1
- Some conscious vision: high contrast stimuli that move
Patient DB
Experiment: Try to guess the position where a light is flashed
1) shift his eyes to the position
2) Reach with a finger
Experiment: Try to discriminate between two stimuli
→Well above chance level
Can also discriminate the direction of motion
Residual functions → no conscious vision but can still do those tasks
MC and the Riddoch phenomenon
• Bilateral visual cortex lesions
• Complete blindness for static objects
• Can still perceive movement
→ Riddoch phenomenon: preserved awareness for
moving but not stationary stimuli
• Can use her sense of motion to navigate
• Can still perceive the affective aspects of faces
Superior Colliculi (SC)
- In the midbrain
- Part of the brain circuit for the transformation of sensory input into movement output.
- Major function : orienting the animal, particularly with eye movements, to objects of interest in the outside world.
Visual agnosia
- 2 Types: Apperceptive and Associative agnosia
- Clinical observations:
- Problems interpreting the information
- Can’t recognise objects or stimuli presented visually
- They did not loose the concept of the image (can recognise by touch or sound)
- Not only a naming problem
- No problem in visual cortex
Apperceptive agnosia
problem with perceptual encoding
- severe shape perception, like blind
- can’t integrate parts of an image into a whole
- can’t copy
- appropriate reaching or grasping
→ bilateral infero-occipito-temporal lesion
• Accurate ability for reaching movement towards objects in specific orientation
Associative Agnosia
problem with associating something perceived with semantical knowledge allowing to interpret it
• unable to identify an object
• can’t draw objects from memory
• can draw or copy but do not know what they have drawn
• can do object matching
• basic perceptual functions intact
• Occipito-temporal lesion disconnecting visual areas with
verbalisation areas or semantic or perceptual memory areas
→ more anterior in the ventral stream
Prosopagnosia
- Face Agnosia
- Deficits in face recognitions
- Can discriminate an object from a face but not between faces
- Sometimes can recognise facial expressions (emotional)
- Fusiform gyrus! FFA
Pure Alexia
- Often with object agnosia but not always
- Alexia is like an agnosia for letters
- Left medial occipitotemporal lesions- white matter
- Where the VWFA is
Simultagnosia
• Visual spatial attentional problem
• Can recognise objects presented one at the time
• Hard when several objects presented simultaneously or a complex scene
• Can count objects
• Can navigate normally in the environment
• Can SEE the scenes but hard to identify them
Reading: Can name words but not non-words, even if they both have a similar spatial span.
Suggestion : words were processed as single objects, while non-word letter strings are processed as
multiple objects (i.e., distinct letters).
→ Dorsal Stream
Unilateral Neglect/Hemineglect
• Patient not aware of what’s in one half of space:
the half contralateral to the lesion
• Mostly lesions in the right hemisphere → parietal
• If you ask them to draw something, they will neglect the left part of the sheet
• Anosognosia: not aware of their deficits
Left vs Right
• The right hemisphere is better for
• spatial orientation
• Distribute attention in space
• The left hemisphere is better for language
• Lesion in the Left posterior parietal area: usually no hemineglect
• They can have Agraphia, Acalculia…
• Why?
→ The right hemisphere posterior parietal area still receives the information by interhemispheric connections and can compensate.
Tests for unilateral neglect
The line bisection test
The Cancellation test
- Visual search
- Must cross out lines
Personal neglect
neglect to shave their left face or to put the left sleeve of their sweater
• Ignore one half of their bodies (contralateral to lesion site)
* Parietal lobe: representation of where your body parts are
→ Personal neglect vs extra-personal neglect
Lesions in Primary Visual Pathways
- Monocular Vision
- Hemianopias
Lesion in V1 (Occipital)
- Cortical blindness (blindsight)
Lesion in Ventral Stream
- Apperceptive and Associative Agnosias
- Prosopagnosias
- Alexia
Lesion in Dorsal Stream
- Simultagosia
- Neglect
