Global brain activity Flashcards
The electroencephalogram
Basic requirements for signal detection
- whole population of neurones must be active in synchrony to generate large enough electrical field at level of scalp
- population of neurones must be aligned in parallel orientation so they summate rather than cancel out
Synchronous activity
Amplitude of EEG signal partly depends on how synchronous activity of underlying neurones is
Number of active cells, total amount of excitation, timing of activity
Four basic EEG phenomena
The alpha rhythm
The beta rhythm
Theta waves
Delta waves
Generation of synchronous rhytms
A one neurone oscillator
Thalamic cells have set of voltage gated ion channels that allow each cell to generate rhythmic, self sustaining discharge patterns, even in the absence of external inputs
Rhythmic activity of each thalamic pacemaker neurone then becomes synchronised with many other thalamic cells
Functions of brain rhythms
Sensory input- thalamus- cortex
Activity coordination (binding) of different cortical regions (synchrony, oscillations)
Meaningless by product of feedback circuits and connections
Behavioural criteria of sleep
Reduced motor activity
Decreased response to stimulation
Stereotypic postures
Relatively easy reversibility
Three functional states of sleep
Awake
Non- REM
REM
CT scans
Based on amount of x-ray absorption in different types of tissue
Bone absorbs the most, cerebral spinal fluid absorbs the least and brain matter is intermediate
Used in clinical settings
MRI
Completely safe, so people can be scabbed many times
Provides a much . better spatial resolution
Provides better discrimination between white and grey matter
Can be adapted for detecting the changes in blood oxygenation associated with neural activity
Sequence of events in acquisition of an MRI scan
Magnetic fields of protons initially random
Add external magnetic field and some protons align
Brief radio wave pulse orients them to 90 degrees and produces a measurable MR signal
The proton returns back and a new brain slice is scanned
Basic physiology of underpinning functional imaging
Brain makes up to 2% of body weight but consumes 20% oxygen
PET measures change of blood flow to a region
fMRI sensitive to concentration of oxygen in the blood
PET
Based on blood volume
Involves radioactivity
Participants scanned once or few times
Temporal resolution: 30’
Effective spatial resolution: 10mm
Sensitive to the whole brain
Use pharmacological tracers
(BOLD) fMRI
Based on blood oxygen concentration
No radioactivity
Participants scanned many times
Spatial resolution: 1mm
Some brain regions are hard to image
BOLD signal
Blood oxygen level dependent contrast
Signal measured in fMRI that relates to concentrtion of oxy and deoxyhaemoglobin
HRF
Haemodynamic response function
Describes changes of BOLD signal over time
