Lecture 5 - Techniques in Neuroscience Flashcards
Multi-unit recording
Composite electrical activity of groups of neurons (e.g., fiber pathway)
Multi Units Recording in Humans Subdural Recording
Used in drug resistant patient epilepsy
o Treatment = remove area of the brain causing seizures
o Add electrode in surgery to see activity when normal activity carried out
Electrode on top of brain to get activity of brain -> each electrode seen in diagram
EEG recording
Electroencephalogram
Low frequency, composite electrical activity of unspecified origin at select brain regions
Measures activity of large numbers (populations) of neurons
Non-invasive and painless
Electrodes measure voltage-differences at the scalp in the microvolt (μV) range
Voltages are recorded with millisecond resolution -> very high temporal resolution
Low spatial resolution
Why Use it?
o Detecting changes in brain patterns
Can average several readings to obtain evoked potential
Often used for detecting epilepsy and other brain disorders (sleep disorders)
Signals reflect global brain states
Event Related Potentials (ERPs)
o An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event
o Remove background EEG brain activity through trial averaging -> only ERP left
What an ERP waveform tell us?
o Neural basis of processing is provided by the difference in activity
Cognitive tasks, somata sensory stimulation, motor stimulation etc.
Altitude and frequency
Brain imaging techniques
Neural activity consumes oxygen and generates electrical signals -> more blood to active region
PET -> blood flow in a region
fMRI -> blood oxygenation
Brains reaction slow (several seconds) -> functional imaging = poor temporal resolution
Detailed imaging -> good spatial resolution
CAT scan -> more invasive (need tracer) -> more expensive
Regional CBF (Cerebral blood flow)
Cortex is densely packed with blood vessels
Distribution of blood vessels mirrors neuronal organisation
Evidence that blood flow is highly locally regulated, potentially at columnar level
When area involved in task -> activated -> blood flow
PET
POSITRON EMISSION TOMOGRAPHY (PET)
Measures local blood flow (metabolic response)
Form of hemodynamically-based functional brain
Subject given a radioactively tagged substance (e.g.,sugar) -> used like that substance in respiration to see activity
Radioactive substance (tracer) -> isotope of substance in body: oxygen, glucose, nitrogen, fluorine
Emits gamma rays as decays in the body -> scanned for
Infer higher regional cerebral blood flow in areas that emit many positrons
Tracer takes up to 30 seconds to peak
Contrast in level of glucose being used in different areas
Red = more activity as more glucose used for respiration -> energy for activity
Use in Dementia (Alzheimer’s and Frontal lobe – 2 types) patients
o Reduced activity in certain brain areas -> behaviour changes, memory loss (dependent on exact areas effected)
o The image shows two types of Dementia (Alzhemers and Frontal lobe), in comparison to normal brain
o Normal brain -> High activity across brain (carrying out daily activity)
o Alzheimer’s Dementia -> reduced activity in temporal and parietal loves (memory loss among other symptoms)
o Frontal Lobe Dementia -> less activity in frontal lobe (behavioural changes and other symptoms)
Strengths and limitations of PET
Strengths
o Relatively direct and easily interpretable measure of brain metabolism
o Was the primary technique for functional imaging
Limitations
o More invasive
o Tracer to see differences (radium) (exposing to radiation)
o Not used much -> MRI better now due to progress
o Tracer to brain -> when activity in brain -> uses tracer like sugar
o Spatial resolution very very poor
o No temporal resolution
o Access to a cyclotron
o Expensive and difficult to maintain cyclotron
Now MRI used more in its place (due to limitations)
fMRI
FUNCTIONAL MAGNETIC RESONANCE IMAGING
Measures local blood flow
Does not use radioactivity, but directly measures the concentration of deoxyhaemoglobin in the blood
This is called the BOLD response (Blood Oxygen Level Dependent contrast)
The change in BOLD response over time is called the haemodynamic response function and it has a number of distinct phases
The Haemodynamic Response Function peaks in 6–8 seconds and so this is the temporal resolution of fMRI
Uses a large magnet
o Magnets
Magnetic field strength (Tesla (T)/ Gauss (G))
1 Tesla = 10,000 Gauss
Earth’s magnetic field: 0.5 Gauss
Refrigerator magnet: 200 Gauss
o MR magnet strengths: 1.5 – 9T (30k-180k times the Earth’s magnetic field)
o Uses superconducting magnets
o Completely safe but must remove magnetic object
How it works
o Increase neural activity induced by external stimulus (e.g. scene showed to participant)
o Neurones need energy -> oxygen -> increased blood flow to area -> less oxygenated blood around that area as needed to be taken into that area of the brain to respire for energy
o Level of boldness dependent on the ratio of oxy to deoxy blood
Strengths and limitations of fMRI
Strengths
No contrast/ tracer needed
Spatial resolution good
Real time movement
Limitations
Must have energy (sugar or oxygen) for brain activity
Loose temporal resolution
Lacks fine spatial and temporal resolution
Measures signals related to the amount of oxygen in brain regions
NOT a direct measure of neural activity
IS related to neural activity but the precise link between
Cerebral Blood Flow and neural activity is not yet known
Facial processing areas in brain
Brain areas activate by male/ female faces
What areas responding to types of faces
Present image to patient whilst in scanner -> syncronise recording
Look at changes in ratio in brain areas, triggered by image
Can veiw which areas of the brain activitated when veiwing faces -> not all activates in an area
Ocipital cortex stongly activiated
Can see specifically which areas -> not expecting all parts in the lobe to be activated as have different functions
Specific face area -> FFA (frontal facial area), OFA (ocipital face area)
Red/yellow = activated
Small activation
Very specific imaging -> very high spatial resolution
Transcranial Magnetic Stimulation
Brain stimulation technique
Use magnet, not for recording, but to stimulate the brain
Maget (coil) placed near different brain areas -> doesn’t have to remove skull, can just be near by the participants head
Magnetic field generated -> cause temporary lesion in certain brain areas -> those nearby
Infers function of a region by temporarily removing it -> measuring effect
No after-effects
Recruit lab model of a brain leasion using this method
Can trigger seizure in epileptic patients
How does TMS work
Coil contains a wire carrying an electric current
Rapid change in the current creates a magnetic field
The magnetic field induces a current in the nearby neurons (causing them to “fire”, i.e. generate action potentials)
Electrical current through brain -> disrupts the cognitive function -> discrupts neurons -> Pressure caused mimics leasion
Single Pulse TMS Repetitive TMS (rTMS) Longer-lasting effects. The mechanism is not clear but likely to reflect changes in synaptic efficacy
Very useful in patients with depression -> trying to use this as a treatment
TMS localisation
Can only stimulate areas near the cortical surface
Spatial resolution difficult to determine, ~10-20 mm
Activates neurone population not single as too big
