Retina To Cortex Flashcards
What is the geniculostriate pathway?
The pathway of the optic nerve from the retina, to the optic tract (where the axons mix and cross over the midline at the optic chiasm) then optic radiation at at the LGN of the thalamus to the visual cortex (V1)
What is the optic chiasm?
It is the first part of the brain where the optic nerve from each eye is divided and where the two nerves first meet
This allows for information from both eyes to be merged and delivered to the same parts of the brain for processing
Half the axons cross the midline to the other side of the brain, the rest stay on the side they originated from
There is no Synapses in the optic chiasm, the first synapse is in the LGN of the thalamus
How is the LGN organised?
Divided into six layers
Each layer receives information from one half of the visual space
Left LGN - info from retinal ganglion cells of the left temporal retina and the right nasal retina
Right LGN - receives info from the right temporal ganglion cells and the left nasal retina
Therefore both sides of the LGN has info from both eyes
Layers 3-6 are the P layers (parvocellular layer) which receive info from the P cells (midget ganglion cells) - associated with high acuity and colour vision
Layers 1-2 are M layers (Magnocellular layer) which receive info from the M cells (parasol ganglion cells) - associated with movement and luminance detection
Describe the receptive field of magnocellular neurons
Centre - surround antagonism in their receptive fields
Light is either exciting or inhibitory depending on if the cell is on centre or off centre respectively
In uniform illumination the antagonism of the two regions cancel each other out.
A response is generated if there is an imbalance of illumination between the two areas
Changing of colour has not effect on the response of the cell
Describe the receptive field of the parvocellular neurons?
Combination of centre-surround antagonism and red-green opponent colour coding
Signals are received from L and M cones
Centre receives input from one type of cone, the surround from the other type of cone causing the neuron to respond to different intensities of illumination
What is the koniocellular layer?
A substructure in the LGN which receives input from the blue-yellow ganglion cells
These cells have large receptive fields and centre only, no surround
Each of the six layers of the LHN has a koniocellular layer - therefore there are effectively 12 copies of visual information. One from each p and m layers and a further 6 koniocellular layers
What are blobs and interblobs?
Blobs are patches of area II of the visual cortex which have neurons from the koniocellular sublayers project into them
Interblobs are the areas between blobs
Describe the organisation of the primary visual cortex?
Receive highly orders input from the layers of the LGN, therefore the layered of V1 are mapped similarly to the LGN. However the map is distorted because the cortical representation of the image is much larger than the physical size of the retina and conversely the representation of the periphery is smaller than the actual image
Cortical neurons are organised into columns at right angles to the layers
Columns for each area of the retina for one eye is adjacent to the column for the area of the retina for the other eye - this forms a hypercolumn
How does the retina observe orientation?
There are simple and complex cells with differing receptive fields
Simple cells have well defined excitatory and inhibitory regions. They can detect an edge or line in an image. Their receptive field is elongated so they are selective for both stimulus position and orientation.
Complex cells have a mixture of regions which are not well defined
They are less specific for stimulus position, they respond to a correctly orientated edge or line anywhere in their receptive field
There are also hyper complex cells which only respond if a correctly orientated stimulus ends somewhere in their receptive field
define the term colour constancy?
The stability of an objects colour under changes in the spectrum of light illuminating it. I.e a red apple appears red under most light conditions
What area of the brain processes colour?
The temporal cortex (V4)
It is also important for colour constancy, shape and attention
Temporal cortex neurons have large receptive fields
How do we measure relative depth?
Prior knowledge/assumptions about a scene which gives inferred depth
Degree of convergence to bring the object into register of both eyes
Amount of accommodation to flatten the lens to focus the eyes and give info on distance
Use information abut motion by comparing images
Capture simultaneous views from both eyes
How is motion detected in the eye?
There are no motion detectors in the eye however the eye utilises the activity of two types of retinal cell
One of which recovers spatial structure from an image by establishing light and dark edges and changes in luminance
The other recovers temporal information from objects by establishing changes of luminance from one instant to the next.
The information from these two types of cell are combines in the visual cortex V1
The neurons in the V1 respond more to one direction of motion than another so are described as directionally sensitive
Theses neurons project into the medial superior temporal area
Define binocular stereopsis?
Solid vision
Recovers depth information and solidity by comparing he images in the two eyes
What is Bayes’ theorem?
The likelihood of a thing
Likelihood that a thing will produce the response / likelihood of the response occurring
