Neurophysiological techniques Flashcards
name types of electrophysical neurophysiological techniques:
- direct measurement of neuronal activity
- direct manipulation of neuronal activity
electrophysiological: direct measurement eg.
- ERG (retina)
- EMG (muscle contractions)
- ECG/ EKG (heart)
- EEG
electrophysiological: direct manipulation eg.
- deep brain electrical stimulation
- transcranial magnetic stimulation - (induced electrical activation of neurons)
- optogenetics
functional imaging: features
- measures changes in neural activity indirectly
functional imaging: eg.
- functional MRI (fMRI) whole brain
- f Near-infrared spectroscopy (fNIRS) limited to cortex
- positron emission spectroscopy (PET)
electrocenphalography (EEG): features
- traditional method of recording brain activity
- diagnostic and research uses
- increasingly replaced by imaging techniques (MRI, CT esp for locating physical damage, tumours)
electrocenphalography (EEG): still useful for diagnosis of
- sleep disorders
- epilepsy
- multiple sclerosis
- optic neuropathy (trauma, diabetes)
electrocenphalography (EEG): types of activity
- spontaneuous
- event-related activity
electrocenphalography (EEG): spontaneous activity
- electrical activity that occurs in absence of obvious stimulus or behavioural manifestation
electrocenphalography (EEG): event-related activity
- evoked potentials: electrical activity triggered by specific stimuli (images, sounds)
- induced potentials: electrical activity related to stimulus processing but w variable timing following event
electrocenphalography (EEG): mechanisms
- measures extracellular electrical currents generated by postsynaptic activity
- sum fo synchronous activity (excitation + inhibition) by neurons w similar spatial orientation
- cortical pyramidal neurons -> aligned in radial columns and fire together
electrocenphalography (EEG): volume conduction -> red arrows
- intracellular primary currents generated by synaptic activity
electrocenphalography (EEG): yellow lines
- extracellular secondary currents generated in surrounding tissues
electrocenphalography (EEG): volume conduction features
- summed w secondary currents generated by other neurons nearby
- charge spreads to surface of cortex beneath skull (volume conduction)
- voltage (potential difference) btw areas detected by scalp electrodes (V)
electrocenphalography (EEG): recording set up
- standardised scalp electrode positions to allow comparison
- 10-20 system
- spacing allows each electrode to preferentially a 6cm area of cortex
- other types of recording often performed simultaneously eg. ECG, EKG to remove artefacts or correlate w behaviours
electrocenphalography (EEG): recording compared by
- bipolar recordings = dif btw pairs of electrodes (A)
- referential recordings = dif of each electrode compared to auricle reference (B)
electrocenphalography (EEG): analysis - oscillation frequency of EEG signal indicates
- indicates normal brain state
electrocenphalography (EEG): types of brainwaves
- gamma
- beta
- alpha
- theta
- delta
electrocenphalography (EEG): recording compared by
- bipolar recordings = dif btw pairs of electrodes (A)
- referential recordings = dif of each electrode compared to auricle reference (B)
electrocenphalography (EEG): analysis - oscillation frequency of EEG signal indicates
- indicates normal brain state
electrocenphalography (EEG): types of brainwaves
- gamma
- beta
- alpha
- theta
- delta
magnetic resonance imaging (MRI): technique initial
- relies on quantum properties of H atoms in body, mostly present as water
- nucleus of H atom spins around axis
- orientation is random
- in MRI machine, magnetic fields generated by electromagnets surrounding patient cause all nuclei to line up in one direction
electrocenphalography (EEG): generalised tonic-clonic ‘grand mal’ seizure
A. normal activity before
B. generalised seizure pattern (no obvious focal origin) across all electroe positions
C. waveforms obscured by muscle spasms (motor activity)
D. immediate postictal period - slow waves
E. normal activity resumed
magnetic resonance imaging (MRI): features
- uses strong magnetic fields (3 Teslas)
- non-invasive tomographic techniques
- 3D reconstruction of internal anatomy from virtual sections
- no radiation
magnetic resonance imaging (MRI): technique
- relies on quantum properties of H atoms in body, mostly present as water
A. nucleus of H atom spins around axis
B. orientation is random
C. in MRI machine, magnetic firelds generated by electromagnets surrounding patient cause all nuclei to line up in one direction
magnetic resonance imaging (MRI): technique contd.
- axes now aligned but still wobble (precession)
- separate (resonant) radio-frequency magnetic pulse at 90º
- cause nuclei line up at 90º but wobble more
- when pulse off, precession first becomes dephased = T2 signal
- nuclei revert to og orientation in magnetic field (H) = T1 signal
magnetic resonance imaging (MRI): technique detected by
changes in phase and orientation detected by sensitive coils in MRI machine
magnetic resonance imaging (MRI): eg. frontotemporal dementia (FTD)
- loss of grey matter (spindle neurons)
- extreme changes in personality
- speech/ language problems
- movement disorders
magnetic resonance imaging (MRI): eg. brain tumour (glioblastoma)
- patient suffers headaches
- compression of brain tissue and ventricles
functional magnetic resonance imaging (fMRI): features
- shows anatomical structures but not functional changes in activity
- sequential images in quick succession to show changes in brain activity
functional magnetic resonance imaging (fMRI): most common fMRI technique
- relies on blood oxygenation level dependent (BOLD) contrast imaging
- detects changes in blood flow triggered by localised changes in brain activity
functional magnetic resonance imaging (fMRI): neurovascular coupling
- stimulus causes increase neural activity
- triggers local increase in cortical blood flow (CBF) = haemodynamic response
- delivers more oxygenated blood to active brain region
- and more glucose (not stored in brain) for generation of ATP to drive activity of ion pumps -> maintain ionic conc. gradients
functional magnetic resonance imaging (fMRI): haemodynamic response
- inflow of oxygenated blood displaces deoxygenated blood
- arterial oxygenation usually high anyway
- dramatically increases oxygenated blood in venous vasculature
functional magnetic resonance imaging (fMRI): haemoglobin
- de/oxygenated blood have different magnetic properties related to haemoglobin protein that binds/ carries oxygen
- oxyhaemoglobin (Hb) = repelled by magnetic fields (diamagnetic)
- deoxyhaemoglobin (dHb) = attracted (paramagnetic)
functional magnetic resonance imaging (fMRI): BOLD signal
- dHb interferes w MRI signal, Hb doesn’t
- dHb decreases strength if alot
- fMRI signal depends on ratio of Hb:dHb in venous microvasculature
functional magnetic resonance imaging (fMRI): voxels
- fMRI records images of brain as stack of slices
- each slice comprises of array of cubes or voxels (3D pixel)
functional magnetic resonance imaging (fMRI): data time series
- each slice is scanned repeatedly w set interval = repetition time (TR)
- BOLD signal in each voxel recorded over time
functional magnetic resonance imaging (fMRI): analysis
- images/ slices taken before/ during stimulus presentation
- averaged BOLD signal in each voxel compared btw image sets
- 3D voxel intensity -> 2D pixel brightness -> contrast map show activity level
- projected on to high res MRI image
functional magnetic resonance imaging (fMRI): pitfalls
- single fMRI volume (3D image) contains as many as 130 000 voxels
- multiple comparisons of before/ after
- false positives inevitable unless proper statistical corrections are applied
- 25-40% prior to 2010 didn’t use proper correction methods
