EEGs measure electrical signals generated by the brain through electrodes placed on the scalp (ibid). Gel or a conduction solution is used to connect the electrodes to the scalp. Electrical signals are produced by partially synchronized waves of neural activity measured in Δ voltage/time (up to 2000 Hz). Signals are able to amplify the waves of neural activity so that sense can be made of them. Waves themselves represent stages of conscious; different frequencies represent different stages. Most of the time our brain is emitting alpha waves, which are of a regular frequency (8-12/sec), high amplitude and represent relaxed wakefulness. Should the wave amplitude decrease, it can indicate neural activity further from the cortex.
When EEG waves accompany physiological events, they are known as event-related potential (ibid). Event-related potentials are calculated by averaging the signal trails epochs, averaging reduces the noise of surrounding activity and increases strength of the signal.
Advantages of the EEG:
– High temporal resolution (accurate at recording fast changes in neural activity)
– Less subject to motion artifacts
– Not claustrophobic
– Can be used on infants
Disadvantages of the EEG
– Weak spatial resolution
– Synchronous firing of 10K neurons is required to produce a magnetic field which is large enough to measure
MRI: Structural and Functional
MRIs produce high-resolution, three-dimensional images from the measurement of waves that hydrogen atoms emit when they are activated by radio frequencies waves in a magnetic field (Pinel, 2011). High spatial resolution means MRIs are able to detect and represent different spatial locations. The images produced are far clearer than CT scans; however, fMRIs are seen as even greater improvement.
The fMRI produces images that represent increased oxygen flow in response energy needs of specific brain regions. Oxygenated blood has magnetic properties due to its high iron content making it sensitive to magnetic fields emitted from protons in the MRI. Deoxygenated blood is not sensitive to magnetic fields; as such brightly light portions of the fMRI reflect high-energy consumption. If you want to read, more about the BOLD fMRI click here, BOLD stands for blood oxygen level dependent signal. The job of the fMRI is to record this BOLD signal.
Advantages of the fMRI
– Accurately depecits structural data
– Reasonable temporal resolution
Disadvantages of the fMRI
– Noisy (literally, not signal noise)
– Very susceptible to movement artifacts
– No metal-based equipment can be around the machine
– BOLD is not a direct measure of neural activity, only oxygen consumption
The MEG measures changes in magnetic fields on the surface of the scalp (ibid). Unlike the fMRI, magnetic fields are produced by changes in neural activity, which activate pyramidal cells of the cortex. Neural activity is not being affected by magnetic fields.
Advantages of the MEG
– High temporal resolution
– Acceptable spatial resolution
– Compared to an EEG, it is less distorted by the scalp
Disadvantages of the MEG
– Just like the EEG, it requires a high baselines firing rate in order for a magnetic field to be produced
– Normally it has to be paired with an MRI
– Not portable
Positron Emission Tomography (PET)
The PET scan is a bit more controversial than some of the other scans because it involves injecting a radioactive substance. Specifically, 2-deoxyglucose is injected in the carotid artery. This substance is used because of its similarity to glucose, a quality which neurons like very much. Neurons take 2-DG into their system, but cannot metobolise it. The result accumulates in active regions of the brain resulting in measurable levels of radioactivity.
Advantages of the PET:
– Reasonable structural accuracy
– Direct reflection of current activity
– No motion artifacts
– Not claustrophobic
Disadvantages of the PET:
– Radioactive substance is involves
– No temporal resolution and no structural information
– Poor spatial resolution
– Expensive and not very portable
Transcranial Magnetic Stimulation (TMS)
In 1985, Tony Barker invented the TMS, which is now known for its ability to prove a particular brain activity causes certain behaviour. A non-invasive technique, the TMS causes depolarisation and hyperpolarisation of neurons in the brain. Electromagnetic induction causes a weak electrical current in the cortex to evoke synaptic potentials. With the TMS it is possible to create a stimulated temporary lesion of the brain by preventing normal brain function without causing any adverse effects.
Advantages of the TMS
– Almost portable
– Can prove causality
– Can simulate a lesion
Disadvantages of the TMS
– Difficult to specify precise regions of the brain
– Only surface regions are detectable