Edge Detection

Question 1

What is the first place along the visual pathway that has orientation-selective neurons?

Answer 1

the primary visual cortex

Question 2

What are the 3 main points of the visual pathway?

Answer 2

1. retina
2. lateral geniculate nucleus (LGN)
3. primary visual cortex (V1)

Question 3

How are retinotopic regions organised?

Answer 3

such that the upper part of the visual field is represented in the ventral retinotopic regions, and the lower part of the visual field is represented in the dorsal retinotopic regions

Question 4

Describe the receptive fields in the retina and LGN

Answer 4

they are round and respond best to small, round spots of contrast

Question 5

What does one retinal ganglion cell generally project to?

Answer 5

one LGN cell

Question 6

What do all V1 neurons have?

Answer 6

oriented receptive fields i.e. they respond best to a bar of particular orientation

Question 7

What are the 2 types of neurons in V1?

Answer 7

simple and complex cells

Question 8

What are simple cells?

Answer 8

phase-sensitive i.e. they respond best to either a bright or dark bar, but not both

Question 9

What are complex cells?

Answer 9

phase-invariant i.e. they respond equally well to a bright or dark bar

Question 10

What is the neocortex involved in?

Answer 10

higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning and language

Question 11

How does the neocortex appear in light microscopy?

Answer 11

a light, unstained sheet of unmyelinated neurons

Question 12

How many layers are in the neocortex?

Answer 12

6

Question 13

What do V1 neurons in the same cortical columns have?

Answer 13

the same preferred orientation

Question 14

Where do LGN neurons mainly project?

Answer 14

layer 4 of the neocortex where most simple cells are found

Question 15

What may simple cell orientation selectivity be due to?

Answer 15

selective connections from LGN cells

Question 16

What may complex cells be due to?

Answer 16

connections from simple cells with varied phase selectivities

Question 17

For every part of the visual field, what is there?

Answer 17

a different complex cell

Question 18

In real life, what must there be to make a response relatively constant?

Answer 18

8-10 simple cells signalling to one complex cell

Question 19

How can a simple cell be created?

Answer 19

an LGN neuron synapsing onto a cortical neuron

Question 20

How can a complex cell be created?

Answer 20

a simple cell synapsing onto a cortical neuron

Question 21

How can phase-invariant responses be created?

Answer 21

from phase-selective responses

Question 22

What are the 2 classical transmitters?

Answer 22

• excitatory glutamate
• inhibitory GABA

Question 23

What is glycine?

Answer 23

an inhibitory neurotransmitter in the brainstem and midbrain, not present in the thalamus and the cortex

Question 24

What are the neuromodulators of the cortex?

Answer 24

• dopamine from the ventral tegmental area
• ACh from the nucleus basalis

Question 25

What is the finest level of synaptic organisation?

Answer 25

monosynaptic connectivity and its synaptic strength

Question 26

What happens in an excitatory presynaptic neuron?

Answer 26

each AP in P1 causes P1 to release glutamate onto P2, causing an EPSP in P2

Question 27

What happens in an inhibitory presynaptic neuron?

Answer 27

AP in I1 causes I1 to release GABA onto I2, causing an IPSP in I2

Question 28

What are the 3 main methods to investigate neural connectivity?

Answer 28

1. anatomical methods 2. paired intracellular recordings 3. correlations between spikes recorded with extracellular electrodes

Question 29

For dyes that do not cross the synapse, how can post-synaptic neurons be identified?

Answer 29

electron microscopy

Question 30

What is the main limitation of purely anatomical methods for investigating neural connectivity?

Answer 30

they cannot tell us the strength of the synapse

Question 31

What is the limitation of paired intracellular recordings?

Answer 31

visual responses from neurons cannot be recorded in a brain slice

Question 32

What is the order of information in a cortical column?

Answer 32

LGN → L4 → L2/3 → L5/6

Question 33

What must be recorded to determine the synapse strength?

Answer 33

the membrane potential

Question 34

What is recorded with extracellular electrodes?

Answer 34

only spikes (APs)

Question 35

What is the size of the tip length of the electrode used for extracellular recordings?

Answer 35

roughly the size of a neuronal cell body

Question 36

How can a single AP be recorded using extracellular electrodes?

Answer 36

filter the frequency of signals to between 300-5000Hz and adjust the electrode placement

Question 37

What is the main idea of extracellular spike correlations?

Answer 37

when a reference neuron excites a target neuron, the target neuron will have an increased probability of spiking for a short time after the reference neuron spikes

Question 38

How does correlation not imply causation with extracellular spike correlations?

Answer 38

the spikes of A and B could be correlated even if there is no monosynaptic connection from A to B