Electroencephalography (EEG) and event-related potentials (ERPs) with human participants

Abstract

Understanding the basic neural processes that underlie complex higher-order cognitive operations and functional domains is a fundamental goal of cognitive neuroscience. Electroencephalography (EEG) is a non-invasive and relatively inexpensive method for assessing neurophysiological function that can be used to achieve this goal. EEG measures the electrical activity of large, synchronously firing populations of neurons in the brain with electrodes placed on the scalp. This unit outlines the basics of setting up an EEG experiment with human participants, including equipment, and a step-by-step guide to applying and preparing an electrode cap. Also included are support protocols for two event-related potential (ERP) paradigms, P50 suppression, and mismatch negativity (MMN), which are measures of early sensory processing. These paradigms can be used to assess the integrity of early sensory processing in normal individuals and clinical populations, such as individuals with schizophrenia.

Figure 1

Schematic overview of EEG setup. Stimulus generation computer outputs stimuli (in this example auditory) to the participant via the stimulus amplifier. EEG data collected by electrode cap on participant’s head is relayed to EEG amplifier and integrated with information about timing of stimulus presentation. Amplified EEG data with stimulus markers is then relayed to acquisition computer for data analysis.

Figure 2

Snapping in electrode. Place electrode over white plastic adaptors on electrode cap, with leadwire placed in the narrow opening between raised plastic ridges. Use a narrow tool such as the wooden end of a cotton swab or an old ink pen to firmly snap electrodes into electrode collar, pushing towards narrow side of adaptor as indicated by arrow. Figure used with permission from Easycap GmbH.

Figure 3

Schematic of EEG amplifier. Specifics will vary according to manufacturer, but will include plugs for EEG channels to be measured. Plug the electrode from the corresponding channel position on EEG cap into the appropriate plug. Some amplifiers will come with channels already labeled, some may need to be labeled by hand. In this diagram the top row of plugs are labeled as an illustration, with the plugs for the Ground, FP1, CZ, REOG and LEOG channels.

Figure 4

Minimizing impedances. Insert wooden end of a cotton swab into center of electrode and gently push hair to one side. You may need to lift electrode and adaptor away from scalp a bit to ensure that scalp is clearly visible. Figure used with permission from Easycap GmbH.

Figure 5

Insertion of foam eartips. A. Roll eartips between fingers to form narrow cylinder prior to insertion. B. Proper insertion of eartips is easier if the ear canal is straightened and enlarged by gently pulling the outer ear (pinna) outward and upward during insertion. Pull the pinna gently but firmly, usually in the direction the ear extends from the head. C. An example of a well inserted eartip, with no foam visible outside of ear. D. An example of an eartip with a shallow insertion. Figures A and B represent insertion of foam eartips and are provided by 3M, Indianapolis, IN. Figures C and D represent E-A-RLink©, a trademark licensed to, and a product manufactured by 3M Occupational Health and Environmental Safety Division, Indianapolis, IN.

Figure 6

Example P50 waveforms. Overlay of neural responses to first click stimulus (black line - First P50) and second click stimulus (grey line - Second P50). Shaded area indicates range of P50 component. This example shows robust suppression of the P50 wave from the first P50 to second P50.

Figure 7

Example EEG trace generated in response to 2 auditory clicks (50 ms duration), one at 0 ms, and the other at 500 ms. Time points relevant to computing P50 suppression ratio are labeled. For 1st peak, point of maximum amplitude between 40 and 80 ms following presentation of first click (at 53.09 ms), and amplitude of negative-going portion of EEG trace immediately prior to that peak (at 35.0 ms). For 2nd peak, point of maximum amplitude between 540 and 580 ms following presentation of first click (at 551.07 ms), and amplitude of negative-going portion of EEG trace immediately prior to that peak (at 539.04 ms).

Figure 8

Schematic for mismatch negativity (MMN) ERP design. Participants are presented with stimuli consisting of primarily “standard” stimuli (90% of trails; grey box labeled “S” in figure) with rare, “deviant” trials interspersed on 10% of trials. ERP waves to Standard (solid black line) and Deviant (dashed black line) are calculated by averaging EEG responses to each stimulus type. Subtracting the Standard ERP from the Deviant ERP yields the MMN-P3a waveform (dashed grey line).

Figure 9

MMN/P3a waveforms for healthy volunteer participants (dashed grey line) and schizophrenia patients (solid grey line). Compared to healthy individuals, schizophrenia patients have reduced amplitude of the MMN and P3a waveforms. This difference is though to be indicative of disrupted early auditory processing in this population.

Figure 10

Example of EEG waveforms when participant has blinked eyes. Sharp spike in activity at SEOG electrode is an eye blink.

Figure 11

EEG trace with blinks and muscle tension. Sharp spikes in activity at SEOG electrode are blinks, and thick black traces at all electrodes shown (though most prominent at SEOG and CA) are periods of muscle tension.

Figure 12

EEG trace with clipping. Flat line at SEOG electrode is an example of electrode clipping.