The grey matter of the cortex is subdivided into 6 layers. What is the main layer of the primary visual cortex that the input neurons first synapse with?
Neurons from the Lateral Geniculate Nucleus will travel through the optic radiations and synapse into Layer 4
There is a segregation of signals from the retina through to the visual cortex.
What is meant by this?
The is a segregation of inputs from the M and P ganglion cells (motion vs. colour vision/acuity) that is retained throughout the pathway to the cortex.
(inputs are not ever mixed and are also processed differently in the brain as a result)
What part of layer 4 do the M and P cell neurons synapse to?
- M cells terminate in layer 4Ca (alpha)
- P cells terminate in layer 4Cb (beta)
What happens after the M and P cells terminate in their respective parts of layer 4 of the primary visual cortex (V1)?
After synapse in the PVC they send out more neurons to other areas of the cortex and subcortex.
Describe the path of the output neurons from layer 4C of the M and P cells
M cell axons go from the synapse in layer 4Ca to layer 4b and then onto areas of the cortex (eg. V2, 3, 4, 5)
P cell axons don't go to area 4b but also go to similar areas of the cortex (eg. V2,3,4,5) from there
What do the major output layers of the visual cortex generally represent? (ie. where are layers 3,4 and 5 located in the brain)
- Layer 3 and 4B: other cortical areas.
- Layer 5: superior colliculus and pons.
- Layer 6: LGN
Describe the physical properties of M ganglion cells
Large diameter ganglion cells
- Wavelength insensitive (they have poor light descrimination as a result)
- Detect motion
- Project to magnocellular layers of the LGN
What is the significance of the M cells projecting onto the 4B region of the visual cortex (while the P cells do not)
Neurons on layer 4B of the cortex hve directional selectivity to be able to process information or "see" motion
What is meant by directional selectivity?
Directional selectivity is a response that is greater for a visual stimulus moving in one direction than for the opposite direction, and its computation in the vertebrate retina is a classical issue in functional neurophysiology.
Thus a stimulus moving in one direction causes the firing of action potentials in some cells of the 4B layer. But using the same stimulus moving in the opposite direction will not cause action potential firing in that same cell.
What is another selectivity mechanism present in the cells of the 4B layer?
Some cells show orientational selectivitity: they respond strongly to lines, bars, or edges of a particular orientation (e.g., vertical) but not to the orthogonal orientation (e.g., horizontal).
What is the importance/significance of having directional and orientation selective cells?
Sets up how we see motion and motion in directions
What is meant by the increasing or heirachial specificity of neuronal response (the stimuli required to elicit a response) moving up the visual pathway?
Moving up the visual pathway, there is the adding information together from all neurons
And each neuron refines the type of stimuli it responds to as it gets closer to higher centres of the visual cortex.
Ie. neuron stimuli specificity become more complex
What percentage of the whole cortex is involved in vision?
The cortical visual system is composed of multiple visual areas with different functions
Around 40% of the cortex is involved in vision
What is the extrastriate cortex?
Any region that goes beyond the primary visual cortex
The region of the occipital cortex of the brain located next to the primary visual cortex. It encompasses multiple functional areas, including V3, V4, V5(MT), which is sensitive to motion
There are two broad and parallel pathways of vision going beyond the primary vision cortex (Extrastriate cortex). How are these formed?
There is a division in the extrastriate cortex based on the types of information the regions encode and what types of stimuli individual neurons respond to. (These two pathways respond/encode two different things)
What are the two parallel visual streams of the extrastriate cortex?
What do they encode?
- A dorsal pathway (That processes where in the visual field an object is)
- A ventral pathway (What it is we are seeing: identification of objects)
What stream is responsible for the sensing and identifying motion
What are the different anatomical areas of the visual cortex involved the different streams?
Common: V1, V2, V3
Dorsal: V5 (Medial Temporal/MT), MST to the parietal lobe
Ventral: V4, Inferior Temporal (IT)
Where and what is area MT of the dorsal pathway?
It is in the middle temporal lobe is an area specialized for processing object motion.
- It Receives retinotopic information from a number of cortical areas including V1, V2 & V3
Area MT receives direct input from cells in Layer IVB of the primary visual cortex (as its main and most important input)
What is the significance of this?
This shows that the neurons of the MY are very directionally sensitive and this stimulus drives the their activity
Describe the neurons of area MT
- They have large receptive fields (which means they have poor resolutions)
- Respond to movement as a stimulus
- Almost all the cells are directional selective
- They respond to different types of motion
Neurons of the MT are able to respond to complex and different types of motion, how is this possible?
Adjacent neurons of area MT are subtley different to one another and highly organised to respond to motion at slightly different angles.
Area MT has direction of motion columns. What is meant by this?
The highly organised, columnar manner of area MT that enable differential detection of direction and orientation
The MT has output to other areas of the brain, what is the major output from the area?
Frontal eye fields of the eye movement system that drives the movement of the eyeballs to follow an object
- 43yo woman
- She only sees snapshots of vision (eg. cars coming out of nowhere, pouring liquid appearing like statue concrete)
- Visual acuity and visual fields normal
What is wrong with the patient?
She has a problem in detecting motion
Small lesion in area MT (unable to see motion)
What stream is responsible for colour perception or visual acuity?
The ventral stream
Note colour vision problems more commolny occur early in the pathway (usually in the retina)
What does how we see mixture depend on?
The mixing of three different colours:
The mixture of red, green and blue light is percieved as white light, why is this so?
Because the mixing causes an equal activation of the three types of cones
How is colour perception achieved?
We have three different types of cone photoreceptors each sensitive to a different wavelenth of light.
- Red: long wavelength
- Green: medium wavelength
- Blue: short wavelength
Each one absorbs a different wavelength spectrum
Perception depends on the ratio of response between the three cones to a stimulus light
What are cone absorption profiles?
A type of cone doesn't absorb only one specific wavelenth of light, rather a spectrum of wavelengths, with their optimal absorption/response to a particular wavelength.
The absorption profile is the spectrum of wavelengths it is able to have some level of response to
What is the main neural mechanism of colour vision?
Colour percieved is determined by the activity of Ganglion cells, in particular P ganglion cells (that detect acuity and colour)
How do P ganglion cells interpret colour?
P ganglion cells exhibit a colour opponent centre-surround visual field
They have a colour ON centre and other colour OFF centre
The way that the ganglion cells give information to the higher centres of the system is based on a comparison between the centre and the surround
What are the four main types of P cells for colour detection?
- Red On centre with Green OFF surrounds
- Green On centres with Red OFF surrounds
- Blue On centre with Yellow OFF surronds
- Yellow OFF centre with Blue ON surrounds
(There is also then Red OFF centre, Green On surrounds, etc)
Draw what happens when a red light is shone in the centre of a Red On centre, Green Off surround and the ganglion cell response to this
Maximal stimulation of the ganglion cell
Draw what happens when a green light is shone in the surround field of a Red On centre, Green Off centre surround retinal field and the response of the ganglion cell to that
Maximal inhibition of the ganglion cell
What happens when a red light is shone on the whole visual field of a Red On centre, Green Off centre
Green cones are still able to respond to the red light in the surround (because of the spectrum) but less so than the red cones.
The brain does not want this, the brain wants a maximally on or maximally off signal to interpret.
Where isArea V4 of the ventral stream of the extrastriatal cortex?
What is it responsible for?
It lies ventrally in the occipital lobe, just inferior to the temporal lobe
It recieves input from V2 and is reponsible for perception of shape and colour
Describe the neurons of V4
- They have large receptive field
- They are orientation selective
- They are colour selective
Describe the major output area of V4, where is it and what is it important for?
The area IT (inferior temporal)
- The neurons respond to a variety of abstract shapes and colours
- Important for visual memory and perception
- Important for the perception of faces and complex stimuli
Colour vision deficiencies are either acquired or inherited. About 8% of males and 0.5% of females are in some way colour blind.
What are the three groups of colour blindness? What is the most common type?
Monocromacy: people only have one type of cone
Dichromacy: sufferers have two functional cones only
Anomalous trichromacy: sufferes have all three cones, but one expresses abnormal pigment and doesnt work the same as normal cones (this is the most common type)
What are the different types of inherited colour vision deficiencies? Which is the most common?
- Protanope-no red cone
- Protanomal-abnormal red cone
- Deutanope-no green cone
- Deutanomal-abnormal green cone (most common)
- Tritanope-no blue cone
- Tritanomal-abnormal blue cone
How is colour vision normally tested?
Psuedoisochromatic plates (Ishihara tests)
There is no such thing as a yellow cone. How is a yellow field created for P ganglion cells?
Within this region of the field, there is activation/absorption of a mixture of red and green cones causing a perception of yellow.
Yellow light stimulates red and green cones equally