ELECTROPHYSIOLOGY Flashcards
How is the world out there perceived, comprehended and acted by neurons in the brain?
*Neuroscience
Neural representation
The way in which objects and properties of the outside world manifest themselves in the neural signal (e.g. different spiking rates for different stimuli)
How is the world out there perceived, comprehended and acted by neurons in the brain?
**Cognitive psychology
Mental representations
The way in which objects and properties of the outside world manifest themselves in the mind, Example: the image of a person (grandmother).
Neuronal recording
(single and multi unit recording)
Electroencephalography
(changes in electrical potential on the scalp)
HOW DOES IT WORK? Single Cell and Multi-Unit Recordings
Invasive: it requires implanting a microelectrode in the brain tissue (normally conducted in animals).
intercellular vs extracellular and measurment of action potential
When neurons fire an action potential, the change in voltage is recorded by the electrode.
We can then relate neuronal activity to experimental events (stimuli/responses). By studying the relationship between neuronal responses and experimental events, we can investigate what cognitive processes the brain area we’re recording from is involved.
fine neuroanatomy - What can we measure?
Activity of a single neuron (single-unit recording)
Activity of multiple neurons (multi-unit recording)
Activity of a single neuron (single-unit recording)
We can look at firing rate: count of action potentials (“spikes”) per sec
Activity of multiple neurons (multi-unit recording)
We can look at the pattern of activity across neurons (e.g. synchronous firing)
What coding schemes does the brain use (Rolls & Deco, 2002)?
1 - Local representation
2- Sparse distributed
3- Fully distributed representation
Local representation
What coding schemes does the brain use
All the information about a stimulus is carried by one neuron (grandmother cell).
Sparse Distributed representation
What coding schemes does the brain use
All the information is carried by a few neurons in a population.
Fully distributed representation
What coding schemes does the brain use
All the information is carried by all the neurons in a population.
Grandmother cells
Single-cell recording in an epileptic patient during surgery. This neuron fires when the
patient sees a picture of a specific person (Halle Berry) but not pictures of other persons.
Electroencephalography (EEG)
Continuous recording of electrical activity in the brain
Event-related potentials (ERPs)
Non-invasive: main electrophysiological technique in humans
Electrodes placed on the scalp
Measures summed electrical potentials
from millions of neurons
Note: electrical signals from a single neuron are too small to record non-invasively and can’t be distinguished from signals from other neurons.
Event-related potentials (ERPs)
?
How/Why can we detect electrical
activity of neurons on the scalp?
Synchronously active populations of neurons generate a large enough electrical field
Not measuring action potentials; instead postsynaptic potentials in dendrites
the signal from a single neuron is too small to detect at the scalp
So how do we detect signals at all?
Synchronously active neurons: spiking at the same time
Physiological origin of EEG signal is the postsynaptic potentials in dendrites of cortical neurons (not action potentials)
Electroencephalography (EEG)
EEG electrode arrangement
The 10–20 system of electrodes used in a typical EEG/ERP experiment.
Predictable EEG for different behavioral states
Important in clinical investigations as an indicator of:
Alertness
Brain Health (e.g. epilepsy)
*NOT widely used in cognitive neuroscience.
USING EEG TO STUDY COGNITION
In a continuous EEG recording, can’t tell what part of the signal has to do with the cognitive process of interest
What we can do is have the participant perform a task and look at the portions of the signal at critical periods (during stimuli or responses)
Say we’re interested in the brain’s response to hearing tones
What might we do if we’re interested in the brain’s response to the tones?
Kind of obvious: just look at the portions of the signal when person is hearing ones
Using EEG to study cognition: Event-related potentials (ERPs)
Activity from several trials is averaged together to extract signal (e.g. electrical activity associated with stimulus onset) from noise (e.g. background electrical activity)
Data for any one trial consists of response to stimulus + random noise
What happens when you average?
increase signal-to-noise ratio
?
Signal to noise ratio: like in math, a fraction
Here when noise decreases, SNR goes up.
How else could we make SNR go up?
Why does averaging increase SNR?
Assumptions
Noise is random, therefore, when averaged across trials
it should cancel out
Signal is not random and it
is similar across trials, so it
should not cancel out.
Noise is random: sometimes positive, sometimes negative
One moment you may be thinking about the weather, another thinking about what you had for dinner last night.
Average of zero
Like if I got you guys to all pick a random number between plus 10 and minus 10, the average would be zero
But signal is not zero – similar every time
So by averaging we’re reducing the noise, just like on the last slide, which increases the signal to noise ratio.