Summary Functional Imaging
Biomedical Sciences Master Course
Radboud University, Nijmegen
Made by: Georgia Graat
,There are 100 billion neurons in the brain which thousands of connections between these. Each
neuron connects to about 10.000 others. The brain represents only 2% of the total body weight, but
receives 15% of the cardiac output, 20% of the total body oxygen consumption, and 25% of the total
body glucose utilization. The neural activity thus consumes a ton of energy. Especially the energy
demand of the glutamatergic neurons accounts for 90% of the total cortical glucose usage.
The main aim of functional imaging is mapping
information about processing of information in the
structure of the human brain onto the physical
organization of the brain. Some structures are known,
but most information is not pinpointed to one location
yet. In any type of imaging, temporal and spatial
resolution matter. Spatial resolution determines which
size of the brain is the smallest you can measure.
Temporal resolution determines at which level in time
you can image processes, depending also on how fast
these processes are in the brain.
Electrophysiology
Electrophysiology is using the electrical activity
generated by the nervous system to study neural
computation, cognition, and behavior. Neurons in the
brain have electrical properties. These are made
because of action potentials over neurons that are
made when influx and outflux of certain ion types
happen over the membrane. The movement of ions
produce an electrical activity that moves over the
membrane like a field. This electrical activity is
produced because the ion movement creates a change
in charge over the membrane.
Electrophysiology measures these changes of voltage throughout certain areas of interest. Actually,
every part of the neuronal network and extracellular space produces electrical activity. There are
different places in which you can record this, like inside the brain or on the skull, that all have
different advantages. The nervous system uses electricity to communicate with itself; between
neurons action potentials are fired to send information back and forth. Every time information needs
to be processed or motor output needs to take place, or the brain is thinking of something, electrical
activity in the neurons is generated. It is useful because electrical activity is fast, easy to route,
controllable, and easy to attenuate, so very good in communication. A
lot of energy is used during this process. Electrophysiology wants to
look at these activities and when and where these happen during
specific tasks.
Different techniques are used in electrophysiology to achieve these
same measurements. The output is measured as the change in voltage
and thus the action potentials, which is displayed as oscillations. The
oscillations are strongly rhythmic as the neural activity is like this. The
, amplitude clearly strongly goes up and down over time. These
oscillations can be converted to the frequency they show.
Different classes of neurons actually have different
frequencies they resonate at and because of this have
different names. This is the temporal scale at which
electrophysiology can measure. For the spatial scale, it can
either measure single units of neurons, cortical columns of
neurons, cortical patches of neurons, and large-scale
interactions for connectivity. These different scales are
measured with different electrophysiology techniques.
- Intracellular recording type or juxtacellular recording type
o Inside the cell or next to one cell
o Patch-clamp technique with a glass pipette on the neuron membrane
o Measures the activity of individual neurons (single unit)
- Extracellular recording type
o In the brain among the neurons
o Electrode technique
o Measures the activity of several neurons at the same time, focusing on individual
spiking
- Local field potential (LFP) recording type
o In the brain in extracellular space
o Electrode technique with a 100 electrode array (connects to external
amplifier)
o Measures the sum of potentials coming from the extracellular space, as
the local field potential is the electric potential in the extracellular space
around neurons
- Electrocorticography (ECoG) (or iEEG) recording type
o On the dura
o Electrode technique placed directly on exposed surface of the brain
o Measures electrical activity from the cerebral cortex, so a cortical patch of
neurons
o Photographic techniques are used to see where the electrodes are placed
- Electroencephalography (EEG) or magnetoencephalography (MEG)
o On the skull, only non-invasive technique
o Electrode technique placed on the outside of the skull
o Measures electrical activity from the cerebral cortex, so a cortical patch of
neurons
o Recordings of large-scale electric signals from the nervous
system
Intracellular, extracellular, LFP and ECoG measurements are all invasive. Only
EEG and MEG are non-invasive. The size of the neuronal cluster that is measured
is the lowest in intracellular, and becomes larger at every technique, with being
over 100.000 neurons in MEG and EEG. The spatial resolution (smallest cluster
you can measure) is the highest in the intracellular techniques, and low in MEG
and EEG. The frequency range that we can measure with MEG and EEG is also
quite low, and the higher frequencies are not clear anymore. The invasive
techniques measure much higher frequencies. All have high temporal resolution.
