Techniques in Neuroscience Flashcards
what does BOLD stand for?
Blood Oxygen Level Dependent contrast
what does CBF stand for?
Cerebral blood flow
define capillary
The smallest blood vessels in the body
define cyclotron
A particle accelerator used to create radioactive isotopes for PET
define EEG
Electroencephalography
Define ERP
event related potentials
define FMRI
functional magentic resonance imaging
define metabolism
the set of biochemical reactions that serve to sustain life, growth and development, especially relating to the delivery of energy.
define PET
Positron Emission Tomography
define TMS
Transcranial magentic stimuluation
define Tesla
Tesla
what is electrophysioology
Single-unit recording studies the electrical activity of individual neurons
Multi-unit recording studies the composite electrical activity of groups of neurons (e.g., fiber pathway)
the branch of physiology that deals with the electrical phenomena associated with nervous and other bodily activity
EEG recording studies the low frequency, composite electrical activity of unspecified origin at select brain regions
We lose specificity- can’t pinpoint
what is single unit recording
Single-unit recording is the use of an electrode to record the electrophysiological activity (action potentials) from a single neuron
An electrode is introduced into the brain of a living animal
It detects electrical activity generated by the neuron adjacent to the electrode tip
The animal is usually immobilized, but awake Single- single to noise ratio too loud to pick up AP
Enable researchers to understand how individual neurons code information
No pain receptors in the cortex
Hasegawa et al., 2015
what is a mirror neurone
Neurones activates both while observing and while performing:
Actions such as grasping
Emotions such as sadness
Watching someone receive painful events such as shocks
Thus, our capacity for empathy– the ability to share another’s emotions and feelings –is based in a simple ‘mirroring’ mechanism implemented by the human MNS that allows us to use the same neural resources to represent states of the self and others in an overlapping way”
what is EEG
Electroencephalogram
EEG measures the activity of large numbers (populations) of neurons
EEG is non-invasive and painless
Electrodes measure voltage-differences at the scalp in the microvolt (μV) range
Voltages are recorded with millisecond resolution
Detecting changes in brain patterns
Can average several readings to obtain evoked potential
Often used for detecting epilepsy and other brain disorders (sleep disorders)
evaluate EEG
Terrific temporal resolution: 1msec recording
Spatial resolution is poor L
what are event related potentials?
Large background oscillations of the EEG trace make it impossible to detectthe evoked response from a single trial
By averaging from hundreds of trials, the background EEG is removed, leavingtheevent-relatedpotentials
Event Related Potentials (ERPs)
An event-related potential(ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event
what does an ERP wave tell you?
info about the neural basis of processing is provided by the difference in activty
how do brain imaging techniques work?
Neural activity consumes oxygen as well as generating electrical signals
In order to compensate for increased oxygen consumption, more blood is pumped into the active region
PET measures the blood flow in a region, whereas fMRI measures the blood oxygenation
The time taken for this response is slow(several seconds) and so functional imaging has a poor temporal resolution, but a good spatial resolution
This is the complementary profile to EEG
why do we need changes in blood flow?
The brain consumes about 20% of energy of body
The brain receives about 20% of blood flow
Why do we need changes in blood flow?
Neural activity increases global blood flow marginally, but dramatically locally (up top 25% change!!!!)
basis of modern neuroimaging techniques
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
what does regional CBF mean?
regional cerebral blood flow
what does PET measure?
Form of hemodynamically-basedfunctional brain imaging
POSITRON EMISSION TOMOGRAPHY (PET) Measures local blood flow (metabolic response)
§ subject is given a radioactively tagged substance(e.g., sugar)
§ compute the locations of decaying radioisotopes that emit gamma rays
§ infer higher regional cerebral blood flow in areas that emit many positrons
§ Tracer takes up to 30 seconds to peak!!
what are the limitations of PET?
PET provides a relatively direct and easily interpretable measure of brain metabolism and it was the primary technique for functional imaging BUT… it has many limitations: Limited spatial resolution! Has no temporal resolution! Have to expose humans to radiation! Access to a cyclotron
how does fMRI work?
§ Does not use radioactivity, but directly measures the concentration of deoxyhaemoglobinin 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
how does the magnetic field work?
Magnetic field strength is measured in Tesla (T) or Gauss (G)
§ 1 Tesla = 10,000 Gauss § Earth’s magnetic field: 0.5 Gauss § Refrigerator magnet: 200 Gauss § MR magnet strengths: 1.5 –9T (15,000 G –90,000 G or 30,000-180,000 times the Earth’s magnetic field)
§ Uses superconducting magnets
Uses superconducting magnets
The magnetic field
These are strong magnetic field!
what does it mean that a brain area is active?
The brain has a constant supply of blood and oxygen; if it didn’t, it would die!
This means we cannot literally stick someone in a scanner and read their thoughts (because the whole brain would look active)
In order to infer functional specialization, one needs to compare RELATIVE differences in brain activity between two or more conditions
A region is “active” if it shows a greater response in one condition relative to another
If the experimenter chooses inappropriate conditions the regions of activity will be meaningless (junk in, junk out) –functional imaging isn’t straightforward!
what are the limitations to fmri?
fMRI lacks fine spatial and temporal resolution
Measures signals related to the amount of oxygen in brain regions It is NOT a direct measure of neural activity It IS related to neural activity but the precise link between Cerebral Blood Flow and neural activity is not yet known Caution should be taken in the interpretation of fMRI measurements with respect to brain activity Very powerful technique with great promise
what techniques can be combined
fmri and EEG
outline TMS
Transcranial Magnetic Stimulation (TMS) Uses electro-magnetic induction via coils to temporarily disrupt cortical function
§ Infer the function of a region (or cognitive mechanism) by removing it and measuring the effect on the rest of the system
§ For example, if damage to a region disrupts reading, but not speaking or seeing, then one might conclude that the region is specialized for some aspect of reading
§ Disruption of brain function comes about through natural damage (strokes, etc.), elicited damage (e.g. animal models), or harmless temporary changes induced electro-magnetically (TMS)
Transcranial Magnetic Stimulation (TMS) • Can impair or facilitate local processing (mechanism not well understood) • Temporary “virtual lesions” effects can last from msec to min brief effects are useful for studying timing of processes in different brain areas • thought to be safe for most subjects some exclusions, e.g., people with epilepsy
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how does TMS work?
I MS: now does It work? Coil contains a wire carrying an electric current
• A rapid change in the current creates a magnetic field ROYAL HOLLOWAY UNIVERSITY
• The magnetic field induces a current in the nearby neurons (causing them to “fire”, i.e. generate action potentials
) This disrupts the cognitive function that they may be doing at that point in time virtual lesion) the cornputer passes a current through a device called the coil two inner copper windings create a magnetic field when a current is applied the coil gives short 50Hz bursts 5 times per Second magnetic pulses stimulate electric current in the brain
this causes cortical neurons under the •te of stimulation to depolarize and charge an action potential • Strong current sent through the coil magnetic field • Passes through scalp and skull creates electrical current in brain underneath coil which disrupts neurons Single Pulse TMS Repetitive TMS (rTMS) Longer-lasting effects. The mechanism is not clear but likely to reflect changes in synaptic efficacy
can only stimulate areas near the cortical surface spatial resolution difficult to determine, —10-20 mm ROYAL HOLLOWAY UNIVERSIT
sumerise TMS
Enables the induction of temporary ‘lesions’ in healthy subjects
§ Single-pulse TMS has produced seizures in patients, but not in normal subjects
§ rTMS has caused seizures in patients and in normal subjects
§ Possible therapeutic applications
