Lecture 3 Methods for studying the brain Flashcards
Lesion Method
A lesion in a localised region of the brain can result in a very specific deficit
Broca’s areas - can understand language but can’t speak it
Strength of the lesion method
Cause and effect found: reveals brain regions that are essential for a given cognitive function and behaviour
Limitations of the lesion method
- Only identifies a single brain region not the network of regions involved in function
- Anatomical variability in the location of brain regions across patients
- Brain reorganization/compensatory behaviours
What happens in animal lesion studies?
- Animal trained in a task
- Surgical removal of a specific brain region
- Observer changes in task performance: - before and after surgery
- between experimental and control animal
Pros of animal lesion studies
Better control over location of lesion than patient studies
Better-matched controls (before and after)
Cons of animal lesion studies
- Ethical considerations
- Differences in brain organisation/behaviour between humans and animals
What can anatomy imaging methods tell us about function?
If there is a correlation between anatomy and behaviour
Key properties of CT scans: based on X-rays
- X-rays in different directions (each giving a 2D image)
- Computer reconstructs a 3D image of the brain
CT scans: measure tissue density
- Can distinguish between skull, brain and CSF/blood
- Little difference between white and grey matter
How invasive are CT scans
X-ray are high energy ionising electromagnetic radiations than can induce cancer at high dose
How does an MRI scanner work?
- Excites hydrogen atoms in the brain using radiofrequency electromagnetic radiation
- Records the electromagnetic radiation emitted by excited atom in different locations of the brain
- Requires a powerful magnet
- Non-invasive
Structural MRI
- Hydrogen atoms in different brain tissues respond differently to radiofrequency excitation
- Can distinguish between skull, CSF, white matter and grey matter
- Fine spatial resolution
Diffusion-weighted MRI:
MRI can also measure the diffusion of hydrogen atoms in water molecules
Where do water molecules diffuse
- Along axons in white matter
- In random directions in other brain tissues
Can measure direction of nerve fibres in white matter
Diffusion tractography
- Derived from diffusion-weighted MRI images
- Follows water diffusion paths along nerve fibres within white matter
- Can identify white matter connections between different cortical regions, including fascicles
Functional techniques: Electrophysiological techniques
- Directly record neuronal activity
- Action potentials
- Postsynaptic potentials
Neuroimaging techniques
- Indirect methods: record metabolic activity associated with neuronal activity associated with neuronal activity
- Energy consumption (oxygen, glucose)
- Blood flow
- Metabolic activity correlated with synaptic activity (PSPs) more than APs
Single-neuron recordings:
Micro-electrode implanted directly near our neuron
Record changes in neuronal membrane potential
Single-neuron recordings: Intracellular recordings
AP and PSPs
Single-neuron recordings: Extracellular recordings:
Only APs are large enough to be recorded
EEG key points
- Neuronal activity
- Only for population of neurons that are aligned so their activity adds up
- Difference in electrical potential near and far from the brain, as a function of time
- Measures PSPs rather than AP
Temporal resolution of EEG
good, recording every millisecond
Spatial resolution of EEG scans
Poor: activity from neuronal populations across the entire brain at each electrode
ERPs key points
EEG response to stimulus is small compared to ongoing EEG
- Present the same stimulus multiple times
- Average the responsiveness
- Unrelated EEG activity averages out, leaving only the brain’s response to the stimulus
ERP time components
Early (<~100 ms): sensory processing
Late: cognitive processing
Magnetoencephalography (MEG) key points
- Record the magnetic fields associated with electrical potentials
- Requires advanced magnetic sensors
- Magnetic field less distorted than electric fields by skull
Slightly better spatial resolution than EEG, otherwise similar
Position emission tomography (PET)
- Injection of radioactively-labelled molecules in blood flow
- Radioactive molecules emits positron
- Positron annihilates with electron, sends two photons in opposite directions
- Coincident photon detection localises radioactive molecule along a given direction
3D image reconstructed similarly to CT scan
Invasive: ionising radiations from radioactive molecules
Use of H20 in PET scan
- Measures blood flow
- More neuronal activity = more blood flow = accumulation of H20
Use of oxygen or glucose in PET scan
Measures oxygen/glucose consumption
More neuronal activity = more oxygen/glucose consumption
Neurotransmitters in PET scan
- More synaptic activity = more neurotransmitter concentration
- Specific to a neurotransmitter system
Spatial resolution PET scan
Relatively low
Temporal resolution PET scan
takes almost a minute of continuous recording for a full functional image of the brain
Functional MRI (fMRI) key points
- Oxygenated and de-oxygenated blood haemoglobin have different magnetic properties
- De-oxygenated blood decreases the MRI signal
- More neuronal activity = increased blood flow = increased oxygenated blood flow = more MRI signal
- fMRI measures the hemodynamic response, rather than neuronal activity
Temporal resolution fMRI
1-10 seconds
- Takes 1-2 second per image of the brain
- Hemodynamic response is slower than neuronal response
Spatial resolution fMRI
≈ 3 mm
Functional near-infrared spectroscopy (FNIRS)
- Send near-infra-red (NIR) light through the skull
- Record it after it scatters through the brain and exits through the skull
- NIR light is absorbed by blood haemoglobin
- More neuronal activity = increased blood flow = more haemoglobin = less signal
- FNIRS measures the hemodynamic response, not neuronal activity directly
Functional near-infrared spectroscopy (FNIRS) temporal resolution
1-10 seconds
Functional near-infrared spectroscopy (FNIRS): Spatial resolution
- A few cm
- Only record from superficial cortex between light source and sensor
Transcranial magnetic stimulation (TMS):
- Electromagnetic coil generates magnetic field through skull
- Magnetic field induces transitory changes in the electrical membrane potential of neurons ( can be inhibitory or excitatory
Transcranial magnetic stimulation (TMS): Limitations
- Low spatial specificity: stimulated regions is relatively wide (several cm)
- Only reaches relatively superficial regions of the cortex below the coil
Transcranial direct current stimulation (TSCS)
- Weak electrical current applied between two electrodes (anode and cathode)
- Created electrical potential between anode (+) and cathode (-)
- Anode: depolarisation -> excitation
- Cathode: hyperpolarisation -> inhibition
Evaluation of Transcranial direct current stimulation (TSCS)
Poor spatial specificity
Superficial cortical regions
Transcranial ultrasound stimulation (TUS)
- Ultrasound pressure waves sent through the skull (> 100 kHz)
- Focused to target a small region with large pressure variations
- Mechanical stimulation of the neuronal cellular membrane can increase or decrease its excitability (exact cellular mechanism still unknown)
Evaluation of
