Recognition & Localisation

Question 1

Describe the hierarchical model of object recognition

Answer 1

1. Detection of edges
2. Detection of combinations of edges and contours
3. Detection of parts of objects, such as a face
4. Detection of object from a point of view
5. Scale and orientation invariant detection of objects
6. Categorisation of objects

Question 2

What are the challenges to the hierarchical model of object recognition?

Answer 2

It doesn’t explain scale and orientation invariance fully. It doesn’t account for top-down feedback, eg, what is the cortex instructing the LGN? It is more likely that a population of neurons encodes a face than a single neuron as suggested by the hierarchical model.

Question 3

Tell me about the arrangement of the Lateral Geniculate Nucleus

Answer 3

It is arranged into 6 layers, alternating in whether the input is from the contralateral or ipsilateral eye. The first 4 layers receive input from the parvocellular ganglion cells and last 2 layers receive input from the magnocellular ganglion cells. Their outputs converge onto either the parvocellular pathway or the magnocellular pathway. LGN projection neurons form 1:1 connections with the P/M ganglion cells, essentially acting as a thalamic relay station (LGN is in the thalamic nuclei). However, 60% of the LGN’s input is from the cortex, it is largely unknown what the function of this feedback is.

Question 4

Tell me about columnar organisation in the ventral pathway

Answer 4

It is organised into layers and columns, the layers being different stages of visual processing in the hierarchical model. The columns are:
a) Ocular dominance columns, these correspond to inputs from either the contra or ipsilateral eye.
b) Orientation columns, they are key in detecting specific orientations.
c) Blob columns, key in the processing of colour.

Question 5

Describe some methods to discover the columnar organisation

Answer 5

Ocular dominance columns were discovered and researched using radioactive proline injected into just one eye. Radioactive glucose can also be used, stimulating just one eye indicates the separation of contra and ipsilateral inputs.
Blobs were discovered using staining via cytochrome oxidase.

Question 6

What is a simple cell?

Answer 6

A simple cell has a receptive field about 2-3x large than a LGN projection neuron, it responds to specific orientations if they’re on the centre of the receptive field. They are located in layers 4 & 6.

Question 7

What is a complex cell?

Answer 7

A complex cell receptive field is made up of many simple cell receptive fields, given that they detect similar orientations. They can detect a specific orientation anywhere in the receptive field and are typically found in layers 2, 3 and 5.

Question 8

What is a hypercomplex end-stopped cell?

Answer 8

They can recognise orientations anywhere within their receptive fields but will produce an inhibitory signal if it extends out of their receptive fields. However, if it changes orientation after it leaves the receptive field, there isn’t an inhibitory signal.

Question 9

What is a grandmother cell?

Answer 9

AKA Jennifer Anniston neuron, can recognise faces and is scale and orientation invariant.

Question 10

What is the orienting reflex?

Answer 10

It is the orientation of the head towards stimuli, salient stimuli are focused on the fovea, this response develops as we age, with none at 3 months but a strong orienting reflex at 10 months old.

Question 11

What is smooth pursuit?

Answer 11

The movement of eyes to keep the moving object on the retina creating a smooth image. Aims to reduce the retinal slip, difference between where the targets motion and the eyes motion.

Question 12

What are saccadic movements?

Answer 12

They are used in object inspection, the eye rapidly moves, anywhere from 0.5 - 30 degrees every 30-60ms focussing on a specific part of an object, for example, parts of a face such as the edges, nose, eyes, ears. It’s worth nothing that saccades are often followed by fixations where more detail can be taken up.

Question 13

Why is object localisation important in hunting/prey capture?

Answer 13

Important in anticipating movement.

Question 14

Tell me about the superior colliculus

Answer 14

The SC receives inputs from ganglion cells, the auditory system and somatosensory system. It is key in regulating saccadic movements. Cats with ablated SCs can’t orientate their heads towards stimuli.
It has a retinotopic map (adjacent spatial locations are represented by neighbouring neurons) made up of layers, each represented by different inputs from different brain areas, such as the retina, visual cortex and somatosensory cortex.
The layers can be visualized through soma staining.

Question 15

What are the differences between mammalian and fish orientation pathways

Answer 15

Mammals use the Dorsal “where” stream:
M-Ganglion cells to LGN M pathway to V1 to V2 to V3 to Middle temporal to parietal lobe
Fish use the tectum, pretectum and hindbrain for their orientation responses.

Question 16

Tell me about patient LM

Answer 16

They had a stroke resulting in difficulty perceiving forms of motion. Object recognition and colour perception was left intact indicating they were in a separate area to motion detection. The patient couldn’t perceive the flowing of liquids and would overflow cups when pouring coffee.

Question 17

How does motion detection have direction specificity?

Answer 17

ON/OFF direction-selectivity (DS) cells in the retina are highly asymmetric, their preferred direction can be guessed from their morphology.
The ratio between excitatory and inhibitory inputs defines the direction selectivity.
Bipolar and amacrine cells input into ON/OFF DS cells.

Question 18

Why is sound localisation important?

Answer 18

It’s key in creating a perception of auditory space, helps us look in the right place in response to auditory salient stimuli.

Question 19

Which systems are used in sound localisation?

Answer 19

The brain uses Interaural level differences, the difference in loudness of a sound between the two ears. The brain also uses Interaural timing differences, the difference in arrival time of a sound between two ears

Question 20

List the brain areas involved in sound localisation

Answer 20

Sound localisation occurs in the brainstem.
Key regions of the brainstem are:
Cochlear Nucleus
Lateral Superior Olive
Medial Superior Olive
Medial Nucleus of the Trapezoid Body

Question 21

What is the LSO EI pathway?

Answer 21

The LSO EI pathway is used to detect Interaural Level Differences (ILDs) of as small as 1-2dB, it is most effective at higher frequency sounds.
LSO neurons receive an excitatory input from the near ear and an indirect inhibitory input from the far ear (initially it’s excitatory but the signal is transformed by the MNTB).

Question 22

What conditions ensure maximal LSO output?

Answer 22

LSO output is maximal when the stimulus is farthest from the centreline towards the near ear. This results in a maximal excitatory input and minimal inhibitory input

Question 23

What is the result of combining both LSO outputs

Answer 23

Combining both LSO outputs fine tunes sound localisation in the horizontal plane. The LSO outputs overlap when sounds are near or on the centreline, this helps to provide maximum accuracy in localising sounds from the centreline, key in hunting and survival.

Question 24

What is the MSO EE Pathway

Answer 24

The Medial Superior Olive excitatory-excitatory pathway is used to detect Interaural timing differences of as little as 10µs.
MSO neurons receive excitatory input from both the near and the far ears. However, as the neuron from the far ear is much longer than it’s near ear counterpart, centreline sounds result in excitatory inputs reaching the MSO at different times. Coincidence of inputs is key to function.

Question 25

What conditions ensure maximal MSO output

Answer 25

MSO output is maximal when the stimuli is closer towards the far ear, this is due to the longer neuron transmission time being compensated for by an increased ITD as the sound wave takes longer to reach the near ear. This results in the coincidence of the excitatory inputs and a maximum MSO output.

Question 26

What is the result of combining both MSO outputs?

Answer 26

Combining MSO outputs allow sound localisation on the horizontal plane. MSO outputs overlap the most when sounds are from the centre line resulting in high levels of localisation accuracy with stimuli on the centreline, like with LSO outputs, key in hunting.

Question 27

Why are barn owls a model organism?

Answer 27

They are amongst the best animals at sound localisation.
They have well defined ITD and ILD circuits
During their development, these circuits are calibrated to the visual system.
Owls eyes are located deep within their sockets with little freedom of movement, owls have to orient their entire heads towards stimuli, a useful trait for experimentation.

Question 28

What were Eric and Phyllis Knudsen’s barn owl experiments?

Answer 28

They aimed to investigate the interaction of the auditory and visual systems to see how interpretation of ILDs and ITDs are modified using the visual system and whether an auditory map would align with a visual map.
1. Owlets wore prism glasses shifting their vision 20° to the left, their hearing was not artificially modified
2. Prisms were removed after 7 weeks
3. Owlets placed in a dark testing chamber with varying horizontal and vertical light and sound stimuli. Their response (orientation towards) the stimuli was recorded

Question 29

What were the results of the Knudsen owl experiments

Answer 29

The visual system quickly readjusted after prism removal. The auditory system had aligned itself to the 20° shifted visual map and took longer to readjust, this indicated that the two systems interacted with a high degree of plasticity.

Question 30

Describe the 3 regions of the owl midbrain involved in auditory and visual system integration.

Answer 30

1. Central Nucleus of the Inferior Colliculus (ICC). Contains neurons tuned to specific ITDs in sound-frequency layers. There is little adaptive plasticity.
2. External Nucleus of the Inferior Colliculus (ICX). Projections from the sound-frequency specific layers of the ICC converge on ICX neurons forming an auditory map of space, this is a site of large scale plasticity.
3. Optic Tectum (OT). Combines the ICX auditory map of space with the visual map of space. Neurons have overlapping auditory and visual receptive fields. There are feedback neurons from the OT to the ICX. It is a site of large scale plasticity.