**2. Event-Related Potentials (ERPs)**

The technique of ERPs is a precise tool regarding time resolution (on the order of milliseconds) that incorporates the recording of ongoing electrophysiological activity using

Event-Related Potential Studies of Cognitive and Social Neuroscience 399

Fig. 1. ERP signal-to-noise ratio. A) ERPs at temporo-occipital scalp in response to face stimuli without preprocessing and (B) with preprocessing. Note how the N170 can be clearly

A continuous reconstruction of electrical activity on the scalp, normally based on spatial interpolation of the electrode sites, is termed a topographical map (or a voltage map or topomap). Each component usually has a relatively specic topographic distribution. The so-called long latency components (cognitive components or endogenous components) occur after 100 ms and are sensitive to changes in cognitive processing, as the meaning of the stimulus, or resources of processing required in the task performed (Hillyard, 2000). In

Eason et al. (1969) found that visual stimuli situated in visual fields with focused attention elicited components with larger amplitude (approximately 100 ms after stimulus onset, P1 and N1), compared with ignored or unnoticed stimuli. This amplitude enhancement is at its maximum in the temporal-occipital region, contralateral to the localization of the stimuli and is sensitive to the specific localization of the stimuli in the visual field (Mangun et al., 1993). Comparable results were obtained in the auditory modality by a dichotic listening paradigm (Hillyard et al., 1973). This auditory early-attention effect reflects a response increase of the auditory primary cortex (Woldorff et al., 1993). The P1 and N1 components are also modulated by several factors in the attentional task, such as emotional saliency,

Is a positive deflection occurring approximately 200 ms after the onset of the stimulus? P200 has been interpreted as reecting selective attention (Hackley, Woldorff and Hillyard, 1990) and visual-feature detection processes (Luck and Hillyard, 1994). Similarly, P2 has been

observed after preprocessing over the right occipito-temporal sites (comparing both ellipses). C) N170 estimation over a representative electrode (T8) demonstrating the signalto-noise ratio reduction in between the subject's average waveform (black line). D). Voltage map reconstruction by interpolation showing the scalp activity at 0, P100, N170, 200 and P2

the following section, we provide a succinct description of several components.

**3. A selective description of main components** 

after the presentation of face stimuli.

**3.1 P100 and N100 (P1 and N1)** 

relevance or familiarity.

**3.2 P200 (or P2)** 

electroencephalography (EEG). ERPs result from the synchronous activation of neural subpopulations that occur in response to events (sensory, motor or cognitive). ERPs are useful not only for their excellent temporal resolution but because recent advances (e.g., dense arrays, single-trial analysis, source localization algorithms, connectivity and frequency measures, among others) provide multiples sources of brain activity in response to cognitive events.

To measure the brain activity, the ERP quantifies electrical fields through the skull and scalp. This last procedure is named electroencephalography (EEG). ERPs are the ongoing electrophysiological activity resulting from the synchronous activation of several neural subpopulations that occur in response to sensory, motor or cognitive events (Hillyard and Picton, 1987). ERPs are the summed activity of excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) activated in response to each new stimulus or subject response. The ERPs are less precise for the anatomical localization of the neural generators than the neuroimaging techniques. Nonetheless, this technique has an exceptional temporal resolution of milliseconds (Kutas and Federmeier, 2000). An ERP's spatial distribution on the scalp is not indicative of its brain-source generators (although some mathematical tools for source algorithm localization can enhance the spatial precision).

Electrodes are attached to diverse points on the scalp relative to bony landmarks. Using a standardized EEG-measurement technique to determine the correct spots, the entire head is measured. Normally, the participants are placed in front of a computer screen with electrodes fixed onto the scalp and connected to electric amplifiers and auditory headsets displaying a pattern of stimuli. One computer records and amplies the electrical peaks elicited by each stimulus onset (or the participant response).

The EEG activity is time-locked to several presentations of similar events (stimuli or participants responses), and the averaging of these segmented EEG traced together is the usual procedure. The average decreases the influence of noisy activity (i.e., EEG not related to experimental events or background noise) while maintaining the event-related activity. Several signal processing steps, such as filtering (e.g., 0.5 to 30 hz), segmentation, artifact detection and correction, bad channel replacements, re-referencing, baseline correction and averaging, are usually required to obtain a suitable signal-to-noise ratio (see Figure 1). After these processing steps, positive or negative changes of voltage constitute ERPs that appear at specific latencies after the stimulus presentation. Most ERP components are referred to by a preceding letter (e.g., "N"), indicating polarity followed by the typical peak latency in milliseconds (e.g., the "N400" ERP component is described as a negative voltage deection occurring approximately 400 ms after the stimulus onset). The timing of the brain processing is measured by the timing of these cortical responses.

The simplest ERP parameters are latency (how long after the event they appear), direction (positive or negative), amplitude (the strength of the voltage change) and topological distribution of the component on the surface of the head (frontal, parietal, occipital, etc.). The standard procedure to visualize and measure the ERP activity consists of quantifying the amplitude and latency (measured in microvolts and milliseconds, respectively) of the waveform associated with a specific stimulus or response. By means of this procedure, different stimuli or conditions can be contrasted in terms of amplitude or latency. It is usually stated that a given ERP "is modulated by," "is sensitive to" or "discriminates" a given condition when statistically significant differences are found in latency, amplitude or morphology, respectively, as a function of such condition manipulation.

electroencephalography (EEG). ERPs result from the synchronous activation of neural subpopulations that occur in response to events (sensory, motor or cognitive). ERPs are useful not only for their excellent temporal resolution but because recent advances (e.g., dense arrays, single-trial analysis, source localization algorithms, connectivity and frequency measures, among others) provide multiples sources of brain activity in response

To measure the brain activity, the ERP quantifies electrical fields through the skull and scalp. This last procedure is named electroencephalography (EEG). ERPs are the ongoing electrophysiological activity resulting from the synchronous activation of several neural subpopulations that occur in response to sensory, motor or cognitive events (Hillyard and Picton, 1987). ERPs are the summed activity of excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) activated in response to each new stimulus or subject response. The ERPs are less precise for the anatomical localization of the neural generators than the neuroimaging techniques. Nonetheless, this technique has an exceptional temporal resolution of milliseconds (Kutas and Federmeier, 2000). An ERP's spatial distribution on the scalp is not indicative of its brain-source generators (although some mathematical tools for source algorithm localization can enhance the spatial

Electrodes are attached to diverse points on the scalp relative to bony landmarks. Using a standardized EEG-measurement technique to determine the correct spots, the entire head is measured. Normally, the participants are placed in front of a computer screen with electrodes fixed onto the scalp and connected to electric amplifiers and auditory headsets displaying a pattern of stimuli. One computer records and amplies the electrical peaks

The EEG activity is time-locked to several presentations of similar events (stimuli or participants responses), and the averaging of these segmented EEG traced together is the usual procedure. The average decreases the influence of noisy activity (i.e., EEG not related to experimental events or background noise) while maintaining the event-related activity. Several signal processing steps, such as filtering (e.g., 0.5 to 30 hz), segmentation, artifact detection and correction, bad channel replacements, re-referencing, baseline correction and averaging, are usually required to obtain a suitable signal-to-noise ratio (see Figure 1). After these processing steps, positive or negative changes of voltage constitute ERPs that appear at specific latencies after the stimulus presentation. Most ERP components are referred to by a preceding letter (e.g., "N"), indicating polarity followed by the typical peak latency in milliseconds (e.g., the "N400" ERP component is described as a negative voltage deection occurring approximately 400 ms after the stimulus onset). The timing of the brain processing

The simplest ERP parameters are latency (how long after the event they appear), direction (positive or negative), amplitude (the strength of the voltage change) and topological distribution of the component on the surface of the head (frontal, parietal, occipital, etc.). The standard procedure to visualize and measure the ERP activity consists of quantifying the amplitude and latency (measured in microvolts and milliseconds, respectively) of the waveform associated with a specific stimulus or response. By means of this procedure, different stimuli or conditions can be contrasted in terms of amplitude or latency. It is usually stated that a given ERP "is modulated by," "is sensitive to" or "discriminates" a given condition when statistically significant differences are found in latency, amplitude or

elicited by each stimulus onset (or the participant response).

is measured by the timing of these cortical responses.

morphology, respectively, as a function of such condition manipulation.

to cognitive events.

precision).

Fig. 1. ERP signal-to-noise ratio. A) ERPs at temporo-occipital scalp in response to face stimuli without preprocessing and (B) with preprocessing. Note how the N170 can be clearly observed after preprocessing over the right occipito-temporal sites (comparing both ellipses). C) N170 estimation over a representative electrode (T8) demonstrating the signalto-noise ratio reduction in between the subject's average waveform (black line). D). Voltage map reconstruction by interpolation showing the scalp activity at 0, P100, N170, 200 and P2 after the presentation of face stimuli.

A continuous reconstruction of electrical activity on the scalp, normally based on spatial interpolation of the electrode sites, is termed a topographical map (or a voltage map or topomap). Each component usually has a relatively specic topographic distribution. The so-called long latency components (cognitive components or endogenous components) occur after 100 ms and are sensitive to changes in cognitive processing, as the meaning of the stimulus, or resources of processing required in the task performed (Hillyard, 2000). In the following section, we provide a succinct description of several components.
