L7 - Stimulus Recognition Flashcards
LGN layered structure
6 layers
4 layers receive input from Parvocellular ganglion cells
2 layers receive input from Magnocellular ganglion cells
What type of input does the LGN have?
Monocular input
In the LGN which layers receive input from which eye?
Layers alternate input from each eye
- One layer receives input from contralateral eye
- Another layer receives input from ipsilateral eye
How is the LGN organised?
Retinotopically
What is the thalamic relay station in the LGN?
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
Where does the LGN receive 60% of its synaptic input from?
From the cortex
What are the two visual pathways in the cortex?
Ventral
Dorsal
What is the ventral visual pathway?
What
Object feature stream
Inferior temporal
What is the dorsal visual pathway?
Where
Spatial location stream
Posterior parietal
Ventral visual pathway method
- Parvocellular ganglion cell
- LGN Parvo
- V1 cortical area
- V2 cortical area
- V4 – cortical area
- Inferior temporal cortex – temporal pathway (what)
What two things increase along the ventral visual pathway?
Increase in complexity of responses of neurons along the ventral stream
Increase in the receptive field size of neurons along the ventral stream
What three things do we need to understand for object recognition?
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
Example of understanding object recognition
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
What is orientation invariance?
Can recognise objects in their natural orientation
Use this knowledge to recognise an unnatural orientation
What is scale invariance?
Can recognise objects independently on their size
Hierarchical model of object recognition
- Detection of edges
- Detection of edges and contours
- Detection of object parts
- Detection of objects from one point of view
- View-invariant object detection – e.g. particular person, a car
- Categorisation
What increases along the hierarchical model?
Increase in stimulus complexity
What experiments were used to test the object recognition model?
Lesions in inferior temporal cortex decrease the ability to recognise objects
Mainly studied by mathematical modelling and electrophysiology
What are the three key features of cortical structure?
Layering
- Different layers have different functions
Columns
- Parts of the brain that have neurons that have very similar functions
Blobs
- Colour
Information flow in the cortex
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
What are the three subtypes of columnar organisation in the cortex?
Ocular dominance column
Orientation columns – direction
Blobs – colour
What experiments where done to study ocular dominance?
Inject radioactive proline in one eye
Inject radioactive glucose in the cortex and stimulate one eye with light
Ocular dominance columns - inject radioactive proline experiment
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
Ocular dominance columns - inject radioactive glucose experiment
Enters neurons that are currently active as they need glucose
Use optic visualisation to observe radioactivity – stripes
Ocular dominance columns in the visual cortex
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
What are orientation columns? – columnar architecture in MT cortex
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
What are blobs?
Parts of the brain have different properties that are important for processing information about colour
What is a hypercolumn?
Three types of columns - Blobs - Ocular dominance columns - Orientation columns This unit can process any type of information
What experiments did Hubel and Wiesel carry out?
Worked on V1 cortex in cat
Recorded response of individual neurones to different direction of stimulus
Example of a Hubel and Wiesel experiment
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
Where are simple cells found?
Localised in layers 4 and 6 of V1 cortex
What do simple cells respond to?
Respond to a bar in certain orientation presented in the center of the receptive field
Receptive fields of ganglion cells and LGN neurones
Receptive field are round shaped
One response in the centre of the cell
Opposite response on the outside of cell
Receptive field of simple cells
Receptive fields are elongated in shape
Different response along midline vertically
Probable circuit mechanism of simple cell receptive field
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
What do complex cells do?
Respond to a bar in certain orientation presented anywhere in the receptive field
Where are complex cells located?
Localised in layers 2, 3 and 5
Possible circuit basis of complex cell receptive field
Complex cells collect information from many simple cells with similar orientation of receptive field
What is a hypercomplex?
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
Receptive fields downstream of V1
Increase in complexity
Receptive fields increase in size
Face sensitive neurons in the temporal lobe
Respond to faces
If you remove characteristic parts of the face the neurons respond less and less
What is a grandmother neuron?
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
What are the problems with the hierarchical model?
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
