Invited reviewA review of the evidence for P2 being an independent component process: age, sleep and modality
Introduction
Event-related brain potentials (ERPs) are induced exogenously by environmental events (such as sensory stimuli) or endogenously by processes such as decision making. ERPs appear as transient changes in the ongoing electrical brain activity (recorded as EEG) within a short time frame following the eliciting event. ERPs typically consist of a series of voltage polarity changes, seen as peaks and troughs in the ERP waveform. Each ERP deflection is a composite of several components that are generated by parallel streams of neural activity, overlapping in time. A component can therefore be defined as a voltage contribution to the ERP which reflects a functionally discrete stage of neural processing, occurring in a restricted cerebral area (Näätänen and Picton, 1987). Peaks within an ERP can be classified according to their magnitude, timing relative to stimulus onset, polarity, anatomical site of generation, or function reflected by them. Depending on the stimulus modality, the ERP will have a series of early cortical peaks, reflective of initial processing in a sensory receiving area, for example, P100 in a visual evoked potential (VEP), P50 in an auditory evoked potential (AEP) or P1 or Nf in a respiratory related evoked potential (RREP). Following these are later peaks such as N1, P2, P3a, P3b and N400, which reflect later more integrative cognitive processing. Unlike the N1 or the other late ERPs, little work has been done on the P2 to elucidate the generation mechanisms, the cognitive/attentional correlates or its functional significance in general. The P2 is usually referred to in the context of the tail end of the N1-P2 complex or the ‘vertex potential.’ Linkage to the N1 has particularly been used to describe the changes that occur to the N1 during drowsiness, sleep onset and stable sleep. Our recent investigations into evoked potentials during sleep in the aged (Crowley et al., 2002) have highlighted the P2 as a component of interest as it is one of the few EEG phenomena that demonstrates a dramatic increase in amplitude with age. Thus, this paper attempts to provide a selective review of what is known about the P2 during wake and sleep and considers whether it is sensible to continue to view it as linked to N1 or whether it should be viewed as a functionally distinct entity.
Section snippets
Phenomenology of the P2
Auditory stimuli elicit a series of long latency ERPs. In the 80–200 ms latency range ERPs mainly consist of two peaks, the first with negative polarity, called N1, occurring roughly 75–150 ms after stimulus onset and the second with positive polarity, P2, with a latency of approximately 150–250 ms. Although the N1 and P2 are most often elicited by auditory stimuli, a response of similar morphology also occurs following stimuli in the somatosensory modality. These two components are assumed to
Scalp topography and source localization
An important approach to understanding the brain processes underlying different ERP components is to determine their intracerebral source. While it is clear that the earlier ERP peaks, including N1, are dependent on stimulus modality, the topography of P2 appears to be similar across auditory, visual and somatosensory modalities. In all 3 modalities it is maximal over the vertex, as shown in Fig. 1 (Oades et al., 1995, Potts et al., 1998, Roth et al., 1976). In contrast to N1, studies on the P2
P2 and attention
An increase in the level of attentiveness of a subject produces an increase in the amplitude of the N1 peak but a decrease in the amplitude of the P2. This effect can best be evaluated by subtracting the ERP from stimuli that have been ignored, from the ERP to the same stimuli when they are attended. Such a subtraction shows a broad negative wave that has been labelled as ‘Nd’ by Hansen and Hillyard (1980) and ‘Processing Negativity (PN)’ by Näätänen et al. (1978). Thus, it has been
P2 and age
Numerous studies have indicated considerable change in AEP morphology across childhood and adolescence (Barnet, 1975, Bruneau et al., 1997, Courchesne, 1978, Sharma et al., 1997). Such studies show that some responses, such as the P2 (see Fig. 2) mature early (reaching adult values by as early as 2–3 years of age), while others, such as the N1, follow a much longer developmental time course, extending into adolescence (Barnet, 1975, Bruneau et al., 1997, Ceponiene et al., 2002, Kushnerenko et
P2 and sleep
During the process of falling asleep, N1 gradually declines in amplitude (see Fig. 3) (Bastuji et al., 1995, Nordby et al., 1996, Ogilvie et al., 1991), due possibly to a decrease in the subject's level of attention. The decrease in the N1 amplitude is paralleled by a slowing of behavioural reaction time (Ogilvie et al., 1991). With progression into non-REM sleep (Stages 2–4), the N1 is usually further attenuated (Nielsen-Bohlman et al., 1991, van Hooff et al., 1997, Winter et al., 1995) or
Wakefulness
Respiratory-related evoked potentials (PREP) are a form of somatosensory evoked potentials that are seen in response to increases in breathing effort, probably due to stimulation of afferent pathways throughout the upper and lower airways as well as the respiratory muscles. P2 was one of the components reported in the first paper showing the existence of the RREP (Davenport et al., 1986), despite the fact that the data reported were from very slow rise time occlusions, presented before the
Summary and conclusions
The initial reactions to Berger's careful and systematic descriptions of human EEG (Berger, 1929) were sceptical. Scientists had difficulty accepting that Berger's recordings were generated by, or even related to actual brain events. While we now have a much greater understanding of the mechanisms underlying the generation of EEG activity, it remains necessary to accept that EEG is limited in the interpretation of its underlying processes. The voltages at a particular scalp site represent the
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