Lecture 2: methods for researching neural activity Flashcards
what is experimental ablation?
destroying part of the brain and evaluation the animals subsequent behaviour
What is the logic of neuropsychological case studies?
If a brain region contributes to a task, then damage to that region should impair performance
H.M
- what were his memory impairments?
- what could he still do?
- amnesia for new information
- could complete maze task and improve each time he did it, although wouldn’t remember ever doing it
Advantages of neuropsychological case studies
-show direct link between brain areas and behaviour
Disadvantages of neuropsychological case studies
- can lead to false conclusions
- case studies are rare
What happens in a CT scan?
Detector measures x-rays passing through patient, different parts of the brain allow different amounts of x ray through.
Constructs 2d image slice of the brain
What happens in a MRI scan?
Strong magnetic field and RF pulses to detect hydrogen atoms that become aligned in a magnetic field and causes them to spin. When the magnetic field is turned off, the protons gradually return to their normal spin, this process produces a radio signal that can be measured by receivers in the scanner and made into an image.
How are brain lesions produced?
- Subcortical brain regions
- Pass electrical current through a stainless steel wire
- Produces a radio frequency current
- Coarse technique: damages quite a large area
How are excitotoxic lesions produced?
- Excitatory amino acid delivered through a cannula (small metal tube)
- E.g. Kainic acid – stimulates glutamate receptors
- Destroys neural cell bodies but spares axons that belong to neurons that are passing by, therefore really specific brain region
What are temporary methods of brain lesions?
observe disruption through anaesthetic
• Sham lesions= a placebo procedure that duplicated all the steps of producing a brain lesion except for the one that actually causes brain damage
What does tracing connections allow?
- the investigation of structures and interactions
- detailed descriptions of neuronal projections from a single neuron or population of neurons to their various targets throughout the nervous system
**mapping of connections between neurons in a particular structure eg the eye and the target neutrons in the brain
How are efferent axons traced?
efferent=axons that leaves the VMH
-• Neurons in VMH send axons to parts of the brain that contains neurons that are responsible for muscular movements
How to trace:
- anterograde labelling
- • Tracing axonal projects from their source (the cell body) to their point of termination (the synapse)
How are afferent axons traces?
afferent=axons that enter to VMH
- Retrograde labelling method (means moving backward)
- Employ chemicals that are taken up by terminal buttons and carried back through the axons toward the cell bodies
- Tracing axonal projections from their termination to their source
Electrical activity
- Action potential=changes in electrical activity
- changes in electrical activity can be used to determine whether a neuron is involved in a behaviour
*Record electrical activity from a particular brain region whilst present a stimulus
If there are changes in electrical activity then that region of the brain may be involved in processing that stimulus
How electrical activity be recorded?
- Implant microelectrodes attached to electrical sockets
* Very fine tip which allows recording from an individual neuron. This is known as single-unit recording
Pros and cons of Single-unit recordings
Advantages:
• Dissociate the roles of different neurons within brain
• Very high spatial resolution
• Extremely precise, high spatial resolution
Disadvantages:
• Invasive so confined to animal research (and very rarely patients undergoing brain surgery)
• Too focused on single brain cells – neglects neural interactions
• Time-consuming
Recording electrical activity in humans: EEG
• Recording electrical activity produced through summation of synchronous firing of post-synaptic potentials
Recording electrical activity in humans: MEG
- Magneto-encephalogram
- Like a ‘magnetic’ EEG – measures changes in magnetic fields caused by electrical activity
- Detect magnetic fields smaller than the earths magnetic core
Pros and cons EEG and MEG
Advantages:
• Non-invasive
• Excellent temporal resolution
• Can be used for neurological conditions and behavioural experiments
• EEG – portable, suitable for infants
Disadvantages
• Costly (EEG > £50,000; MEG > £1,000,000
• MEG requires special equipment
• Poor spatial resolution – summation of many neurons: not good at telling where something in the brain happens as summing the activity of thousands of neurons together
Measuring metabolic changes
- telling WHERE
- if a brain region involved in a particular task=neural activity
- increases in neural activity=increases in metabolic activity
*measure changes in metabolic rate as an indirect measure of brain activity
Measuring metabolic changes: PET
-Inject radioactive 2-DG
• Most active cells will take up the highest concentration
• Place head in scanner
• 2-DG decays emitting positrons which meet nearby electrons
• The two annihilate each other emitting two photons which travel in opposite directions
• Sensors detect the photons and the scanner plots the location from which the photons were emitted
Measuring metabolic changes: fMRI
- Indirect measure of brain activity: Detect changes in the levels of oxygen in the brains blood vessels
- Increase neural activity in a brain region = increase blood flow to that region = increases in local blood oxygen level
- Oxygenated blood has magnetic properties due to its high iron content
- BOLD signal: blood oxygen level dependent (increased blood flow to particular part of the brain)
Pros and Cons PET and fMRI
Advantages
• Good spatial resolution
• fMRI: non-invasive
Disadvantages:
• Expensive
• PET: use chemicals with short half-lives which are made on-sight
• fMRI: expensive set-up and running costs
• PET: invasive, radioactive materials
• fMRI: certain patient groups excluded: metal; claustrophobia
• Indirect measures: metabolic activity
• Poor temporal resolution, poor timing information, takes a while for blood flow to increase to the certain parts of the brain
Stimulating the human brain TMS
- Rapidly changing electrical current to create a magnetic field
- Sends a magnetic field which alters neural activity
- Inhibit neural activity: reversible lesion
- Increase neural activity
- Link brain regions with performance
TMS pros and cons
Advantages:
• Safe, non-invasive
• Causal link between brain and behaviour
• Can reveal time-course of neural processing
• Better spatial resolution compared to lesions studies
• Participants can be used as their own control
Disadvantages
• Auditory and somatosensory stimulation – effects neural and task performance
• Limited to structures a few cm from skull
• Some group of participants unable to take part
• Cant stimulate deep brain sources
