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Kog.Neuro. (Fach) / Neuroimaging Techniques (Lektion)
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- purpose of neuroimaging techniques -determine how brain activity relates to cognitive processes (eg memory, vision, emotions...)
- how many neurons/ synapses? around 100 billion neurons, communicating via around 250 trillion synapses!
- Electroencephalography (EEG) -detects the opposite volume current, generated by electric current flowing through an apical dendrite of a large pyramidal neuron -flow of volume current depends on the tissue conductance -skull has low conductivity, so on the scalp we can only observe low amplitude currents (around 50 micro volt) -> if you want to detect them >> at leat around 10,000 pyramidal neurons with similarly orientated dendrites need to fire at the same time within close proximity to the skull
- Why does EEG detect mainly pyramidal cell activity? -pyramidal cells are the only cell type with many local unidirectional dendrites
- How does EEG measure? -via an EEG electrode x and an EEG reference electrode, the difference amplifier measures the EEG signal for electrode x -due to high temporal resolution, we can measure the Event-Related Potential (ERP)
- What means the inverse problem in connection to EEG? We cannot know what causes the brain activity !
- What are the advantages of the EEG? -high temporal resolution -relatively inexpensive -non-invasive/ suitable for nearly any type of participant! -portable!
- What are the disadvantages of using EEG? -low spatial resolution ->> inverse problem/ volume conduction effects -limited sensitivity to activity in deep brain structures -fiddly to setup
- What are volume conduction effects? When measuring activity via EEG, you have to consider the effects which will appear, because the electric potentials you want to measure are in relative distance to their source generators!
- Intracranial Electroencephalography (Intracranial EEG) -electrode grid is placed inside the skull of epileptic patients, to identify the source of seizures
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- Advantages of Intracranial EEG -high temporal AND high spatial resolution -EEG currents have higher amplitude
- Disadvatntages of intracranial EEG -invasive! only in epileptic patients -partial brain coverage -severely limited access -non-typical neural tissue
- Magnetoencephalography (MEG) -measures the magnetic field evoked by potentials in the apical dendrites pf pyramidal neurons -measured via a helmet, which consists of a lot of squids, which localize the magnetic potentials via feedback coils
- Advantages of MEG -high spatial resolution - no volume conduction effects! :) -not fiddly to setup - put on the helmet, sit in the chair
- Disadvantages of MEG -even less sensitive to deep sources (compares to EEG) -blind to radial sources -not portable -very expensive (the scanner as the device, plus it uses a lot of helium)
- Functional magnetic resonance imaging (fMRI) -this techniques uses the MRI scanner, which creates high resolution 3D pictures of the brain, by measuring the BOLD response -intensity of the pixel (voxels) is influenced by A) hydrogen levels and B) local magnetic field homogenity
- What is the BOLD response? BOLD response = Blood Oxygen Level Dependent response -when neurons are active, they need more oxygen. Therefore active neurons cause nearby capillaries to dilate, which increases blood flow and blood volume. -in turn,the concentration of deoxygenated blood cells decreases
- Advantages of fMRI (3T) -high spatial resolution -full brain coverage -non-invasive/ for nearly all kinds of participants -
- Disadvantages of fMRI (3T) -poor temporal resolution (BOLD response is very slow response) -BOLD response sums up in large draining veins ->> compromises specificity of spatial resolution ->> misallocating effects -very loud! -very expensive
- fMRI - high field strength (7-9T) ->> Advantages -even higher spatial resolution -less prone to misallocating effects due to large draining vains -higher signal-to-noise-ratio (SNR)
- fMRI - high field strength (7-9T) - Disadvantages? -image distortion possible, due to greater magnetic gield inhomogeneity -even more expensive!
- Functional Near Infrared Spectroscopy (FNIRS) -also measured BOLD response -but via increased near infred light reflection of oxygenated blood cells through the skull!
- Advantages of Functional Near Infrared Spectroscopy (FNIRS) -portable -cheap -suitable for infants
- Functional Near Infrared Spectroscopy (FNIRS) - Disadvantages -lower spatial resolution -cannot detect deep sources
- Positron Emission Tomography (PET) -predecessor of fMRI -participants are injected with radioactive tracer (fluorodeoxyglucose), which emmits gamma rays, which the PET can detect -concentration is greater in tissues with larger blood volume
- Why is fMRI better than PET? -higher spatial resolution! -does not require radioactive material! -actually, also cheaper!