Lecture 1 Flashcards
Interfacing Brain and Body
What does the cup-stacking competition demonstrate?
Demonstration of how vision is required to guide action and the link between the two. Everything is worked out in advance and requires a lot of preplanning.
How does this link to the brain?
There is a lot going on in the brain outside of our awareness to help us execute those tasks in the real world.
What does the samurai sword video show?
There is a lot of decision making going on outside of conscious awareness to help us perform these types of tasks.
Anticipatory control takes place in the brain to help us interact with the world in way that our movements are smooth and executed.
Why is the cerebellum an important part of the brain?
Plays an important function in making decisions outside of conscious awareness, to make movements in a timely manner.
The cerebellum only knows that you are doing 8 seconds before you are consciously aware of it.
Really important in motor control, involved in learning and plays a role in cognitive function.
The neural basis of vision and action
Our actions are largely driven by brain processes that unfold outside of our consciousness.
How do we typically carry out these movements?
In a very stereotypical manner. We perform actions in the same way time and time again.
What kinds of decisions does the brain need to make?
How am I going to move my limbs in space?
Which path am I going to take to pick up an object?
How fast am I going to move my limbs?
What kind of angles my joints are going to bend to execute that action?
What type of muscle activity do you have?
You have got different amounts of muscle activity, the contraction reaction of muscles is the same, as long as the movement is the same.
Have got different patterns of muscle activity which is accompanied by different neural firing patterns. When a motor command activates muscles, it is characterised by changes in action potentials which activate your muscles. So stronger forces would generate a faster rate of action potentials than a less intense movement.
What did the experiment which looked at characteristics of movement want to understand?
Wanted to understand, is there a comfortable, preferred way in which people will perform a very simple action?
What did the experiment find?
People carry out very simple task in a very similar, stereotypical manner. Suggesting that there does appear to be a preferred manner in which we execute our movements.
There is a way in which the movements that we make are characterised by a preferred or comfortable manner of execution.
We do things in a preferred manner, why is that?
We don’t want to do something too quickly because it would use more energy, you will become more tired quickly. but you don’t want to do things slowly because then you may not be able to execute a task on time.
What is the optimum goldilocks hypothesis?
The idea that there is an optimum preferred amount of time that people should spend on their mobile phones.
Same principles apply in the motor world as well. There seems to be a comfortable, preferred way in which we perform out actions.
We perform our actions in a comfortable and preferred way, how does this link to energy expenditure?
No point in using too much energy because you are tired more quickly.
Why do we experience optical visual illusions?
Because the brain has a preferred way in processing the world around us to save energy and to reduce the amount of information that needs to be sent from the eye to the brain.
What mechanism is this linked to?
Compression. This is an important concept because compression underlies the reason why we experience many visual illusions.
What is a motor invariant?
The fact that when you carry out a movement, many of the decisions that we make stay the same.
What type of decisions stay the same?
Same speed, same velocity, same angles.
What type of stereotypical trajectories do humans show for eye and arm movements?
Path: sequence of positions of the hand in space
Velocity: time sequence of along a path
What is a velocity profile?
Your movements speed up as you get halfway to the target and then they slow down as you hit that target.
Many of our movements are characterised by parameters that stay the same, what type of parameters?
The path you take, the time it takes you to get to the target, duration and the smoothness of your movements (called a jerk) stay the same between the same repetitive movements.
So, you have duration, velocity, jerk which are the first, second, third derivatives of position.
What is the interface between the brain and our muscles?
Neuro-muscular junction (NMJ)
What is the neuro-muscular junction?
A connection between brain-limb to help interact with the world around us.
What do you have at the top of the neuro-muscular junction?
Axon terminal, where an action potential is sent to a muscle to activate it.
What do you have at the bottom of the neuro-muscular junction?
Muscle fibre where it has receptors to receive the chemicals which are released from the axon terminal which then allows a signal to transfer from a neuron to the muscle to excite it.
In what type of surface does the axon meet the muscle fibre?
Convoluted surface
Why is there a a convoluted surface?
It means that you can increase the surface area just to get more receptors to receive more neurotransmitters
In the visual system you have the retina, what does that contain?
The retina contains light sensitive cells which detect sensory information from the visual world
What does the output from those sensitive cells do?
The output from those sensitive cells forms the optic nerve, which then sends information from the eye to the brain.
What type of decision making is already going on at the back of the eye?
There is a lot of processing going on at the back of the eye.
A lot of decision making about where something is located in space, the colour of an object, the edges of objects.
It is reconstructed the further along the visual pathway that you go towards.
How does the brain handle information like that?
The brain has to put all this information together like a jigsaw puzzle to reconstruct what reality looks like.
What does information look like at the level of the retina?
Very different to how we consciously experience the world.
It is blurred, soft and upside down looking image and the brain has to correct all of that information to give us a conscious perception of reality.
What is really out there is nothing like what we consciously perceive.
The idea of compression, how does it apply to the visual system?
The same principle applies to the visual system, the visual system also needs to reduce how much information goes from the eye to the brain.
What happens if the compression mechanism doesn’t work correctly?
Sometimes its a little too efficient and it tricks us into seeing things that aren’t really there.
Why do we see the bowl of strawberries as red?
Because the berries are in a scene where everything is cyan - the visual system discounts the cyan.
The “opposite” of cyan is red
How could top down information be playing a role in the strawberries illusion?
We are familiar with what the colour of a strawberry looks like so top down information is playing a role there.
But that is not the key mechanism at play here.
How does the brain process the cyan tint to the picture?
The picture is surrounded by this cyan looking film. Later on, this is something that our brain is taking into consideration in trying to work out the colour of objects.
Our brain has the capability of detecting the level of background light and subtracting that from the actual colour of objects.
What is white balance correction?
(Like when you take a picture on your phone, you notice that when you move it around, the colour of the image changes depending on whether you are pointing it at a warmer or cooler object).
The white balance correction mechanism is something that the brain does as well. It is basically subtracting the colour cyan from the image and the opposite of cyan is red. S
So, the brain is subtracting cyan and we see the complementary or opposite colour to that, which is red.
What tricks you into seeing a complementary colour?
When you experience one colour for a long period of time, you adapt to that particular colour and the consequence of that is, because you adapt to that colour, the cells which are responding to that colour reduce their activity but the cells which detected the opposite or complementary colour weren’t adapted and the output from those cells which weren’t adapted is tricking you into seeing the complementary colour.
What are the 2 problems that the brain needs to solve?
1) The resolution problem
2) The energy problem
What is the resolution problem?
That there is too much information in the world for the brain to record all of it. Even though the brain is big, it still isn’t big enough to record the world as it is. It has to reduce the amount of information in the world to send to the brain.
The brain needs to reduce the amount of information it sends from the eye to the brain.
What is the energy problem?
If the brain did record absolutely everything, it would require a lot more energy which means a lot more information has to be sent from the eye to the brain which takes a longer amount of time.
The optic nerve would have to be much fatter to carry all of that information - but if it was any fatter, it would be bigger than the size of the eye making the eyes redundant.
(Blind spot is at the back of the eye where the optic nerve goes through the back of the eye to the brain).
Optic nerve bigger = blind spot is bigger = you don’t see anything.
Its a compromise between good design and just processing information from the world efficiently.
What would happen if all of the cells in the retina were active all of the time?
The amount of energy (and therefore blood vessels/blind spot) required would be huge.
What is a solution?
Compression is a mechanism that solves these problems in not requiring more energy and processing everything in high-res detail.
What happens when the brain compresses?
Only transmit important information
Don’t transmit things humans don’t need to react to
Changes are more important than the stuff that stays the same
Changes across space = only detect edges
Changes over time = only detect things that move (new objects)
What is the brain really good at detecting?
Edges are really important information, the brain is really good at detecting edges or contrast between things (contrast between colours, contrast between shapes, contrast between motion, fast and slow, contrast between texture - rough and smooth).
The brain is really good at detecting differences between stimuli.
Why is the brain good at detecting contrast or differences between things?
That can capture most of the information in the world around us. The brain is only interested in things that are really important, like edges.
It dt doesn’t pay too much attention to things that you don’t need to react to - things that are moving capture our attention and we are more likely to react to them, there is no point in capturing the things that stay the same in high-res detail (inefficient).
You can reduce how many cells are active if they are all responding to exactly the same colour.
What are the simultaneous contrast illusions?
Where one colour influences your perception of another colour.
The surrounding colour is influencing your perception of the horizontal bar in the centre.
Demonstrates that surrounding colours can influence your perception of a neighbouring or target colour (the horizontal bar in the middle).
Why are colours affected by a neighbouring colour?
Might have some green sensitive receptors, for example, and associated with every one of these cells is another type of cell which plays a role in inhibiting or turning off those cells - these are called spatial inhibitors (every ‘G’ cell has a spatial inhibitor cell linked to it).
Spatial inhibitors turn off a cell if their neighbour is also active. This process is called lateral inhibition.
Thus the brain compresses signals that stay the same over space.
So, what is lateral inhibition?
(Adaptation) It is a process whereby if one cells is active, you switch off the one standing next to you. This type of adaptation is quite fast but also disappears and fades quite quickly.
What is one way that the brain can compress or reduce how much information it sends from the eye to the brain?
To switch off certain types of cells.
How can this explain the illusion?
- You would get activity of red and green photoreceptors but all the neighbouring cells also have a cell which can switch them off.
- When you surround that colour with a massive block of red, just the red cells would be active so we can perceive the colour red.
- When you have a contrast or a difference between red and that centre square, to reduce how much information the brain sends from the eye to the brain, the red cell in the centre is inhibited or reduced to save energy.
- Consequence of that is that the green photoreceptor has not been reduced tricking you into thinking that centre square is greener than what it actually is because the surrounding colour red has switched off the red photosensitive cells in the centre.
- Green remains tricking you into seeing greener than what it actually is.
What is the concept of a receptive field?
An area of sensory space which responds to a stimulus. They respond to pairs of colours which is why illusions are usually the complementary or the opposite of what you have adapted to.
The brain is capable of processing many pieces of information in pairs simply because of the way receptive fields work and process information about the world in pairs and complements in pairs.
What does lateral inhibition do?
Disables the spreading of action potentials from excited cells to neighbouring cells.
Enhances the contrast between stronger and weaker signals (it amplifies the signal the brain sends from our senses to our brain).
Spatial enhancement of contrast improves the localisation of objects
Each receptive field inhibits its neighbour
Tactile inhibition actually occurs “upstream” in the spinal cord.
Temporal inhibition -> after-effect type illusions
This is a compression mechanism which occurs over time.
What happens when you stare at something for a long period of time?
You adapt to it such that the cells which are active reduce their activity to save energy.
The illusion: the photoreceptors that are sensitive to those colours have adapted and reduced their activity but the cells which are the opposite of those colours were not inhibited so now their output is greater than the cells that were inhibited. The only information left is about the opposite or complementary colours which explains why you see the image in real-world colours.
Encoding changes over time
Temporal inhibitors turn off cells if they are active for a long time
Adaptation is slow, it takes time to build up and takes time to fade away.
Thus the brain compresses signals that stay the same over time.
How does the brain compress signals that stay the same over time?
White = Red+Green+Blue (Additive colour mixing)
If we look at red for a long time, our “R” cells will be inhibited
If we then look at something white then green and blue cells respond (because there is always baseline activity from cell) but the red cells don’t.
White now looks blue/green
(The cells which produce the opposite colour weren’t inhibited, giving a rise to this colour-after effect).
What is a rate code (neural adaptation)?
Firing frequency codes for intensity.
The rate of firing codes for the intensity of the object that we are interacting with and the relationship between the firing rate.
So, there’s a relationship between the frequency of action potentials that are being generated, which is then sent to the muscles to activate them, depending upon the intensity of the object that we are interacting with.
E.g. if you lift up a heavy object -> more frequent firing rate or generation of action potentials.
lighter object -> fewer action potentials being generated
What are the action potentials at the neuromuscular junction, which activate muscle to help lift up objects, described as?
Non-linear, it is curved.
The amount of action potentials being generated actually decreases as the intensity of a stimulus increases.
Not the case that you get 200 action potentials when lifting up 2 kilogram and 300 for lifting up 3 kg (if it was a linear relationship) - this assumed that the brain is capable of generating an infinite number of action potentials which it can’t do.
How do neurons adapt to different stimulus intensities?
It is possible for neurons to adapt the shape of that stimulUs intensity curve so that the cell can adapt to a much wider range of stimuli.
Neurons are flexible, they can adapt and change how they respond to changes in the environment - this enables the motor system to adapt to changes in the environment, like the visual system can adapt to changes over space and time.
This flexibility that neurons can exhibit means they can change their sensitivity to stimuli to make them more flexible and helping them to adapt to a much wider range of stimuli. Rather than having certain cells that respond to a fixed amount of intensities.
How is this more flexible?
Better to have one cell which changes and adapts to a wide range of intensities rather than a lot which detect a range of light and heavy objects.
What is the filling in illusion called?
Craik O’Brien-Cornsweet illusion
What happens in the filling in illusion?
The ability of the visual system to fill in missing information as a way of reducing how much information is sent from the eye to the brain.
What do we see in this illusion?
We are tricked into thinking the face is a lighter colour than the hair and beret in the surround, when in fact they are exactly the same colour.
What is the reason for seeing this?
Because the brain would be sensitive to edges at the boundary, where there is contrast, its actually lighter on one side of the line and darker on the other side of the line.
What the brain is doing is only using information at the boundary and using that information to fill in missing information.
The brain doesn’t need to process all of the cell which are responding to the face, don’t all need to be active. Only the ones at the boundary between the hair and the face can be used to tell us what colour the hair is and what colour the face is.
How does the information at the boundary link to the illusion?
Only the information at the boundary is being used to trick us into thinking the face is white and the hair is dark. The brain is filling in the missing information from the hair in the face to trick us into thinking the face is lighter than the hair, when they are the same.
