around the visual cortex region is sensitive to the difference between two and three-dimensional images, without using actual depth or information integrated across both eyes. much in line with observations of traditional visual search paradigms, the application of p2 studies to language research has shown that the amplitude of the p2 is sensitive to both the orthographic combinability and phonological consistency (neighborhood sizes for similar appearing and similar sounding words) in the reading of chinese phonograms. high combinability and consistency chinese characters elicited lower p2 amplitudes than low combinability and low consistency characters. the suggests that characters with high combinability or high consistency facilitated early stages of orthographic and phonological processing which lowered activation at the perceptual level and resulted in a less positive p2. sources the neural source of the visual p2 is difficult to ascertain given the limited spatial resolution of the erp technique. since the recordings obtained from the scalp reflect only the dipole moments created by post-synaptic potential changes, they are subject to several factors including orientation, magnitude, and number of generator dipoles. thus, the observed topographies of the p2 observed in experimental conditions may not be indicative of their true source. it is thought that the visual p2 encompasses both a frontal and a posterior source component; in particular some of the neural activity may originate from the visual cortex in the occipital region, while the similar auditory p2 is likely generated at least in part in the auditory cortex in the temporal region and the reticular activating system. ross and tremblay recently showed different source locations for auditory evoked n1 and p2 sources using meg. in a visual semantic priming paradigm, p2 amplitude differences are associated with phase-locked theta brain wave oscillations. among the complex of the p1, n1 and p2, the p2 shows the strongest task-related modulation of theta wave oscillations between congruent and incongruent tasks. source analyses in this study and others showed that local generators of the p2 may originate in parieto-occipital regions. also, it is known that the visual p2 in monkeys is generated by neurons in area v2 of extrastriate cortex. researchers used a combination of erp, current source density (csd) and multiunit activity (mua) methods to locate the source of the p2 in the v2 layer of the visual cortex between 100–300 ms. theory at present, the p2 has been well-characterized in studies that focus primarily on visual sensation, such as the visual search paradigm. however, due to the wide range and diversity of factors that have been found to affect the characteristics of the p2, reaching a comprehensive theory of the underlying neural processes that the p2 reflects has been difficult. one theory is that the p2 indexes some form of selective attention which identifies meaningful stimuli through feature suppression. one study suggests that the increased p2 found during trials of efficient visual searches reflected the ability for the brain to reduce search space. such a hypothesis appears intuitive, as in the efficient visual arrays the targets share no features with the distracters. thus, the distinct features of the distracters can be ignored for the purposes of a particular trial. meanwhile, in the inefficient trials, the presence of shared features complicates such suppression, which might explain the observation that the amplitude of the p2 is decreased for such conditions. likewise, this rationale may apply to contexts beyond traditional visual search paradigms, including language. some studies of the p2 have cited the presence of a repetition effect as evidence that the p2 in part represents some facet of a perceptual-matching process. additionally, one could link previous observations in other studies such as those utilizing either visual search and visual priming paradigms to this perceptual-matching process, suggesting that the activity related to the p2 represents some sort of a top-down process in which prior associations is accessed in the presence of stimuli. it would appear that by incorporating relevant associations into working memory, presented stimuli can be evaluated as being either similar or dissimilar to mental representations. clinical applications the visual p2 has been proposed to have clinical utility with regard to alzheimer's disease diagnosis. researchers have found that the latency of a p2 elicited by flashes of light is significantly increased in patients with dementia and early onset of alzheimer's disease. it is also significantly delayed and interval between the p1 and the p2 was found to be significantly longer in patients with alzheimer's disease compared to controls. this may suggest a defect in the pathway between visual cortex and the visual association center, where some believe the p2 originates from. specifically, this latency pattern has been found over posterior electrode sites. as such, the latency of flash evoked p2 waveform may be useful as an early diagnostic tool for alzheimer's disease or alzheimer's risk, particularly when seen over the characteristic posterior sites.