Event-Related Brain Potentials (ERPs)

 
 
 

   The 100 billion or so neurons in the human brain communicate by generating small electro-chemical signals. If the probes from an instrument for measuring electrical energy (such as a voltmeter) are placed near such a brain cell it will register a voltage change whenever the neuron is active. Neurons can be active or generate potentials up to several hundred times per second. Although these electrical potentials are relatively small and cannot be monitored individually from a distance, because there are a lot of neurons and because neighboring neurons frequently are active close together in time, the behavior of the group can be measured with probes placed at some distance from the group -- in fact, since 1929 it has been known that groups of neurons can be monitored with probes (called electrodes) placed on the scalp, outside the brain!

    The on-going electrical activity of the brain measured from scalp electrodes is called the electroencephalogram or EEG.   The EEG is present from before birth until death. In fact, in some places death itself is defined by the absence of an EEG. The EEG is usually monitored using a device called a polygraph and is displayed as continuous changes in voltage over time. In a typical EEG session a number of electrodes are attached to the scalp of the subject -- either by glue or, more commonly these days, by wearing an elastic cap (electro-cap). The electrodes are then plugged into the polygraph and the EEG is displayed on a moving sheet of paper or on a computer screen. The EEG is used clinically to help diagnose brain disorders such as epilepsy and sleep disturbances. It gives relatively good information about the general mental state of the individual. Changes in state are associated with a change in the amplitude and frequency distribution of the EEG. For example, alpha waves are 7 to 12 Hz relatively large amplitude EEG waves that are associated with a relaxed but awake state, while beta waves are 13 to 20 Hz waves of lower amplitude than alpha, which are associated with a more alert or "active" mental state. Unfortunately, while the raw EEG can distinguish between such gross changes in state, it has not proven to be specific or sensitive enough to distinguish between more fine-grained changes in mental activity. For example, the EEG looks very much the same whether a person is reading, listening to music or watching TV. Therefore, raw EEG has been of limited use in studying moment-by-moment human cognitive activity (note however, that more recently, progress has been made in using the EEG signal to study more fine-grained cognitive processing -- see e.g., Hald, et al).

    Buried within the EEG is a signal which is more revealing about information processing in the brain. This signal can be obtained by time-locking the recording of the EEG to the onset of events such as a person reading a word on a computer screen, listening to a musical note played on an instrument, or viewing a picture in a magazine. The resulting activity is called an "event-related potential" (ERP), which can be readily distinguished from the raw or background EEG by its more consistent morphological structure (shape).  Unfortunately, ERPs are of relatively small amplitude, measuring from less than 1 to as many as 10 microvolts (a microvolt is a millionth of a volt). This is in comparison to the background EEG which can be from 10 to 100 microvolts. As a result of this size disparity ERPs cannot be readily seen in a raw EEG tracing! One point to keep in mind is that the raw EEG is made up of all brain activity (visible at the scalp) at a particular point in time, while the ERP is that part of the activity associated with the processing of a specific event.

    ERPs are usually obtained in a specialized laboratory consisting of a set of physiological amplifiers and one or two computers.  The subject has a number of electrodes affixed to the scalp (electro-cap) which are in turn connected to the physiological amplifiers. The subject is then exposed to a number of stimulus events (e.g., words displayed on a computer monitor) while their EEG is recorded or digitized by a computer.  To visualize ERPs the experimenter must use signal processing techniques to eliminate non-event activity. Typically this involves recording EEG activity time locked to multiple presentations of the same or similar events and then averaging these tracing together. The averaging process tends to decrease the influence of random activity (i.e., the background or non-event related EEG) while maintaining the consistent event-related activity. With enough repetitions (trials) the ERP emerges and the contribution of the background EEG subsides. The number of trials necessary to obtain an ERP depends on a number of factors, the most important being the "signal-to-noise ratio", that is, the relative size of the signal (the ERP) relative to the size of the noise (the background EEG). In cognitive experiments 30 to 50 stimulus presentations are typically required to obtain a good clean average ERP.

    ERPs can be record from all of the primary sensory modalities (visual, auditory, somatosensory and gustatory) and from motor events (e.g., a button press). Morever, they can be recorded from multiple locations on the scalp. While there are formidable challenges to determining the location within the brain from which ERPs emanate, recording from multiple sites does afford some information on the locus of the underlying relevant brain systems.

    By convention ERP researchers break down ERP waveforms into several basic parts or components. Components are the positive and negative-going fluctuations that can be seen in any ERP waveform. Viewed on different time scales one can see that the ERP is a rich source of temporal information. In general, the components that occur prior to 100 ms are thought to reflect information processing in the early sensory pathway. For example, the auditory brain stem ERP arises from neural impulses traveling from the cochlea through auditory brain stem centers, while the middle latency components seem to reflect activity in the thalamus and possibly the earliest cortical processing. Cognitive scientists have been most interested in the so-called long-latency ERP components: which include the P1, P2, N1, N2, N400 and P3 components. These components are named by there polarity (N for negative) and either their ordinal position after stimulus onset (P1 is the first positive peak), or their latency after stimulus onset (the N400 is a negative-going component peaking at 400 msec). In general, the long-latency components occurring prior to 200 msec are thought to reflect late sensory and early perceptual processes while those after 250 milliseconds or so are thought to reflect higher level cognitive processes (e.g., memory and language).

    ERP researchers tend, for convenience sake, to identify the positive and negative fluctuations in the overall scalp ERP as the actual components themselves. This is, to some degree, misleading. Any given ERP waveform recorded at the scalp is actually the summation and cancellation of neural activity from a large number of neural generators from a number of different brain regions.

For a review of language sensitive ERPs see:

Osterhout & Holcomb