Week 3, Lecture 3- Methods of Cog Neuro Flashcards
Methods of Cognitive neuroscience- Action potentials, single cell recordings, EEG, ERPs and MEG
Explain how Chemical Signalling works?
- Axon terminal releases neurotransmitters into the synaptic cleft
- Protein receptors in the dendritic membrane of the postsynaptic neurons bind to the neurotransmitters
- A synaptic potential results
How does electrical signalling work?
- An action potential is a sudden change in the electrical properties of the neuron membrane in an axon via voltage-gated ion channels.
- Na+: positively charged sodium ions * K+: positively charged potassium ions
1- Neurotransmitters binding to dendrites make charge at dendrites slightly more positive.
2- If this is strong enough (voltage rises above threshold; usually around -50mV) the cell
depolarises.
Na+ channels react to the voltage change by opening up and letting Na+ into the axon (i.e., voltage-gated ion channels)
3- So much Na+ comes in that there is a positive potential: “firing”
4- Na+ channels close, K+ channels open to let K+ out.
That is, potassium starts being pumped out of the cell to restore it back to its more negative resting state (repolarization)
5- Leads to refractory period; undershoot which make it hard to fire again (hyperpolarization)
What are single cell recordings?
- Very small electrode (small device) implanted into axon (intracellular) or outside axon membrane (extracellular)
- Directly measures an action potential (electricity going on) in response to a stimulus
- Count the number of spikes (“firings”) per second in response to a stimulus
- Sometimes, multi-cell recordings
- Invasive
- Done on rats, primates, humans (patients undergoing surgery for
epilepsy)
How do neurons code information?
- Amplitude (size/strength) of the action potential can vary, but the variation does not carry information
- Number of action potentials propagated (transmitted/spread along the axon) per second varies along a continuum – the spiking rate (i.e., firing rate)
- Spiking rate relates to the informational “code” carried by that neuron
- Some neurons may have a high spiking rate in some situations (e.g., listening to speech) but not others (e.g., during vision)
What is rate vs temporal coding?
Rate coding = neurons encode information in the rate of neural firing
* E.g., single-cell recording studies show increase in firing rate in response to a stimulus
* Temporal coding = neurons encode information in the synchrony of neural firing
* E.g., neurons fire in greater synchrony in response to a stimulus * Could be measured by multi-cell recording
Explain the idea of the ‘grandmother cell’
- Studies by Hubel and Wiesel showed us that there was a selective response of cells (certain cells responded to certain things) in primates when shown different orientations of lines.
This led to the idea of the ‘grandmother cell’
The ‘grandmother cell’ = a hypothetical neuron that responds to a specific concept or object - Example of local representation
- In human studies neurons have been found that are selective to concept or objects, but generalise across appearance (This means that these neurons respond to a particular concept or object even when its visual appearance changes)
- E.g. A cell in the medial temporal lobe (hippocampus) selectively responded to depictions of Halle Berry (Quiroga et al., 2005)
What does this selective responding (grandmother cell) reflect?
Ison et al. (2015) found cells that were selectively responsive to pictures of a particular person
* They paired those pictures of the person with a picture of a landmark
* The person-selective neurons started responding to the landmark (increased neural firing) – this suggested an association had been formed between the person and landmark
* Suggested these neurons are involved in learning and memory
What are the pros/cons of single cell recordings?
- Very invasive
- Only very localised
- Translational if between animals and human (can we apply animal results to people)
How do EEG’s work?
Use EEG to look at rhythmic oscillations (repetitive back-and-forth movement from electric brain stuff) over time
* “Waves” oscillate at different frequencies
* Rate of oscillation found to correlate with different behavioural activity
* Measures summed electrical potentials from millions of neurons
* Sensitive to dendritic currents
* Measures from electrodes on head/scalp
Explain the electrode locations and reference electrodes of electroencephalography (EEG).
Electrode Locations:
10-20 system is common
* Letter = location
* Number = hemisphere * Odd = left
* Even = right
Reference Electrodes:
Reference electrodes provide a basis for comparison
*From CHATGBT as explanation: The reference electrode is placed somewhere on the scalp (or sometimes on the earlobe or mastoid bone) where brain activity is minimal or “relatively quiet.”
What the reference electrode does is measure the general electrical activity in that area. Since this area doesn’t usually show brain activity like the areas being studied, it can be used as a baseline to compare against the brain activity picked up from other electrodes.
Explain what Dipoles are.
Difference in positive and negative charge in the space outside the neuron
* As positive ions flow into dendrite = space outside becomes more negative
* Therefore, reflects dendritic currents
* Dipole = pair of positive and negative electrical charges separated by a small distance.
* If a population of neurons is aligned in a similar direction and firing in synchrony the dipoles can be summed together
* Measured at scalp as distribution of +ve and -ve charges
* EEG signal comes from changes in +ve/-ve charge over time
What are event related Potentials?
- Use EEG to study event-related potentials (ERPs) by linking average change in EEG signal to the timing of a cognitive event.
How are ERPs meausred?
EEG signal is averaged over many events to increase signal to noise ratio
* Noise = random neural firing, outside interference
* Noise ‘cancels’ out
* Consistent, systematic neural firing remains= +ve and –ve peaks
* Electrodes on the scalp record a series of positive and negative peaks
* Timing and amplitude of the peaks is related to different aspects of the stimulus and task
* Polarity of peak not meaningful
* E.g., positive does not equal more cognitive activity or higher performance
What are ERP components of measurement?
Names based on polarity & timing (we care about timing): * Pxxx = positive peak xxx ms (miliseconds) after event
* Nxxx = negative peak xxx ms after event
Some are exogenous (i.e., based on the stimulus properties, automatic and more of a reaction e.g. loud noise making someone scared) and some are endogenous (i.e., based on the person’s reaction, high processing like solving a puzzle)
Explain the ‘oddball’ paradigms.
- A persons response to something novel/special/unexpected
- Not vision-specific (can be a sound)
- Endogenous
- Real-world applications include brain-computer interfaces and lie detection
What is Magnetoencephalography (MEG)?
- Measures magnetic fields associated with electrical activity in the brain
- More sensitive to activity at sulci (grooves at the surface of the brain)
- Potentially good spatial resolution * 2-3mm (can distinguish between different locations)
Explain the physics of MEG>
- Dipoles created by neurons have a magnetic field perpendicular to the direction of the dipole
- Dipoles run parallel to the skull in sulci- Magnetic field leaves the skull
- Dipoles are perpendicular to the skull at gyri- Magnetic field does not leave the skull
- Look at photos
Explain event related field (ERF)
- MEG eqivalent of ERP
- Has the same concepts
Explain the pros and cons of EEG/MEG
- Excellent temporal resolution: Millisecond temporal accuracy
- Poor spatial resolution: Signal is derived from different sources in the brain and it is not possible to infer exactly where these sources are
What are the benefits of EEG?
- Sensitive to both gyri and sulci activity
- Relatively cheaper and widely available * Participants can move more
- Good for children & infants
What are the weaknesses of EEG?
- Signal can be affected by skull, meninges, etc. * Often includes gel, somewhat messy
What are the weaknesses of MEG?
- Poor at detecting deep dipoles & at gyri
- Expensive
- No metal allowed
- A bit claustrophobic
- Movement restriction
