Neuroimaging Flashcards
What 3 methods can be used for brain lesion studies?
- Physical
- Pharmalogical
- Reversible (cooling or transcranial magnetic stimulation )
What are the advantages of brain lesion studies?
- Can provide information on function
- Carry out otherwise unethical studies
What are the disadvantages of brain lesion studies?
- Difficult to say how precise/specific they are
- Is the effect due to solitary contribution or an imbalance effect
What is transcranial magnetic stimulation?
- The induction of an electric current in nearby neurons using a rapidly changing magnetic field
- Can activate (e.g muscle reponses) or inhibit b
What are the advantages of TMS?
- Ability to study healthy patients with a controlled stimulation
- Inexpensive
- Excellent temporal resolution
What are the disadvantages of TMS?
- Low spatial resolution
- Possible risk of inducing epilepsy, effects on mood, local headaches
- Long term effects are not well established
What is transcranial direct current stimulation (tDCS)?
Using + and - patches to induce an electron flow in the brain which can be
Cathodal - hyperpolarisation of neuronal membranes decreasing firing rate
Anodal - depolarisation of neuronal membranes increasing the firing rate
What are the advantages of tDCS?
- Inexpensive
- Allow for controlled neuromodulation
What are the disadvantages of tDCS?
- Low spatial resolution
- Cannot be used on patients with epilespy, implants or who are on medications
What are the two categories of neuroimaging techniques?
- Hemodynamic
- Electromagnetic
How does PET work?
- Ingestion of radioactively labelled material, emit positrons which are picked up
- Active brain accumulates oxygen and glucose
- Provides functional view with 45-60s integration period
What are the advantages of PET?
- Allow measurement of metabolism and blood flow
- Can use in conjunction with behaviour and pharmalogical studies
What are the disadvantages of PET?
- Expensive
- Invasive (eposure to radiation)
- Moderate spatial and poor temporal resolution
How does fMRI work?
- Uses huge magnet to induce nuclear magnetic resonance phenomenon where protons align in parallel or anti-parallel in response to strong magnetic field
- When RT is switched off measure time longitutional magnetization (T1) and transversal relaxation (T2) retun to normal
- High concentration of H has high sensitivity to NMR therefore there are differences between oxy-haemoglobin and deoxy-haemoglobin
What are the possible issues with fMRI?
- Indirect measure
- Inhibitory activity may increase or decrease energy consumption
- Susceptible to noise
- Arbitrary thresholds for significance
What are the two methods in fMRI?
Subtraction method - experimental minus control activated areas
Region of interest - look at patterns of activation across different experimental conditions
What is diffusion tensor imaging (DTI)?
- Used to determine anatomical connections
- Uses stats to examine interrelationships between brain areas
What are the advantages of fMRI?
- Safe and therefore repeatable
- High spatial resolution
- Moderate temporal resolution
What are the disadvantages of fMRI?
- Expensive (uses He)
- Movements are restricted
- Claustrophobia
- Large ammounts of acoustic noise (can’t carry out auditory experiments)
- Fields can generate body heat
What is spatial/temporal summation?
When action potentials summate either
temporally - close activity
spatial - summation of neurons in similar area firing at the same time
What is EEG?
Measure summation of post-synaptic potentials
What are the limitations of EEG?
- Must be measuring an open field which is synchronously active and arranged perpendicular and close to the skull
- Polarity sometimes not interpretable
How are EEGs recorded?
- Using reference electrode of minimal activity and measuring the difference
- Standardised system “International 10-20 system” with 4 reference points: inion (back), nasion (eyes), and left and right perpendicular points
What is the difference between EEG and ERP?
EEG - large voltage from large area of the brain
ERP - averaging across similar events timelocked to onset of the stimuli
What are the assumptions of ERP?
- That background varies randomly and is not time local
- Repetition of the same event will ellicit the same behavioural/brain response
What is the potential issue with the assumptions of ERP?
Neuronal signals change when stimuli are presented repeatedly due to adaptation effects
What are the components of an ERP?
- Anticipation response
- Early brain stem response
- Midlatency components related to the perceptual response
- Late commponents (+100ms) which are tak-related potentials
How are ERPs labelled?
- Polarity
- Latency
- Topography (spatial distribution across scalp)
What are the advantages of EEG?
- High temporal resolution
- Cheap and non-invasive
What are the disadvantages of EEG?
- Poor spatial resolution
- Poor conductivity of certain tissues can cause distortion
- ‘Inverse problem’ where we know how activity is distributed but are unclear on the source
- Absence of difference does not necessarily mean there is no effect: could mean method is not sensitive enough
How have ERPs called into question ‘free will’?
- Libet experiment
- Readyness potential starts -550ms before consciously reported intention to act
What is Magnetic-encephalography (MEG)?
- Recording of electric/magnetic activity elicited by neurons
- Measures mostly tangentially orientated neurons (parallel to the skull)
- Very sensitive sensor (only small magnetic field)
What are the benefits of MEG?
- High temporal resolution
- Easier to use than EEG
- Magnetic fields aren’t as distorted as other brain tissues
What are the disadvantages of MEG?
- Expensive
- Susceptible to magnetic artifacts
- Participant head movements are highly restricted
How does optical imaging work?
- Use lazer diodes to shine light through tissue
Can interact by:
Scattering - neural tissue becomes more transparent when activated
Absorption - in light spectra is related to bloos oxygenation
What are the advantages and disadvantages of optical imaging?
Advantages: - Inexpensive and safe - Excellent spatial and temporal resolution Disadvantages - Limited penetration of tissue - Low transparency of white matter - Low signal to noise ratio
