In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synaptic activity of a specific neural process. The P2 component, also known as the P200, is so named because it is a positive going electrical potential that peaks at about 200 milliseconds (varying between about 150 and 275 ms) after the onset of some external stimulus. This component is often distributed around the centro-frontal and the parieto-occipital areas of the scalp. It is generally found to be maximal around the vertex (frontal region) of the scalp, however there have been some topographical differences noted in ERP studies of the P2 in different experimental conditions.
Research on the visual P2 is at an early stage compared to other more established ERP components and there is much that we still do not know about it. Part of the difficulty of clearly characterizing this component is that it appears to be modulated by a large and diverse number of cognitive tasks. Functionally, there seems to be partial agreement amongst researchers in the field of cognitive neuroscience that the P2 represents some aspect of higher-order perceptual processing, modulated by attention. It is known that the P2 is typically elicited as part of the normal response to visual stimuli and has been studied in relation to visual search and attention, language context information, and memory and repetition effects. The amplitude of the peak of the waveform may be modulated by many different aspects of visual stimuli, which allow it to be used for studies of visual cognition and disease. In general, the P2 may be a part of cognitive matching system that compares sensory inputs with stored memory.Freunberger, R., Klimiesch, W., Doppelmayr, M & Holler, Y. (2007). Visual P2 component is related to theta phase-locking. Neuroscience Letters, 426, 181–186.Luck, S. J., & Hillyard, S. A. (1994). Electrophysiological correlates of feature analysis during visual search. Psychophysiology, 31, 291–308.
History
The first mentions of an ERP component similar to that of the modern P2 were characterized in studies of basic visual and auditory evoked potentials. One of the first of such studies involved the presentation of flashing lights. Using this method, researchers found that a series of potential changes were consistently observed across repeated trials. These would later be classified as components of the visual evoked response (VER), part of which includes the P2.
The P2 follows the visual N1 (or auditory N100) and P1 waveforms (negativity and positivity at 150 and 100ms respectively) and is followed by the N200, P3, and N400 waveforms. Other components may overlap with the P2 to some extent, making it difficult to distinguish clearly between them, depending on the location of measurement. Originally, the P2 was characterized as a sub-component of a complex involving the N1, P1 and P2, which was known as the vertex potential and which was classically studied as a unitary phenomenon. In particular, the relationship between the N1 and P2 was thought to be important. The difference between the N1 and P2, known as the vertex amplitude, was found to be significantly larger for target than non-target stimuli and for rapid attention switching task.Furutsuka, T. (1989). Effects of rapid attention switching on the N1-P2 amplitude of the visual event-related potentials. Research and Clinical Center for Child Development Annual Report, 11, 55–64. Further studies have subsequently examined the P2 separately from the N1 and have found that the amplitude of the P2 itself is larger for target stimuli that are less frequent. This is similar to the P3, though the P2 is usually seen for more simple features than the P3.Crowley, K. E., & Colrain, I. M. (2004) In the auditory domain, there is evidence of enhanced P2 amplitudes even when a target stimuli is not embedded in a series of identical stimuli. In these instances, enhanced P2 amplitudes have been associated with auditory learning and repeated stimulus exposure.Tremblay et al. 2001 Central auditory plasticity: Changes in the N1-P2 complex after speech-sound training. Ear and Hearing 22,79–90 Enhanced P2 amplitudes have been reported in musicians with extensive listening experienceShahin et al. 2003, Enhancement of Neuroplastic P2 and N1c Auditory Evoked Potentials in Musicians, Journal of Neuroscience, 2 July 2003, 23(13):5545–5552 as well as laboratory based auditory training experiments.Tremblay et al. 2009, Auditory training alters the physiological detection of stimulus-specific cues in humans. Clinical Neurophysiology. 120,128–135 A significant finding is that P2 amplitude changes are sometimes seen independent of N1 amplitude changes,Ross and Tremblay 2009. Stimulus experience modifies auditory neuromagnetic responses in young and older listeners. Hearing Research, Feb;248(1–2):48–59 again suggesting some degree of independence of N1, and P2 latencies and amplitudes appear to be affected by old age.Tremblay et al. 2003 Effects of age and age-related hearing loss on the neural representation of speech cues. Clin Neurophysiol. Jul;114(7):1332–43A review of the evidence for P2 being an independent component process: age, sleep and modality. Clinical Neurophysiology, 115, 732–744.
In terms of modality, the visual P2 is similar to the auditory P2 and both have been studied in similar contexts. There are most likely multiple distinct P2s in different modalities, including both frontal and posterior visual P2s, which may or may not have similar origins or functional similarities. It is not yet understood whether the visual, auditory or other P2s reflect the same functional and neural activities.
Component characteristics Like other evoked-response potentials, the presence of the P2 is revealed in the waveform of the EEG recorded by time-locking data from trials to the onset of the stimulus, in appropriate paradigms. As data from the recordings of multiple trials are averaged together, the persistent characteristics of the P2 become apparent. The fact that this waveform appears stable across similar trials is what suggests that it is a meaningful response to a given stimulus.
Using electrodes attached to the earlobes of participants as a reference the visual P2 can be found over anterior and central sites on the scalp, and is usually maximal over the frontal region. The more posterior P2 has been studied in relation to visual complexity in language processing, visual search tasks and memory and repetitions paradigms. The component is evoked as part of the normal response to visual stimuli, but the amplitude and latency (delay between stimulus and response) may be affected by exogenous factors, such as repeated visual stimuli. This component has been linked with higher-order perceptual and attentional processes, including feature analysis of geometric figures and visually presented words. The exact function and neural source of the P2 is not yet known, but some evidence indicates that the P2 may reflect general neural processes that occur when a visual (or other sensory) input is compared with an internal representation or expectation in memory or language context.Evans, K. M., & Federmeier, K. D. (2007). The memory that’s right and the memory that’s left: Event-related potentials reveal hemispheric asymmetries in the encoding and retention of verbal information. Neuropsychologia, 45, 1777–1790.
Main paradigms The P2 has traditionally been studied in the context of perception, with specific emphasis on how stimulus evaluation takes place. As such, multiple paradigms have been used in experiments seeking to understand how manipulations of sensory stimuli modulate the characteristics of the P2.
The visual P2 has been studied in the context of visual priming paradigms, oddball paradigms (where the amplitude is enhanced to targets), and studies of repetition in language. One of the more well-studied paradigms with regards to the visual P2 has classically been the visual search paradigm, which tests perception, attention, memory, and response selection. In this paradigm, participants are instructed to focus their attention at a central point on a screen. It is then that participants are given a cue indicating the identity of a target stimulus. Following a delay, participants are then presented with a set of items. Instructed to identify the location of the target stimulus, participants respond by button-pressing or some other method. Trials are classified as either "efficient" or "inefficient" based upon the relationship between the target stimuli and non-target stimuli, known as "distracters". In the case of efficient search arrays, the target object or stimuli does not share any features in common with the distracters in the array. Likewise, in an inefficient array, the targets share one or more features with the "distracters".Phillips, S, & Takeda, Y. (2009). An EEG/ERP study of efficient versus inefficient visual search. Journal of The Cognitive Science Society
The visual P2 has also been studied in the context of the visual priming paradigm, which seeks to understand how prior information shapes future response. In this experimental design, participants are briefly presented with an image or word, followed by a delay, and a subsequent stimulus upon which participants must make a classification. Researchers have used the visual search paradigm with stimulus arrays and found that target stimuli elicited larger anterior P2 components compared with standards. This evidence suggests that top-down information processing about feature classification affected processing at the visual perception stage. Thus, the P2 may index mechanisms for selective attention, feature detection (including color, orientation, shape, etc.) and other early stages of item encoding.
With regard to the auditory P2, the primary paradigm used to study manipulations of this type of sensory information is the auditory oddball task. In this procedure, participants are presented with a stream of auditory stimuli: including frequent, standard stimuli as well as infrequent, target stimuli. Participants of such studies are asked to ignore the frequent standards and respond to the infrequent targets.
In general, increases in the attentiveness of the subject lead to decreased amplitude of the P2. Increased attention decreases the amount of search space, or number of associations that need to be made, and may facilitate feature classification in visual search at the stage of perceptual processing. More probable targets also lead to decreased amplitude of the P2, which is sensitive to the number of non-target (distracter) features in a visual search. The amplitude of the P2 is greater when the visual search is more efficient (selective attention), but this does not affect the latency.
Functional sensitivity
General features of stimuli
Research using the visual search paradigm has shown that features such as color, size, and orientation of the stimulus have a necessary role in eliciting the P2 effect seen during trials of efficient search. Other characteristics, such as attention, repetition and probability of the stimulus also impact the amplitude of the P2. The diversity of these factors tends to suggest that the P2, as a response, is multidimensional with respect to its sensitivity to stimulus features.
Memory