L7 - Stimulus Recognition

Question 1

LGN layered structure

Answer 1

6 layers
4 layers receive input from Parvocellular ganglion cells
2 layers receive input from Magnocellular ganglion cells

Question 2

What type of input does the LGN have?

Answer 2

Monocular input

Question 3

In the LGN which layers receive input from which eye?

Answer 3

Layers alternate input from each eye

• One layer receives input from contralateral eye
• Another layer receives input from ipsilateral eye

Question 4

How is the LGN organised?

Answer 4

Retinotopically

Question 5

What is the thalamic relay station in the LGN?

Answer 5

Ganglion cell axons make 1:1 connections with LGN projection neurons
Receptive fields of LGN neurons are similar to receptive fields of retinal ganglion cells

Question 6

Where does the LGN receive 60% of its synaptic input from?

Answer 6

From the cortex

Question 7

What are the two visual pathways in the cortex?

Answer 7

Ventral

Dorsal

Question 8

What is the ventral visual pathway?

Answer 8

What
Object feature stream
Inferior temporal

Question 9

What is the dorsal visual pathway?

Answer 9

Where
Spatial location stream
Posterior parietal

Question 10

Ventral visual pathway method

Answer 10

1. Parvocellular ganglion cell
2. LGN Parvo
3. V1 cortical area
4. V2 cortical area
5. V4 – cortical area
6. Inferior temporal cortex – temporal pathway (what)

Question 11

What two things increase along the ventral visual pathway?

Answer 11

Increase in complexity of responses of neurons along the ventral stream
Increase in the receptive field size of neurons along the ventral stream

Question 12

What three things do we need to understand for object recognition?

Answer 12

Find single neurons (or population of neurons) that specifically respond to presentation of a specific object and understand their presynaptic neuronal circuits
Understand how activation of these neurons causes particular behavioural reactions
One neuron vs population of neurons

Question 13

Example of understanding object recognition

Answer 13

Subject looks at an image of a particular object.

• Few neurons in higher cortical areas fire
• Stimulation of the same neurons causes perception of the same object

Question 14

What is orientation invariance?

Answer 14

Can recognise objects in their natural orientation

Use this knowledge to recognise an unnatural orientation

Question 15

What is scale invariance?

Answer 15

Can recognise objects independently on their size

Question 16

Hierarchical model of object recognition

Answer 16

1. Detection of edges
2. Detection of edges and contours
3. Detection of object parts
4. Detection of objects from one point of view
5. View-invariant object detection – e.g. particular person, a car
6. Categorisation

Question 17

What increases along the hierarchical model?

Answer 17

Increase in stimulus complexity

Question 18

What experiments were used to test the object recognition model?

Answer 18

Lesions in inferior temporal cortex decrease the ability to recognise objects
Mainly studied by mathematical modelling and electrophysiology

Question 19

What are the three key features of cortical structure?

Answer 19

Layering
- Different layers have different functions
Columns
- Parts of the brain that have neurons that have very similar functions
Blobs
- Colour

Question 20

Information flow in the cortex

Answer 20

In parts of the cortex there are more cell bodies than in other areas
Different types of upstream cells send their projections to different layers
Interneurons receive input from one layer and send projections to a different layer
Different layers send their outputs to different areas of the brain

Question 21

What are the three subtypes of columnar organisation in the cortex?

Answer 21

Ocular dominance column
Orientation columns – direction
Blobs – colour

Question 22

What experiments where done to study ocular dominance?

Answer 22

Inject radioactive proline in one eye

Inject radioactive glucose in the cortex and stimulate one eye with light

Question 23

Ocular dominance columns - inject radioactive proline experiment

Answer 23

It enters ganglion cells and diffuses along the axon
Enters LGN and crosses the synapses
Enters LGN neurones
Use optic visualisation to observe radioactivity – stripes

Question 24

Ocular dominance columns - inject radioactive glucose experiment

Answer 24

Enters neurons that are currently active as they need glucose
Use optic visualisation to observe radioactivity – stripes

Question 25

Ocular dominance columns in the visual cortex

Answer 25

Different parts of the brain within the same cortical area receive input from only one eye Each column in the V1 cortex receives inputs from either ipsilateral or contralateral eye Different layers of LGN already receive input from different eyes

Question 26

What are orientation columns? – columnar architecture in MT cortex

Answer 26

Neighbouring parts of the brain have neurons with slightly different properties Therefore respond differently to motion in different orientations This also occurs in the LGN

Question 27

What are blobs?

Answer 27

Parts of the brain have different properties that are important for processing information about colour

Question 28

What is a hypercolumn?

Answer 28

``` Three types of columns - Blobs - Ocular dominance columns - Orientation columns This unit can process any type of information ```

Question 29

What experiments did Hubel and Wiesel carry out?

Answer 29

Worked on V1 cortex in cat | Recorded response of individual neurones to different direction of stimulus

Question 30

Example of a Hubel and Wiesel experiment

Answer 30

Response of V1 neuron to bars of different orientations Can plot on graph of stimulus orientation/neural response Gives a bell shaped curve These V1 neurons cells are called simple cells

Question 31

Where are simple cells found?

Answer 31

Localised in layers 4 and 6 of V1 cortex

Question 32

What do simple cells respond to?

Answer 32

Respond to a bar in certain orientation presented in the center of the receptive field

Question 33

Receptive fields of ganglion cells and LGN neurones

Answer 33

Receptive field are round shaped One response in the centre of the cell Opposite response on the outside of cell

Question 34

Receptive field of simple cells

Answer 34

Receptive fields are elongated in shape | Different response along midline vertically

Question 35

Probable circuit mechanism of simple cell receptive field

Answer 35

Several neurons that have receptive fields orientated in one line in the LGN feed to the same neuron in the visual cortex Allows elongation of receptive field in one direction

Question 36

What do complex cells do?

Answer 36

Respond to a bar in certain orientation presented anywhere in the receptive field

Question 37

Where are complex cells located?

Answer 37

Localised in layers 2, 3 and 5

Question 38

Possible circuit basis of complex cell receptive field

Answer 38

Complex cells collect information from many simple cells with similar orientation of receptive field

Question 39

What is a hypercomplex?

Answer 39

End stopped complex cell If you have a bar in a certain orientation you have a response As the bar goes away from receptive field – cell stops responding If you change the orientation of the bar away from the receptive field – cell starts responding again

Question 40

Receptive fields downstream of V1

Answer 40

Increase in complexity | Receptive fields increase in size

Question 41

Face sensitive neurons in the temporal lobe

Answer 41

Respond to faces | If you remove characteristic parts of the face the neurons respond less and less

Question 42

What is a grandmother neuron?

Answer 42

Neuron in brain fires when a specific thing is seen or heard – e.g. Grandmother - 1 or a few neurons recognise a particular object The neurons also show adaptation – initial firing is high then drops very quickly

Question 43

What are the problems with the hierarchical model?

Answer 43

Still poor in scale and orientation invariance Does not take into account feedbacks from higher cortical areas Need experimental validation which is difficult because need to record pre and postsynaptic responses