Event-Related Potentials, or “ERPs”
By non-invasively attaching electrodes to the scalp, it is possible to pick up the brain’s naturally-occurring electrical activity. Not all brain activity can be measured this way, but we can obtain signals from large populations of neurons that have the right spatial configuration and that are active at about the same time. Luckily, this is true for many neurons in the cerebral cortex, which is critical for perception, attention, memory, language, and higher cognition.
The on-going electrical activity of the brain is known as the electroencephalogram or EEG. Recordings of the EEG play an important role in clinical contexts, as they reveal information about brain health and states of alertness. For psychological research, however, we are interested in that part of the EEG signal that is specifically related to an event of interest — for example, the appearance of a sensory stimulus or a participant’s response. These are small signals that are hard to see in the on-going EEG. Thus, we use procedures to extract these signals to create Event-Related Potentials (ERPs), i.e., averaged brain activity that is time-locked (lined up in time) to events of interest.
To obtain ERPs, electrodes are attached to the scalp at various locations (at least two) and are connected to amplifiers. The outputs of the amplifiers are converted to numbers by a device for measuring electrical potentials, an analog-to-digital converter. The potentials are sampled and stored for later analysis. Because, as already mentioned, ERPs are small compared to the EEG, analysis generally begins with a procedure to increase discrimination of the “signal” (the activity of interest) from the “noise” (the rest of the EEG). The most common way to do this involves averaging (i.e., adding together and taking the mean of) a number of segments of the EEG that are time-locked (consistently related in time) to the same kind of events (e.g., all EEG segments that immediately follow the presentation of an anomalous final word of a sentence). Because aspects of the EEG that are not time-locked to the event are assumed to vary randomly from sample to sample, the averaging procedure should result in a reduction of the noise, making the signal ERPs visible. The result is a waveform of voltages across time that contains a number of positive and negative peaks, which can then be subjected to a variety of measurement operations. Years of research have helped to link different aspects of this waveform to specific cognitive processes, making ERPs a powerful technique for examining the nature of cognitive and neural processes and tracking these with millisecond level temporal resolution.
In practice, in our lab, ERPs are measured by putting a stretchy cap with a set of electrodes embedded in it on a participant’s head. Additional electrodes are attached with tape to the face and neck. These electrodes make contact with the skin through some conductive gel. Having the cap put on the head takes between 30 and 60 minutes as the electrodes are lined up, the area under each electrode cleaned (a little “exfoliation” treatment) and the gel applied (“moisturization”). During this “spa treatment” participants may study, read, listen to music and/or chat with the experimenters. Most people find the procedure a little strange but not unpleasant. Once the electrodes have been applied, participants move from the preparation area to an experimental booth, where they are seated in front of a computer. The electrodes are attached to the amplifiers, and the participants then spend around an hour doing a task at the computer. Participants are also given the opportunity to see their brain activity as it unfolds. At the end of the experimental task, the electrodes are removed and participants have the opportunity to clean up before leaving the lab. The data will be processed by the experimenters over the next few days or weeks.