Sight and Visual Pathways Flashcards
how can vision be experienced differently?
through
- motion
- 3D vision
- HD
- technicolour
where does sensory processing start?
in the eye, as the retina is part of the brain
how do retinal images begin?
the wrong way around and flipped side to side - variable resolution
what part do axons play in processing information
- The axons of the retinal ganglion cells (that receive output from the photoreceptors ) are very sensitive to edge info
- e.g. contrast between objects excites retinal ganglion cells
Why are edges important
- Not enough photo receptors to produce a more high res version of the world
- This is why edges are important
how does the eye grab information?
through receptive fields
receptive fields
- region in the sensory periphery within which stimuli influence the electrical activity of sensory cells
- e.g. excite/inhibit firing rate of a retinal ganglion cell
- this is because each photoreceptor receives a small part of info from the visual landscape
How much space is a receptive field able to take in , what does this mean for the brain?
- only a small sample of sensory space is sampled, no one retinal ganglion cell can see the entire world
- So a single retinal ganglion cell will only respond to a single colour , they do not have access to everything we consciously see
- It shows how our brain has to pull together all of the output of many different ganglion cells to construct an image about the world
When will photoreceptors respond
- when light falls on their receptive field
-output takes form of AP in the retinal ganglion cells
-their axons then form the optic nerve
what happens to receptive fields as they get deeper into the brain?
get more structured and specific
how are receptive fields organised?
to detect changes in vision, by having different regions which turn cells on and off
where are retinal ganglion cells?
- on the inner surface of the retina
- at the bottom
whats the pathway that leads us to retinal ganglion cells?
Photoreceptors+ rods+ cones –> bipolar cells –> retinal ganglion cells
what function to retinal ganglion cells perform?
- lateral inhibition to cells that arent processing anything of interest
- this leads to compression
types of ganglion cell receptive fields
- on centre and off surrround
what are on center receptive feilds
- type of ganglion cell
- when receptive fields receive information from the bipolar cells, (like light)shines on the centre part of the receptive field
- it excites the firing rate of the retinal ganglion cell
what are off surround receptive feilds
- info that shines on the surrounding part will inhibit/turn off the retinal ganglion cell
- So the on and off regions act as switches
the consequence of centre-surround antagonism
“simultaneous contrast illusion”
what do we see in a simultaneous contrast illusion
-The bar in the bottom is more darker on the right and lighter in the left
- in reality the lower bar is the same shade of grey all the way across
“simultaneous contrast illusion”
- on-centre receptive fields (on the darker background) are not inhibited, as there is less light to activate the off-surround, so the bar appears lighter= more info about light being sent to our brain tricking to us seeing brighter
- on-centre cell (on the lighter background) is inhibited because there is more light to activate the off-surround, so the bar appears darker= less info about light being sent to our brain tricking to us seeing darker
what do simultaneous contrast illusions show us
shows how background information plays a role in influencing what we see (because of our receptive fields )
- This is the same for colours (we see the opposite as consequence of receptive field )
how is retinal output used to signal changes?
- by detecting edges over space,
- we break pictures apart in our brain into edges (colour comes later on)
Our retinal ganglion cells and edges
- because of their sensitive nature , areas with changes would strongly excite our retinal ganglion cells = sharper edges
- no change= same static color
- so edges are enhance when changes occur
What is the craik O’brien cornsweet illusion
tricks us into thinking the face we see is constructed of a brighter face and a darker hat + hair colour even though theyre the same
craik o’brien cornsweet illusion is a consequence of…
seeing only edges, as images containing only edges pass through the retina unchanged, whereas normal images are converted into edges
What is the end result of the Craik O’Brien -Cornsweet illusion
end result= retina output looks the same between both pictures, this means brain will interpret as the same and fill in the missing greys
what are troxler fading and after-images?
Looking at the top cross for 10ish seconds–> moving eyes downwards you should see the opposite/complementary colour to the one you stared at initially ( at the bottom)
how are after-images produced?
- Because there’s no clear boundaries between the stimuli, there is little physical change taking place and so to save energy the brain inhibits activity of cells that receive info about luminance over time
- so we opposite brightness youve adapted to because of time lag
- Over a short period of time the blobs should fade because info about these colours is being inhibited/decreasing ( brain is receiving less info over time about what we are looking at)
what does output from the retina reach?
the lateral geniculate nucleus
what is the LGN?
- where information from the left and right VF combines in the brain, and RGC become specialised for motion or colour
- so this happens before enters any of the V’s
Structure of the LGN
- Info starts to get separated in terms of color and motion so the LGN is constructed of different layers (6)
- each layer has a complete retinotopic map of half the visual field
- In between these layers we have certain types of cells
LGN and how info is represented (retinotopic map)
The way which info is represented in the LGN has the same spatial relationship as info presented in the retina
- So objects that appear side by side in the retina will activate activity that appears side by side in the brain (1:1 mapping from retina & LGN )
- However this is half the visual field because the left LGN only receives info from the right side of space (vice versa)
- So there is no single cell within a layer of the LGN which has bionocular vision (both left and right side of space)
- But each LGN receives input from both eyes
what cells appear in the LGN?
koniocellular , parvocellular , magnocellular cells
Koniocellular cells
K cells process info abour blue and yellow
Magnocellular cells
- Large cell body
- fast response
-sensitive to motion/depth - clustered in the periphery (periphery is where we usually need to detect movement )
-course detail
parvocellular cells
-small cell bodies
- respond to red-green colour,
- slow response to colour
- closely concentrated in the fovea (part of the retina where there is high res vision, responding to more finer detailed info)
the LGN is the main…
relay point from the optic nerve on the way to the cortex
The visual pathway
info in the visual system is processed along to separate pathways , exciting different parts of the visual system
division of visual pathways
dorsal stream
ventral stream
dorsal stream
where objects are processed in space, motion, and depth
-helps motor cortex to helps us move our limbs
V3, V5/MT, V6, V7
ventral stream
recognising faces, places, and objects
-what things are
V1, V2, V4, V8, LOC, OFA, FFA, PPA
The ventral system
V1,2,3
-highly important in colour, deconstructing info into position and mapping out lines and spatial outlines of objects , info about motion
-these are the first and main areas of our visual cortex
The occipital facial area (OFA)
info about faces processed
Fusiform facial area (FFA)
facial processing area
Para hippocampal place area (PPA)
process info about places
Lateral occipital cortex (LOC)
process objects
Medial superior temporal cortex (MST)
processing advanced motion (e.g. rotation) + self movement
Superior temporal sulcus (STS)
extracts info about emotions being expresses via face (this is separate from the area that recognises faces
V5/MT
- motion
How can we use a mathmatical model in relation to our vision?
- Using this model, they could represent how our brains see this illusion as its simply using knowledge of receptive fields
- Supports the idea that many optical illusions are simply a consequence of the physiological behaviour of retinal ganglion cells
- They did not need any higher order top down processing to explain visual illusions
Why may mathematical models contradict the explanation
- Very lower order functions of retinal ganglion cells can contribute to this
- But there is a more higher order mechanism (V4) with white balance correction (colour constancy )
