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Electroencephalography and Event-Related Potentials 89
The ERP waveform of voltage plotted against post-stimulus time consists of a series of
positive and negative peaks; these are typically compared to a pre-stimulus baseline.
Voltages are thus only negative or positive with respect to the baseline. The ERP peaks
are typically labelled according to their polarity (negative [N] or positive [P]) and their
latency in milliseconds relative to stimulus onset (e.g., N100, P230, P300). Occasionally,
peaks are designated by their polarity and ordinal position in the waveform (e.g.,
N1, P1, and N2). Sometimes, the labels denote a functional description (e.g., mismatch
negativity), refer to its presumed neural generator (e.g., auditory brainstem response) or
its most reliable scalp location (e.g., left anterior negativity). Particular ERP “components”
denote that, in response to an external stimulus, different parts of the nervous
system with different function are involved at different time points. Thus, different
temporal intervals of the waveform likely reflect different anatomical locations and
different functional processes. Moreover, any particular interval may reflect more than
one underlying process.
Usually, the amplitudes, latencies, and scalp distributions of the earlier ERP components
(with latencies up to 100 ms) are highly reproducible across sessions within an
individual (Halliday, 1993). Systematic variations in the physical parameters of the
evoking stimulus (e.g., intensity, frequency, duration) lead to predictable changes in
these early components reflecting the altered activation of sensory pathways. Hence, the
earlier evoked components are considered to be “exogenous” or stimulus bound. For
research on cognitive processes, the more informative brain waves are the “endogenous”
components. The relative insensitivity of endogenous components to variations in
the physical stimulus parameters contrasts with their exquisite responsiveness to task
demands, instructions, and subjects’ intentions, decisions, expectancies, strategies, etc.
Thus, the same physical stimulus may (or may not) be followed by particular endogenous
components depending on how the subject chooses to process it. Endogenous ERP
components are not “evoked” by a stimulus but elicited by the perceptual and cognitive
operations that are engendered by that stimulus.
EEG pre-processing: ERPs are so small that an artefact in a single trial may influence
the appearance of the average. Time-linked artefacts may be emphasized by averaging.
Thus, it is necessary to remove these artefacts before averaging. There are at least three
different strategies to remove artefacts in EEG data: [1] filtering, [2] removing particular
ICA components that represent artefacts, and [3] rejecting artefact-loaden trials.