Event Related Potential Data

Despite the wide variety of questions addressed by ERP researchers, and the many different experimental paradigms employed to answer them, there are several characteristics of ERP data that are seen as common across diverse studies. ERP data consist of the series of positive and negative changes in voltage across the time segment comprising the experimental trial (see Figure 3.1 for sample ERP data). The relative amplitudes and latencies of these voltage changes comprise what are referred to as waveforms (e.g., P300, Nc, etc.), the broad unit of analysis of the ERP (more specific details on the nature and meaning of these waveforms will follow in subsequent sections of this chapter). The length of the averaged time-locked segments of the EEG comprosing the ERP data typically ranges from 1 to 2 seconds (this segment corresponds to an experimental trial) and is determined by several parameters, largely dependent on the type of stimuli, the length of time the stimuli is to be presented, and the amount of time allotted to recording after the stimulus has been presented. However, given that brain electrical potentials are being recorded in "real time" and the relative rapidity of neural responses to quickly presented,

Amplitude (microvolts)

16 12

16 12

PSW

P260

r

n r-

N150 \

J^Nc

-200 0 200 400 600 800 1000 Time (Milliseconds)

1200 1400 1600

-200 0 200 400 600 800 1000 Time (Milliseconds)

1200 1400 1600

figure 3.1. Sample grand mean illustrating event-related potential (ERP) data. Waveforms shown were elicited from 30-month-olds as they viewed face stimuli. Nc: negative central; PSW: positive slow wave.

discrete stimuli, the maximum useful time for recording a given trial typically does not exceed 2 to 3 seconds.

In most ERP investigations, these segments of EEG signal are averaged together for each participant, across multiple trials from a given condition, and each electrode site separately, to make up what are usually referred to as individual averages. Typically, each participant in an ERP experiment will have several individual averages, each representing data collected during one or more types of trials. For example, in a simplified hypothetical ERP experiment where the participant is shown two types of visual stimuli (faces, objects), each participant would have two individual averages, one representing an average of the ERP responses for all of the trials where faces were shown and another for the trials that consisted of objects. Data derived from the individual averages typically make up the unit of analysis in ERP studies. Very generally, for each individual average, the amplitudes (usually reported in microvolts) of the peaks of relevant waveforms are determined through a combination of software algorithm and manual inspection. Likewise, the latencies (i.e., time of occurrence) of the peak amplitudes of the waveforms are also determined. These values are then submitted to appropriate statistical analyses.

Although data at the level of individual averages are utilized for statistical analyses, the grand mean is typically what is illustrated in write-ups of ERP experiments (see Fig. 3.1). The grand mean is a mean of the individual averages from each relevant group or condition. Thus, continuing the hypothetical experiment from above, the authors of this study would most likely include two grand means in their write-up, one illustrating the face condition and one of the object condition. The grand mean is useful in illustrating the morphology of waveforms and, as an average over many participants, is less "noisy" and provides a composite picture of the waveforms elicited by the ERP paradigm.

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