EEG stands for “electroencephalography”. This is a technique for recording “brain waves”, or more technically the electrical activity produced by the brain. EEG does this using electrodes that are placed on the scalp. Measuring electrical activity from the brain is useful because the brain uses electrical impulses to operate – electrical signals travel along brain cells (neurons). The basic way that information is processed by the brain is by very complex connections (networks) of neurons communicating with each other using electricity.
ERP stands for “event-related potentials”. This is a specific use of EEG in which we look at brain waves that are generated in response to particular events, such as seeing a word or picture on a computer screen. Within the first second of seeing a stimulus, like a word or a picture, the brain generates a series of “blips” in the ERP brain wave that indicate the activity of different brain systems that are involved in perceiving and making sense of that stimulus. By looking at the timing of these blips (technically called “components”) we can gain a better understanding of how different types of information are processed, and when. One of the biggest advantages to EEG/ERP is the ability to see brain activity as it unfolds in real time, at the level of milliseconds (thousandths of a second).
One of the big disadvantages of EEG/ERP is that it’s hard to figure out where in the brain the electrical activity is coming from. By putting lots of electrodes all over the scalp (in our lab we use 64 or 128 electrodes), we can get some idea of where the ERP components are strongest. This doesn’t really tell us where in the brain the signals are coming from, but it can be useful in telling us whether two ERP components come from the same place or not. For example, let’s say we want to know if different parts of the brain are used when you see a picture of an apple, and when you read the word “apple”. Maybe the ERP components to the picture and the word are biggest at the same time, but they are biggest over different parts of the head. This would tell us that different parts of the brain were being used.
Modern analysis techniques can give us a better clue to where in the brain ERP signals are coming from. Using lots of electrodes and complex models of the shape of the head and the brain, these techniques allow us to make a guess as to where in the brain the components are generated. However, this is still a “best guess” and lots more work is needed to improve these techniques.
The NeuroCognitive Imaging lab is equipped with three EEG/ERP recording systems:
One is a 128 channel Electrical Geodesics GES-250 system. This system differs from traditional EEG systems in that rather than using metal disc electrodes and goopy gel, the electrodes are embedded in sponges that are soaked in an electrolyte solution. It uses high input impedances, which means that it’s very quick to put on. An experienced technician can get the “net” on in 10-15 minutes, whereas putting on 128 electrodes in low impedance systems takes about 60-90 minutes. Wearing the “net” is also very comfortable. NetStation software is available for data recording and analysis.
Our second system is a 64 channel system with Brain Products ActiCap electrodes. This system is very compact and portable, which makes it ideal for studies where it’s easier to go to research subjects than to have them come to us. Another advantage to this system is that the “active” electrodes are much less susceptible to interference from electrical noise in the environment, such as that generated by lights and computer monitors. EEG is often recorded in a specially shielded booth to avoid this contamination, but with the actively shielded electrodes there’s no need for this.
Our third system is a 64 channel ActiChamp system also from Brain Products. Uniquely, this system can be set up as a pair of 32 channel caps, which allows us to simultaneously record brain activity from two people as they engage in a conversation.
NCIL is equipped with an RF-shielded, sound attenuating booth for EEG recording, made by Eckel Industries of Canada. This is where the EGI system is located. It’s very quiet in there, which is nice for minimizing distractions as well as for the occasional nap.