Stimulus Recognition: Flashcards
Remember photoreceptors are stimulated by light, this sends signals to bipolar cells then to ganglion cells. Ganglion cells then send signals to the LGN
What does LGN stand for, how many layers of cells does it have?
This is the lateral Geniculate nuclues
It has 6 layers of cells
How many layers of the LGN receive inputs from the magnocellular and parvocellular ganglion cells ? And what is meant by the LGN receiving monocular inputs?
2 layers of the LGN receive input from the magnocellular cells - these are the inner layers of the LGN
The outer 4 layers receive input from parvocellular ganglion cells
The LGN layers has monocular inputs. Thus each LAYER receives input from a single eye!
The outer most layer receives contralateral input, then the next one receive ipsilateral, then the next in receives contralateral.
What type of neurones is the LGN? And what kind of connections are made between ganglion cells and LGN neurones? And what is special about the receptive fields on LGN’s in comparison to Ganglion neurones?
Interneurones
1:1 connections are made.
And the receptive fields on the LGN’s are similar to those on ganglion cells
What do parvocellular and magnocellular ganglion cells do?
Parvocellular ganglion cells detect space
Magnocellular detect motion
Where does information from the LGN go?
The the visual cortex or the V1 cortex
Information from the visual cortex travels to other parts of the brain via which two streams and to which two place?
Information either goes along the ventral pathway to the inferior temporal lobe or it goes via the dorsal stream via the posterior parietal CORTEX (not lobe).
How much synaptic input in the LGN comes from the cortex? What does this mean? And how many interneurones are in the LGN?
60% of synaptic input in the LGN comes from the cortex
This means the cortex tightly regulates the LGN
There are also many interneurones in the LGN
Information which travels from parvocellular ganglion cells travels through the ventral stream. Describe the route:
Parvocellular ganglion cells send signals to the LGN, then to the V1 cortex, then via the ventral stream this info is sent to the V2 cortex, then V4(to process colour)
then it helps us to understand what we are seeing by going to the INFERIOR temporal lobe
Information which travels from magnocellular ganglion cells travels through the dorsal stream. Describe the route:
The Magnocellular cells send info to the LGN, the the V1 cortex. Via the dorsal stream info is then sent to the v2 and v3 cortex before movement is processed.
What must we do before we use neurones which respond to object recognition in tests?
Firstly find single neurones which specifically respond to presentation of specific objects
Stimulate these neurones - and observe perception from these neurones
Everything pre synaptic to these neurones need to be studied in detail.
You must understand how activation of these neurones cause behavioural changes and look at how one neurone (jennifer anniston neurones) and populations of neurones work
What are the key principles of object recognition by the brain?
We have orientation invariance - we can easily recognise objects in orientations we dont usually experience
We also see scale invariance = we recognise objects independently of size.
We can also recognise objects which dont have typical features - e.g. recognise lego.
What is the hierarchal model of object recognition? As in what are the steps in object recognition?
Firstly we have detection of edges
Then we have detection of a combination of edges and contours
Then there is detection of the object parts - e.g. the face (this detection is deeper in the brain)
Even later we have neurones which respond to specific objects from a single specific point of view.
We then have view invariant object detection - this helps us to view a particular person for instance
You then categorise this - identify humans and animals
What is the issue with damage in the cortex and how do we study this?
Lesions in the inferior temporal cortex (not lobe) for instance decrease the ability of a person to recognise objects
We can study this through electrophysiology
Whats important to note about modelling the brain?
You dont tend to look at the entire brain, you tend to focus on certain parts of it with imaginig
Along the ventral stream, what happens to receptive fields?
The receptive field size on neurones increases as you go along the ventral stream.
What are two features of the cortical structure in ref to the visual cortex?
There is layers of neurones in the cortex
There is also columns and blobs (detect colour) in the cortex - the column detect ocular dominance, orientation and direction
How many layers does the v1 cortex have? (note this is not asking about the layering of the LGN neurones! Its talking about the visual cortex)
And how do the neurones in each layer differ?
How do the inputs differ?
There are 6 layers in the visual cortex
They differ in the neurotransmitters they release. And different layers have different properties.
Different layers receive different inputs either from the dorsal or ventral pathways.
What is the different columnar organisation in the cortex?
There are ocular dominance columns Orientations (direction) columns And blobs (which detect colour)
How to find ocular dominance columns in the visual cortex?
You can either:
Inject radioactive proline into one eye:
This diffuses into the LGN and then into the visual cortex
In the cortex you then see stripes where there are lots of proline and stripes where there isnt. This tells you where the columns are.
OR
Inject radioactive glucose in the cortex and stimulate on eye
This again shows stripes in the cortex which correspond to where the ocular columns are
This is because glucose is consumed by firing neurones in the visual cortex. These firing neurones are where the ocular dominance neurones are.
Why are stripes seen in the ocular dominance experiements?
Remember one eye is used
And the cortex has contralateral then ipsilateral inputs
Thus all contralateral or ipsilateral inputs are highlighted
What do oreintation columns tell you?
The orientation of an object
Important to note neurones close to each other have different orientation selectivities
One neurone may be selective to one particular orientation of an object. If you move to the next neurone it detects the object in a slightly different orientation and so on.
Blob columns?
Found shape
They receive input from parvocellular cells from the LGN. These discriminate between different colours
Hypercolumns?
- this is just your oreintation columns , ocular dominance columns and then blobs
What is the orientation test which helps us to distinguish which neurones in the v1 cortex respond to certain orientations of light?
Cat test - hubel and weasel test
Gently insert electrodes into cats brain and monitor visual area of brain
Different neurones light show a response to different orientations of light. These flashcards will refer to the orientation of the objects are bars as these are straight
Individual oreintations columns in the v1 respond to a single orientation of an object in sight
What are simple cells? What is special about their receptive fields?
Simple cells are found in layers 4 and 6 of the visual cortex and respond to orientation again
Their receptive fields are elongated and in one long line. These respond to the same orientation of an object
This is because they receive a large input from LGNs.
However these tend to detect orientation in the CENTRE of their receptive fields
What are complex cells? Where are they found? How do they differ from simple cells?
These are found in the v1 cortex
These detect object orientation away from the centre of their receptive fields . Whereas simple only detect orientation in the centre of their receptive fields.
But this is usually in one orientation, changing the orientation of the bar means the cells stop responding
Where are simple and complex cells found?
Layers 2,3,5 of the visual cortex
What make up complex cells?
Complex cells receive multiple inputs from simple cells
Yet they respond to objects like bars in slightly different orientations. Collectively these simple cells help complex cells to identify objects like bars in a range of different positions
What a hypercomplex cells? And what happens when objects are shone half way in and out of the receptive field?
These are called end stopped complex cells
They respond to certain orientations of objects but if the objects slightly go out of the receptive field due to change in orientation then you see some of the object in normal orientations and then not the rest
After the v1 cortex where does information go?
Layers 2 and 3 of the v1 send info from the v1 to other cortical regions
Layers 5 and 6 send info to deeper structures
Remember layer 3 is for object locations and 4 is for colour
How do receptive fields change downstream (After) v1?
They increase in size
So v1 receptive fields are smaller than v2
What are neurones in the temporal cortex sensitive to? And what happens if you change the properties of these things these neurones are sensitive to?
This cortex is sensitive to faces
If you remove some properties (such as facial features) from the face, faces become harder to detect
What are grandmother cells? Why are they related to jennifer aniston?
These are cells in the temporal cortex and detect facial features
They are also called jennifer anniston neurones
This is because patients who had parts of their brain removed to stop epileptic seizure had their facial recognition abilities tested . Jennifers annistons face was used to see if the individuals were correctly identifying faces
Electrodes were inserted in their brains to see if Jennifer Anniston face was being detected adequately. At different orientations and sizes
Whats wrong with the hierarchal model of v1 ect?
This model is poor at explaining scale and orientation invariance
And doesnt take into account feedback from other higher cortical regions
