Lecture 2 Flashcards
What is the retinal image like?
Variable resolution (mostly poor) or slightly sharper in the centre than on the side.
Wrong way up
Flipped side to side
How we see the world is dependent on what?
Purely dependent upon how our visual system has been evolved or constructed.
How is the retina sensitive to edges?
The retinol ganglion cells, receive output from the photoreceptors then its the axons of those retinol ganglion cells which are very sensitive to edge information.
Edges excite retinol ganglion cells.
Why are edges important for retinol ganglion cells?
Because edges represent a physical change in the world that the brain is highly sensitive to. And its typically things where there are changes which excite cells within the visual system, its these changes which represent really important information that we need to respond to and about how the world looks to us.
How much energy does the brain use?
20% of the energy that the body consumes occurs within the brain.
Simultaneous contrast type illusion explanation
To save energy, going to reduce the firing rate of cells which are all receiving information about the same colour but because those neighbouring cells are inhibited, it also inhibits cells responding to that central colour in the middle influencing how it is perceived which is why we see it slightly darker than what it actually is because the surrounding context is inhibiting some of the photosensitive cells in the centre making it look darker than it actually is.
What do temporal inhibitors do?
Turn off cells if they are active for a long time
Explain the after-effect type illusions
Looking at the same picture for a long period of time -> to save energy, going to turn off some of those cells, inhibit their activity. The consequence is that the cells that weren’t inhibited now are giving off more information, which is why you see the opposite colours to those that were temporarily inhibited.
What is the watercolur illusion?
Demonstrates the importance that edges are to the brain, and how edge information is used by the brain to fill in missing information.
Edge information is being used by the brain to fill in missing information, tricking us into thinking the centre of those square doughnuts have some colour to them but they don’t, it is simply the border, the edge information, which has a slight colour to it.
What is a receptive field that the retinol ganglion cell has?
A receptive field is an area of sensory tissue which excites a particular cell.
Just like a small part of the retina might excite a retinal ganglion cell, that small area of sensory space on the retina is that retinal ganglion cell’s receptive field - an area of sensory periphery which excites a neuron.
The way that these receptive fields are constructed help retinol ganglion cells detect edges.
What is the lateral geniculate nucleus?
Relay centre where information from the eye is collected before moving further along the visual system, into deeper cortical areas and the different areas of the visual system, which are specialised for detecting certain types of stimuli.
What does each photoreceptor recieve?
Receives a small part of information from the visual landscape
What is the cell’s receptive fields
Proportion of the retina that affects the signals of that photoreceptor is called the cell’s receptive field
What sends output to retinol ganglion cells?
A large number of rods and cones send their output to retinol ganglion cells.
Small patch of space on the retina is described as the receptive field for the single retinal ganglion.
What does the receptive field do?
An area of sensory space which can excite or turn on, increase the firing rate of a retinal ganglion cell.
Explain the concept of a receptive field
Only samples a small area of sensory space. No one retinal ganglion cell can see the entire world or has access to everything that we consciously see (only gets light from a small part of space), it only receives information from a very small part of space.
The brain has to pull together all of the output from many retinol ganglion cells to construct an image of the world.
What happens when light falls on the receptive field of photoreceptors in the retina?
Photoreceptors will respond when light falls on their receptive fields whose output takes the form of action potentials. These outputs are transmitted through retinal ganglion cells whose axons form the optic nerve.
Where are the receptive fields?
Brain cells throughout the visual system have receptive fields
What happens to the receptive fields as we go deeper into the brain?
Get more structured, more specific and more interesting.
Explain the output of the eye
Receptive fields have these centre surround properties. The retinol ganglion cells are the output from the retina. They also receive input from another class of cells the bipolar cells. Your photoreceptors - rods (for low light vision) and cones (For colour) send their output to the bipolar cells and bipolars send their output to the retinal ganglion cells. Its the axons from the retinal ganglion cells which makes the optic nerve.
What function do the spatial and temporal inhibitors perform?
Performing the function of lateral inhibition. Important concept because it is responsible for this idea of compression (Reducing how much information is sent from the eye to the brain by inhibiting the activity from some cells when they are not processing anything of interest or any change or contrast within the world). Anything which stays the same, the brain inhibits activity from those cells to save energy.
Which cells perform the function of lateral inhibition
Amacrine and horizontal cells.
What happens when light activates the centre part of the receptive field?
Focus on on-centre sensory type:
Idea is that these receptive fields, because of the way they receive information from the bipolar cells, they are constructed in such a way that when information like light activates the centre part of the receptive field, it will turn on or excite, increase the firing rate of this retinal ganglion cell.
What happens when it shines on the surround part?
Whereas information which shines on the surround part of a receptive field will inhibit or turn off the retinal ganglion cell.
How do the on and off regions act like switches?
You can turn them on when they perceive information in the centre or turn them off when they receive information in the surround.
What happens for the other classification (the off-centre)?
They simply function the opposite way. These cells switch off when they receive information in the centre and turn on when they receive information in the surround.
What could you record from a recording electrode in the cell body of a retinal ganglion cell of the firing rate in the on-centre and off-centre?
You will see an increase in firing rate for an on-centre cell, which is receiving information which excites the centre part of the cell’s receptive field.
Whereas in an off-centre, that retinal ganglion cell would be turned off if it was receiving information in the centre part of its receptive field.
What happens in the on-centre cell when there is no light?
There is nothing to sense or detect about the world, just get a small amount of background firing rate from these cells.
What are you actually recording?
The generation of action potentials when you are picking up this spiking activity.
What happens in the on-centre cell when it receives information in the centre?
It strongly excites the cell, it turns it on and then you can see an increase in firing rate.
What happens with information in the surround?
Switches it off, you see no output from that cell because it has been turned out.
What about a full light?
Information across both the centre and the surround would average itself out, and then you will see something that is indistinguishable from the normal background activity, as if there is no light at all.
What does this tell you about the receptive fields?
The on and off-centre cells function like switches. You can turn them on and off depending on the type of information that they receive.
