Visual Pathways & Visuo-perceptual Disorders

Question 1

From the retina to the primary visual cortex

Answer 1

Eyes –> Thalamus (Lateral Geniculate Nucleus) –> occipital striate cortex (V1)

Question 2

The optic radiations

Answer 2

• 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

Question 3

Visual map

Answer 3

• 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…

Question 4

Occipital cortex

Answer 4

1) Primary visual areas : striate cortex

2) Secondary visual areas: extrasriate areas

Question 5

The ‘where’ pathway

Answer 5

• 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

Question 6

The ‘what’ pathway

Answer 6

• Ventral pathway: occipito-temporal
• Follows the course of the inferior longitudinal fasciculus
  * Recognition of objects
  * Colors
  * Read text

Question 7

Fusiform face area (FFA)

Answer 7

• Perception and recognition of faces
• Area for visual expertise?
–> Some studies also reported FFA activation in experts in cars or birds

Question 8

Visual Word Form Area (VWFA)

Answer 8

• In literate adults: a region specialized for letter strings
• Those ventral area become specialised for specific visual categories with the development of expertise

Question 9

Neurons in V1 only respond to

Answer 9

• small receptive fields (they don’t care for the whole shape)
• They code for a specific point of light on the retina

Question 10

Posterior IT Primary cells respond to

Answer 10

orientation, shape, colour

Question 11

Scattered throughout the IT: Texture cells respond to

Answer 11

specific patterns

Question 12

Anterior IT

Answer 12

• 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

Question 13

Grandmother cell

Answer 13

a term that refers to a neuron that would respond only to a specific, complex, and meaningful stimulus

Question 14

Mishkin’s experiments with monkeys

Answer 14

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

Question 15

Hemianopia

Answer 15

blindness in one hemifield

Question 16

Bitemporal hemianopia

Answer 16

• Section (or tumor) the optic chiasm
• Each eye sends to the brain information from the controlateral visual field
→Loose periferal vision

Question 17

Homonymous hemianopia

Answer 17

• Section of the Optic tract or Optic radiations

→Loose completely one visual field

Question 18

Visual cortex lesions

Answer 18

• 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

Question 19

Blindsight

Answer 19

• 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

Question 20

Patient DB

Answer 20

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

Question 21

MC and the Riddoch phenomenon

Answer 21

• 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

Question 22

Superior Colliculi (SC)

Answer 22

• 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.

Question 23

Visual agnosia

Answer 23

• 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

Question 24

Apperceptive agnosia

Answer 24

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

Question 25

Associative Agnosia

Answer 25

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

Question 26

Prosopagnosia

Answer 26

* 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

Question 27

Pure Alexia

Answer 27

* Often with object agnosia but not always * Alexia is like an agnosia for letters * Left medial occipitotemporal lesions- white matter * Where the VWFA is

Question 28

Simultagnosia

Answer 28

• 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

Question 29

Unilateral Neglect/Hemineglect

Answer 29

• 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

Question 30

Left vs Right

Answer 30

• 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.

Question 31

Tests for unilateral neglect

Answer 31

The line bisection test The Cancellation test - Visual search - Must cross out lines

Question 32

Personal neglect

Answer 32

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

Question 33

Lesions in Primary Visual Pathways

Answer 33

- Monocular Vision | - Hemianopias

Question 34

Lesion in V1 (Occipital)

Answer 34

- Cortical blindness (blindsight)

Question 35

Lesion in Ventral Stream

Answer 35

- Apperceptive and Associative Agnosias - Prosopagnosias - Alexia

Question 36

Lesion in Dorsal Stream

Answer 36

- Simultagosia | - Neglect