Lecture 9: The visual pathways and visuo-perceptual disorders

Question 1

How does the information travel from the retina to the visual cortex?

Answer 1

Eyes –> Thalamus (Lateral Geniculate Nucleus) –> Occipital striate cortex (V1) –> V2 and so on…

• The eye lens projects an inverted (upside down) representation of the visual field on the retina (back of the eye ball)
• visual space is inverted in the brain. Info from outside world is being transfered upside down to the retina*

Question 2

The optic chiasm

Answer 2

• Visual information is processed contralaterally
• Left occipital lobe receives information from right visual field and vice versa.
• information from the right visual
  field will strike the nasal part of the right eye but the temporal part of the left eye and
  this information will be sent to the left occipital cortex
• The optic chiasm is the place where some of the information is processed
• Information on the nasal part of each eye crosses the optic chiasm –> this is how one 1⁄2
  of the visual field is sent to only one half of the brain
• The optic chiasm is OUTSIDE of the brain, after this the optic tract sends the information
  throughout the brain

Question 3

Primary Visual Pathway

Answer 3

Retina –> optic nerve –> chiasm –> optic tract on the contralateral side –> Lateral Geniculate Nucleus (LGN) on the contralateral side (Thalamus)

After thalamus, information is related to occipital libe from white matter (optic radiation).

• Retina receives information
• Optic Nerve transfers information to LGN
• LGN in contralateral side

Question 4

Optic Radiations

Answer 4

From the thalamus to the visual cortex, optic radiation translates the information
- The image sent to the LGN is still inverted –> this is called retinotopic map
- A specific part of the visual cortex is associated with a specific part of the visual field
- Axons with information from the upper quadrant of the visual field make a loop called Meyer’s loop and they reach the lower part of the visual cortex
- The information from the lower quadrant is sent through the fibres of the optic radiation that sends it to more dorsal parts of the occipital cortex.
- if you get a specific defect in the meyers loop = lose upper part of visual field.

Question 5

Occipital Cortex

Answer 5

1. Primary visual areas: striate cortex (V1)
   - receives info: low level processing of info
2. Secondary visual areas: extrastriate areas (V2 – V5)
   - Need several steps where information travels from V1 to other areas towards the
   temporal lobe before the information can be fully processed

Question 6

Visual Map

Answer 6

• Each neuron has a specific receptive field
• Map of the visual field on the visual cortex –> retinotopic organization (info in the middle is in the middle in V1)
• Each neuron or group of neurons respond to very specific stimulation
  E.g., one neuron responds specifically to only line orientation vs movement.

How do we put all of this together to interpret a visual scene?
- This is what we will see later on!

Question 7

Integration

Answer 7

• As the information is transmitted to the next visual area: V1 to V2 to V3, which responds to more and
  more complex stimulation, the integration of what is there takes place.
• The information also crosses the CC (corpus callosum) as it is transmitted for a more
  complete integration of the visual stimulus.
• Information is treated more and more complex as it moves to higher levels of visual cortex: V1 –> V2–> V3…

Question 8

The “where” pathway

Answer 8

• The dorsal pathway for vision
• From the occipital to the parietal lobe.
• called the where pathway because this is
  where the spatial relation among objects and their position in space is integrated
  Interprets:
• Location of objects
• Movement direction, velocity of objects in space
• Spatial orientation
• Guide actions directed at objects
• evaluate the distance
• integration of body-environment spatial relationships
• V5 (MT): perception of movement and direction

Parietal lobe is important for the integration of body environment spatial relationships –> This is not necessarily conscious though

Question 9

The “what” pathway

Answer 9

The ventral pathway for vision
- From the occipital lobe to the temporal lobe –> also called the occipito-temporal pathway
- This pathway follows the course of the inferior longitudinal fasciculus (white matter tract).

What it does:
- Recognition of objects
- Colours
- Read text (VWFA) (the regions responsible for word reading are in the inferior temporal area)
- link with memory –> previous knowledge

Inferior temporal <- Occipital
Temporal Occipital (TEO) and Temporal (TE) <- V4 <- V3,V2 <- V1
Subsequent connections with temporal and frontal limbic structures: cognitive associations (memory, emotions…)

Yellow = inferior temporal -> synthesis of all physical properties Blue = occipital

Question 10

Fusiform Face Area (FFA)

Answer 10

• It is right lateralized in right handed individuals.
• Important area for the perception and recognition of faces
• FFA is located at the junction of the occipital and temporal lobe (ventral pathway)
• Pretty small area that responds specifically to faces. Region activated when faces are shown.
• Some studies have also reported FFA activation in experts in cars or birds
• Humans are experts in faces (detecting facial features).
• Does this area light up when we recognize something we know REALLY well? Visual expertise?

Question 11

James & James Study

Answer 11

Also another very interesting study (James & James 2013)
- Studied kids who were experts in Pokémon
- Showed images of faces to the kids in a scanner to be able to identify the FFA in the brain
- Extracted the BOLD signal within this area for when they were viewing faces vs. viewing Pokémon
- A lot more brain activation within the FFA for the Pokémon experts when they
were viewing Pokémon characters than did controls.
- Results suggest that this type of relative early experience has a dramatic effect on activation in the FFA

Question 12

Monkey study from readings

Answer 12

Lecture 6 Reading: shown in monkeys that there are areas in the inferior temporal cortex (TEO and TE) that respond to a very wide range of colours and geometric shapes –> this is where the synthesis of information is made for object recognition
- Just to hammer this point home, the synthesis of physical properties happens in
this area (TEO and TE)
- Subsequent connections with temporal and frontal limbic structures are essential for cognitive associations (i.e., memory, emotions…)

Question 13

Visual Word Form Area (VWFA)

Answer 13

• In literate adults: a region specialized for letter strings –> located in part of the fusiform gyrus.
• According to the recycling hypothesis of Dehaene, those ventral areas become specialized for specific visual categories with the development of expertise.
• Whether you are an expert in faces, written words, or even Pokémon’s the idea is that you will have a specific area for it.
• Damage to the inferior occipital areas (that are part of the ventral stream of vision) cause deficits in complex visual processing but also in attention, learning, and memory

Question 14

Experiments: Recording in neurons

Answer 14

• When you record Neurons in V1 you can see that they only respond to small receptive fields (they do not care for the whole shape of an object).
• Neurons in the primary visual area (V1) only code for a specific point of light on the retina
• If you record from neurons outside of V1 –> ex. electrode in the temporal cortex on the ventral pathway (not in V1)
• You see that the neurons respond to very specific stimuli, but more complex (ex: they respond to the shape of a star but won’t respond to the shape of a circle or a triangle).

Summary:
In V1 –> specific point of light
Not in V1 –> shapes, specific stimuli

Question 15

Along the ventral pathway -> experiment Tanaka et al. 1991

Answer 15

• Cell recording in the inferotemporal (IT) cortex of monkeys
• Posterior IT primary cells (neurons): responded to specific orientations, shape, colour. They responded to more basic information.
• These cells were closer to the occipital areas
• Another neuron responded specifically to the colour red

Texture cells (scattered throughout the IT): responded to specific patterns (i.e. texture)
- Responded to more complex stimuli
- These cells were scattered throughout the IT cortex
- Combination of orientation and pattern that they responded to.
- As they went to different neurons they say that there was a specific response to different aspects (ex: polkadot pattern)

When they moved more anteriorly they found elaborate cells.
- In the anterior IT these cells responded to shape of the contour, shape + texture, shape + colour, texture + colour
- It was much more complex, hence the name ‘elaborate’. Only activate when they see the precise info.
- Ex. a neuron needed a combination of a disc and a bar to fire
- Anterior IT (TE) neurons have larger receptive fields than posterior IT (TEO) neurons
(V4 -> TEO -> TE)

Question 16

The ‘Grandmother cell’

Answer 16

• A term that refers to a neuron that would respond only to a specific, complex, and meaningful stimulus (such as one’s grandmother)
• Population coding or a single neuron? Some neurons respond to complex info. Some neurons respond to one specific person.
• Technical term for the grandmother hypothesis is “sparseness”

At earlier stages in the brain’s ventral visual pathway (object-representation), the neural code for an object is a broad activity pattern distributed across a population of neurons (ex. in V1), each responsive to some discrete visual feature.
- At later processing stages, neurons become increasingly selective for combinations of features, and the code becomes increasingly sparse – that is, fewer neurons are
activated by a given stimulus, although the code is still population based (does not reach the level of one single neuron)
- Sparseness has its advantages, especially for memory, because compact coding maximizes total storage capacity of the brain.
- This paper concluded that grandmother cells are the theoretical limit of sparseness, where the representation of an object is reduced to a single neuron

Question 17

Quiroga Paper

Answer 17

• Recorded neural activity from structures in the human medial temporal lobe that are associated with late-stage visual processing and long-term memory
• i.e., entorhinal cortex, the parahippocampal gyrus, the amygdala, and the hippocampus
• the recordings were made in the course of clinical procedures to treat epilepsy
• One neuron selectively responded to Jennifer Aniston, and different images of her too (no matter the orientation, colour…). Responded to no one else - must be responding to a memory of the person.
• Must be more memory related than strictly visual or perceptual

Question 18

Vision or memory?

Answer 18

• One of the most difficult aspects of vision is that any given object must be recognizable from the front or side, in light or shadow… and somehow, given those very different
  retinal images, the brain consistently invokes the same set of memory associations that give the object meaning
• Storing those associations in memory

What do the ‘Grandmother’ or ‘Jennifer Aniston’ neurons respond to?
- Abstract representations of the identify of the individual or object shown.

The medial temporal lobe (where they recorded from) is usually associated with long term memory formation and consolidation, but here what they see is high-level visual responses from those neurons
- So what happened?
1. There are strong anatomical connections between the ventral visual pathway and the medial temporal lobe.
2. fMRI in humans and electrophysiology in monkey’s show activations in the medial temporal lobe in responses to faces, objects, places…
3. The hippocampal system is needed for a visual percept to be consciously remembered –> the visual information is stored for association with memory

Concluded:
- That the invariance based on learned associations is not geometric transformation of the visual structure, rather, these cells encode memory-based concepts rather than visual appearance. AKA based on memory more than vision

Question 19

Mishkin’s experiments with monkeys

Answer 19

You can train a monkey by giving them a reward if they select the right stimulus (e.g. each time they select a + sign they will get a reward)
- In this study, they had monkeys with lesions in the L and R inferior temporal areas
- These monkeys could not learn visual discrimination
- BUT, they could learn auditory and tactile discrimination
- They could learn positional discrimination (where the object is in respect to another).
- Ex. learn to select a stimulus when a + was presented to the right of a square but not when it was to the left of a square
- Learning the position in space is associated more with the dorsal pathway

• When the monkeys had parietal lesions, the opposite was seen:
• They could do visual discrimination tasks
• They could not do positional (or landmark) discrimination
• Could not learn the relationship between objects
• This paper was able to really learn more about the dorsal vs. ventral pathways for vision

Question 20

Blindness

Answer 20

• Occurs when the eye’s do not work

What if only one eye does not work?
- Each eye can see both visual fields
- You will loose only part of one visual field (due to overlap only loose peripheral vision)
- This is called monocular vision
- You can move your eyes to compensate if you want to look towards something you cannot see

Question 21

Hemianopia

Answer 21

• blindness in one hemifield
• Only either the left visual field or the right you can see
• Damage in the primary visual pathway
• Two types:
  1) Can happen if you have a lesion in a section (or tumor) of the optic chiasm (B)
• The information from both temporal sides of each eye can only make it through the brain
• This is called Bitemporal hemianopia: each eye sends the brain
  information from the contralateral side
• Loose peripheral vision
  2) Can also happen if you have a lesion in a section of the optic tract (C) or optic radiations (E)
• This is called Homonymous hemianopia: you lose one visual field completely
• (D) shows a lost part of the upper visual field –> this means that the right Meyer’s loop was sectioned in this example
• And you only loose the left upper quadrant because the lesion
  was lateralized on the right

*The main thing to remember is that the right visual cortex only gets information from the left visual field and vice-versa

Question 22

Visual cortext lesions: Blindness?

Answer 22

• Ablation (lesions) of V1 = blindness (even if your eyes work secretly because there are no cells in
  your cortex to interpret that information)
• No initial cortical processing of visual information necessary for perception
• Lesions in the right striate cortex means you are not able to see what is in the left visual field. However, a person can still be aware that there are things!
• Even in bilateral lesions of V1: Unconscious residual vision (in people who lose both occipital lobes = some unconsious processing)

Question 23

Blindsight (or cortical blindness) –> not learned in class

Answer 23

• Unconscious residual vision following lesions or an ablation of the primary visual areas.
• Patients are still able to detect and identify visual stimuli, in the total absence of perceptual awareness following lesions to V1 (Overgaard, 2011).
• People with blindsight will have some conscious vision: can see high contrast stimuli that moves.

Helen the monkey:
- Had bilateral destruction of the striate cortex
- Could still orient towards, follow, grasp, detect, localize, and discriminate visual objects.

Patient DB (Weiskrantz et al. 1974):
- Lesions in the visual cortex
- Asked the patient to try and guess the position where a light is flashed.
- When they did flash the light, even if the patient could not guess, he could still:
- Shift his eyes to the position (weak performance though).
- Reach with a finger –> clear correspondence between the location of the flash and the finger

• Another experiment: try to discriminate between two stimuli (e.g., X vs. O, orientation of a line)
• He tested well above the level of chance
• He could also discriminate the direction of motion
• He still had residual functions –> not conscious, but enabled him to do the tasks

Case: MC and the Riddoch phenomenon
- MC had bilateral visual cortex lesions (extensive lesions)
- MC had complete blindness for static objects –> could not see anything
- BUT, she could still perceive movement
- This is the Riddoch phenomenon: perceived awareness for moving but not stationary stimuli
- Like the monkey Helen, MC could still use her sense of motion to navigate
- Could also still perceive the affective aspects of faces –> could feel an emotion if she was presented with a sad face
- “Vision isn’t an all or none thing – there are different aspects of vision”
- fMRI study of MC
- Stimuli in the scanner: moving versus stationary checkerboards
- Looked at the brain activation while she was watching the stimuli
- Found some activation in MT (sometimes also called V5)
- Which is an area specialized for motion perception
- But how does it get information if there is no V1 – V4 to transfer information to this region?

• Concluded that there was another subcortical pathway from the eyes to MT/V5 and it was from the eyes –> superior colliculi —> pulvinar (thalamus) then to be transferred to V5
• Good chance that plasticity occurred after her injury that reinforced her pathway
• Preserved abilities:
• Navigating objects –> associated with the posterior parietal area for the visual guidance of movement
• Emotion perception –> the amygdala is still functional and is responsible for this (the SC are in close connection to the amygdala and the anterior cingulate cortex for emotion processing)

Question 24

Agnosia

Answer 24

From the Greek, meaning “to lack knowledge of”
- Visual agnosia (can also have tactile, auditory, etc.)
- 2 types: Apperceptive and Associative agnosia

Clinical observations with visual agnosia:
- Problems interpretating visual information.
- Cannot recognize objects or stimuli presented visually
- They did not loose the concept of the image (can recognize by touch or sound)
- Not only a naming problem –> not a language issue
- They have no problems in the visual cortex –> how do we know that?
- Not only from looking at their brain but also from tests showing that they CAN see things, they just cannot recognize objects.

Question 25

Apperceptive agnosia

Answer 25

problem with perceptual encoding
- Severe shape perception, like they are blind
- Cannot integrate parts of an image into a whole picture.
- Cannot copy an image down –> BUT they can draw something from memory
- They are still able to do reaching or grasping
- If you ask them to grab an apple they will grab it with precision even if they cannot “see” it.
- Verbalizaion of what they see perceptually is not there (cannot tell you anything about what they see).
- Usually this agnosia is associated with lesions in the ventral pathway of vision, in the bilateral infero-occipito-temporal area.

Question 26

Patient DF

Answer 26

Patient DF (Goodale et al. 1991)
- Apperceptive agnosia patient
- Unable to report the orientation of single lines or objects.
- Showed an object with a slot in it, could not say what the orientation was.
- But when asked to put a card through the slot, she did perfectly (accurate orientation)

• Patient DF had an accurate ability for reaching movement towards objects in specific orientation
• Dissociation between inability to report the angle of the slot and the performance of the visually-guided action
• was really good evidence in 1991 supporting distinct what and where pathways

Question 27

Associative Agnosia

Answer 27

Problem with associating something perceived with semantical knowledge allowing to interpret it
- Unable to identify an object
- Unable to draw objects from memory
- Can draw or copy but do not know what they have drawn
- Can do object matching
- Basic perceptual functions are intact
- Usually this is due to occipito-temporal lesions disconnecting visual areas with verbalization areas or semantic or perceptual memory areas
- So, due to a lesion in the anterior ventral stream

Question 28

Prosopagnosia

Answer 28

• also called face agnosia
• Deficits in face recognition
• Can discriminate an object from a face but not between faces
• Sometimes they can recognize facial expressions (emotional aspects because thats processed by the amygdala)
• Due to a lesion in the FFA (fusiform face area –> fusiform gyrus)
• Again, in the ventral stream

Question 29

Pure Alexia

Answer 29

• Often occurs with object agnosia but not always
• Alexia is like an agnosia for letters (form of agnosia specific for letters)
• Usually associated with lesions in the left medial occipitotemporal areas (often in
  the white matter connecting the different areas)
• This is where the VWFA is

Question 30

Brain Plasticity

Answer 30

Lost of vision or congenitally blind (born blind).
* Representation of cross-neuroplasticity in congenital visually impaired patients.
* Observed an expansion of areas not related to vision to the visual cortex resulting in an improvement of non-visual senses —> case or reorganized and reallocation of function
* Visual brain areas recruitment during the sound and auditory processing. Also can be used to enhance touch, and smell abilities.
* As children grows because the occipital is not used as much the somatosensory area takes over (plasticity).

Question 31

Congenital Deafness: what happens to the auditory cortex?

Answer 31

• Congenital deafness (any cause): auditory cortex never receives input.
• Neurons respond to seeing someone signing (ex: American Sign Language) → plasticity
• Removal of one sensory modality in humans leads to neural reorganization of the remaining modalities.

Question 32

Auditory Agnosias

Answer 32

• Patients aware of sounds but difficulty with
  sound identification.
• Not attributable to hearing or cognitive deficits
• Typically behave as if they were deaf (despite normal peripheral hearing and brainstem evoked potentials)
• Less frequent than vision → in contrast to vision, where each hemisphere receives information from only half of the visual field, auditory information arrives to each hemisphere from the entire auditory scene.

(not studied very much/hard to study)

Question 33

Different types of auditory agnosias

Answer 33

General auditory agnosia → bilateral temporal lesions
* Impaired recognition of all types of auditory stimuli, including speech, environmental sounds, and music.

• Agnosia for speech (verbal auditory agnosia or pure word deafness) –> regions close to primary area
• Agnosia for music (amusia)
• Auditory affective agnosia and Agnosia for environmental sounds (nonverbal agnosia)

Question 34

Verbal auditory agnosia (word deafness)

Answer 34

• Agnosia specific to speech sounds (Often called pure word deafness). Often evolves from Wernicke’s aphasia, in that paraphasias and reading and writing
  deficits resolve without corresponding improvement of speech perception.
• Not only for words per se, but rather agnosia for speech sounds.
• Patients with verbal auditory agnosia are aware of speech but describe it as sounding like a foreign language, cartoonlike etc
• Speech production in verbal auditory agnosia is relatively normal
• Left-sided (or bi-lateral) temporal lesions in the superior temporal cortex

Question 35

Amusia

Answer 35

• Acquired agnosia for musical perception
• Common disorder after a middle cerebral artery (MCA) stroke, (incidence of 35 to 69% )

Apperceptive: perceptual deficits in musical perception in the face of preserved musical production
→ right-hemisphere lesions

Associative: preserved processing of pitch, timbre, and rhythm, but an impaired ability to recognize melodies and sing from memory (despite preserved ability to recognize familiar song lyrics)
→** bilateral temporal lesions. **

Question 36

Lesions in ____ cause what disorders?
1) Primary Visual Pathways
2) V1 (occipital lobe)
3) Ventral Stream
4) Dorsal Stream

Answer 36

Primary Visual Pathways:
- Monocular vision
- Hemianopias

V1 (Occipital lobe)
- Cortical blindness (blindsight!)

Ventral stream
- Apperceptive + Associative Agnosia’s
- Prosopagnosia’s
- Alexia

Dorsal stream
- Simultagnosia
- Neglect