Lecture 1 - CNS Flashcards
CNS
brain = spinal cord
EEG invention
1924 by Hans Berger
took 5 years before publishing (age of slow science)
lot of backlash initially
already discovered things like alpha waves
committed suicide after his lab was shut down
fMRI invention
1990s
TMS invention
1985
Galvani
first discovered electrical activity moving bodies
in frogs
18th century
Eduard Hitzig
showed stimulation of cortex in dogs
1870
-> direct brain body connection
Caton
spontaneous, ongoing electrical activity in rabbits
1875
EEG measurement properties and origin
extremely high temporal resolution
combined activity of thousands of neurons
measures in distance from actual neurons
voltage
potential of a current to flow
like water pressure
a relative measure
measured in EEG
current
number of charged particles / electrodes / ions
in a given time
the actual flow
cells measured in EEG
pyramidal cells
oriented vertically / perpendicular to surface of head
potential measured in EEG
postsynaptic potential
at the ends of axons
LFP
local field potential
larger difference between net negative and net positive charge
what is measured with EEG
mostly LFP
sometimes also spikes = action potentials
equipotential lines
points along circles where voltage is the same
measurable by EEG
equivalent current dipole
sum of many neurons together
with the same direction
= unidirectional voltage field
= open field
problem with brain folds
local cancellation
closed field
no single orientation of neurons
smearing
distortion and spread of voltages
due to the tissues between brain and device
and volume conduction
what we cannot say about peaks
whether signal comes from excitation or inhibition
the direction of the neurons
orientation of the reference eletrode
where the PSPS comes from, soma or axon
what EEG cannot detect
tangential dipoles
because they don’t have a voltage differencew
topo(graphic) map
colourful map depicting the amplitude/the dipoles around the dhead
over time
plots every 2 to 5 ms
source reconstruction
from topomap to underlying neural activity
inverse problem
which activity leads to the dipoles we measure?
cannot be solved
is underdetermined / ill-posed
because there are too many possible solutions
superposition problem
infers actual dipoles
C1, C2 etc = component one etc
is a generalization
weight matrix
approximates which dipole contributes what to what is measured
problem: often many combinations possible
forward problem
which dipoles will this neural activity create?
easily solvable with sufficient head model
common solutions to the inverse problem
limit number of sources
or limit the position
use other techniques with higher spatial resolution
or data like single cell recordings
ECoG
electrocorticography
directly from brain surface
only during surgery
e.g. when localising sources of epilepsy
MEG
brother of EEG
measures magnetic equivalent of dipoles
with Squid sensors, very large coild
right hand grip rule: thumb is direction, fingers point in direction of magnetic field
MEG advangtages
less blurring because no smearing
slightly better spatial resolution
can measure tangential dipoles
opposite of EEG! Combining both however is not very informative
MEG disadvantages
very expensive, between EEG and fMRI
huge device, huge running costs
same temporal resolution as EEG
signal summation
many sources contribute to the voltage measurement at each point
analogous to local field potential
