Neural code Flashcards
What is neural code?
Information is encoded to communicate & process (we pick up information continuously, information is encoded everywhere – in books, e-mails, conversations) Neural code (neurons use AP signals to communicate with each other) is at the core of brain function.
What are the different spatial principles of brain networks?
Local: small generic processing units (small networks, e.g. three neurons which communicate with each other through synapses).
Global: aggregate related processes in units: dedicated whole brain regions: location code (map).
Large-scale connectivity: intra-cortical pathways/ streams = information highways.
What is the basic principle in the visual system?
The basic principle in the visual system is you have a stimulus that is thrown into the eye and it responds by creating neural activity in the optic lobes.
What is neural response?
It can be interpreted as the excitation from the soma of a neuron.
(It’s the exponential multiplication of a tiny signal).
What is rate code?
Rate code is the the average number of spikes per unit time.
As the intensity of a stimulus increases, the frequency or rate of action potentials, increases.
Information is encoded in the firing rate. There is a coding in the stimulus in terms of the number of spikes per second.
What are the first steps of encoding, from light to spikes?
First the stimulus falls on the retina.
Photoreceptors in the retina have graded potentials. Which connect to interneurons through synapses which often operate on graded potentials.
Then they go on to a neuron which ‘counts the number of spikes.’ The spikes represent the intensity of the stimulus. They are carried forward to the next level of information processing.
What are the features of a direct one-to-one representation of each sampling point?
The neurons detect the single point and transmits this further into the next level as a single neuronal activity.
There is no interaction between the information coming from different points in the visual field.
• size not encoded (response level stays the same)
• low sensitivity
• high resolution
What are the features a direct many-to-one representation of each sampling point?
These neurons have many inputs and create one output.
The neurons interact and converge through summation.
• graded response (response gradually increases for every additional stimuli point)
• sensitivity increased
• reduced resolution
What happens after the information processing from the sensory input system?
After information processing from the sensory input systems which is spatial pooling (summation), you can build other elements such as neural computation (e.g. subtraction).
What is lateral inhibition?
A neural computation in which responses from neighbouring regions are subtracted is called lateral inhibition.
What are the two fundamental types of activity in neural computation?
Two fundamental types of activity: excitation from the summation in the centre which is positive and inhibition from the right side which is subtracting the output from that.
What is the first level of processing in the brain?
• Direct representation (one-to-one) of each sampling point (pixel by pixel to the first level of processing in the brain).
What is the next level of processing in the brain?
• The next level is summation (Many-to-one)
• This can be a simple summation, and everything is added or it can be a subtraction so it can be processed further.
Involved in lateral inhibition.
What is the definition for the visual receptive field?
The location in space where the presence of a visual stimulus can produce a change in the response of a neuron. (This is where the neuron receives information from the outside world)
What is the simple mapping of receptive fields?
Shining small spots of light and checking where a change in response occurs.
The number of light spots you detect, gives you an idea about the size of the receptive field.
What is a two-dimensional receptive field?
Neural computation in two dimensions: in a receptive field, light is collected from a restricted region of the visual field (a single line of receptors) but they are spread all over in two dimensional images. There can be two types of that: Some of them are excitatory surrounded by inhibition. Or inhibitory (giving negative responses) and being surrounded by excitatory positive responses. (On Off centre)
What is an on off centre?
This is a centre where the response of the neuron increases and there is a surrounding area where response decreases when we put a light there.
How can you record a two dimensional receptive field?
Plotting receptive fields with a spotlight. E.g. retinal ganglion cells
The stimulus is the monitor and the person looking at it is given point stimuli. Sometimes there is a positive response, no response or a negative response.
There is a centre where the response of the neuron increases and there is a surrounding area where response decreases when we put a light there. This is why it’s called a centre surround organisation.
Alternatively, it can also be recorded by a larger stimuli instead of point by point using an electro physical recording from neurons. (See page 9 of notes for diagram).
What is the spatial integration in two dimensional cases?
Adding up excitation and inhibition.
It’s almost like balancing:
The more light in the centre, the more positive the response and the more light into the surround, the more negative the response.
You balance this up to find your final result.
What happens when there is a difference between what happens in the centre and surround.
This is in some way a level of intelligent encoding.
By balancing positive and negative responses, you take out the average which isn’t interesting, and you know where the differences/ contrasts are.
What is meant by contrast (where the differences are)?
Contrast is the relative difference in the stimulus intensity between a bright and a dark region of a visual stimulus.
What happens at a luminance boundary (contrast border)?
- In regions of equal luminance, excitation and inhibition cancel each other.
- At the boundary the excitation and inhibition are not balanced and thus increase the relative difference of perceived brightness.
- At the bright side of the boundary, you have more excitation than inhibition and that’s why you get a positive response.
- On the dark side of the contrast border, you have more inhibition than excitation because the excitatory neuron is not stimulated. So, you get a negative response here and a positive response there.
• Boundary encoding is a very clever way of encoding because it saves us a lot of activity – you only present electric activity in the brain when things change.
What is meant by the process of filtering?
Filters are selection mechanisms for information. The filter mechanisms reduce uniform information and focus on the important thing.
• The information is being transformed
• Regions of change are emphasised
• The receptive field acts as a filter – signal from around boundary regions are transmitted, but not from uniform areas.
What is meant by redundancy reduction?
We don’t transmit information which doesn’t contain any change. Only when something is changed, you transmit the information, and this is maximising redundancy. E.g. when setting an alarm, it only rings when something happens and if it was ringing all the time it would be useless.
