Studies of the effects of classical conditioning led to the identification of the contingent negative variation (CNV), one of the first of the endogenous responses clearly linked to a cognitive process. In these experiments, an initial stimulus was delivered, followed by a delay and then a second stimulus. During the interval between the stimuli, a slowly growing negative potential was observed at the scalp. In many experiments the second stimulus was intended to cue a behavioral response, although a response is unnecessary and, as outlined above, would presumably elicit motor potentials that would complicate interpretation. Studies with variable intervals between the stimuli suggest that the CNV reflects at least two processes: an orienting response (o-wave) associated with the initial (warning) stimulus and an expectancy response (e-wave) that develops in anticipation of the imperative stimulus.

2. P300

One of the most extensively studied cognitive event-related responses is the P300 (or P3) complex, sometimes referred to as the family of late positive responses. The form of this response is relatively independent of the sensory modality used to elicit it. The generic paradigm associated with the response is an oddball discrimination task: for example, two or more stimuli are presented in a random series so that one occurs infrequently. This oddball elicits a response that begins around 300 msec poststimulus and may last 100 msec or more. Since the initial reports, the response has been resolved into at least two components that are differentiated on the basis of scalp topography and sensitivity to paradigm manipulation. The P3A is larger in amplitude over the central and frontal electrode sites. The response may be an alerting response arising in frontal cortex; in any case it is the component most strongly associated with the oddball response. The P3B appears to reflect subsequent allocation of attentional resources and encoding of stimulus memory. Some authors have argued that the response reflects information transfer via the corpus callosum with subsequent activation of hippocampal and parietal processes.

3. Selective Attention

The processing of information from the environment can be effectively suppressed or significantly enhanced depending on the state of attention. Event-related responses have been used extensively to study the time course and anatomical basis of selective attention in the human brain. Such experiments are typically conducted with contingencies that are manipulated within or between blocks of stimuli, so that the same physical stimulus may be the target of a discrimination task, may share some features with the target, or may be irrelevant. Data are typically analyzed by taking differences between responses to the same stimuli that differ in assigned task. The so-called negative difference or processing negativity is constructed by subtracting the response to an unattended stimulus from the response to the same stimulus while attended. In focused auditory attention, this effect may be observed as early as 20 msec poststimulus and is most apparent around the latency of the N1; however, the time course does not reflect a simple scaling of the N1.

In visual evoked responses, tasks involving spatial selective attention, spatial cueing, or visual search produce enhanced P1 and N1 components, although controversy remains regarding whether the sources of the difference component are the same as for the underlying evoked response component. Visual selection on the basis of features such as shape, texture, or color elicits a difference signal termed the selection negativity and is observed from 150 to 250 msec poststimulus with a maximum over posterior electrodes. Taken together, these observations suggest that attention to location in the visual field is an early process, perhaps mediated by facilitation and/or suppression in prestriate relays such as the thalamus. Attention to other features or conjunction of features probably is mediated by specialized visual areas further along the processing chain.

In discrimination trials in which a fast choice is enforced by a reaction time task, an error-related negativity (ERN) is observed in trials in which the wrong response is performed. This response, predominantly observed over midline frontal areas, appears to reflect a process of rechecking that is initiated in parallel with the response. In many cases the ERN appears before the behavioral response; presumably when the response is optimized for speed, the motor system is committed before the decision-recheck cycle is complete.

5. Language-Related Responses

Event-related potential studies have been used to identify a number of responses associated with language processing. Word stimuli presented in spoken or written form elicit a response peaking around 280 msec (N280 or lexical processing negativity) typically characterized by a negative peak over left frontal regions of the scalp. This is considered to be a generic correlate of word processing, though in an experimental context the latency can be manipulated as a function of the frequency of the word. Another prominent response is elicited by semantic incongruity in written text or speech. The N400 is a right posterior scalp negativity the begins around 200 msec and peaks around 400 msec following the presentation of a word that violates semantic expectation. Because the response is associated with meaning and context, the N400 is considered an index of higher order language processing. A

number of response components have been associated with processing of the form and structure of language. The syntactic positive shift (P600) is a late positive response elicited by certain types of grammatical errors (such as subject-verb agreement), even though the meaning of such a sentence may be clear. Figure 10 illustrates an example of a language-related ERP.

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