The P1 is called the P1 because it is the first positive-going component (when also using a mastoid reference point) and its peak is normally observed in around 100 ms.
The different components within the category of VEPs were first described by Spehlmann in 1965, who compared human ERPs when viewing patterned and diffuse stimuli that were quickly flashed on the screen while a person was viewing the general area where the flash was to appear.
[2] Since its initial discovery, the common theory about the C1 continues to state that it is an early component when viewing stimuli and that it represents activity in the primary visual cortex.
In his experiment, Spehlmann showed participants patterns of black and white squares, arranged in a checkerboard manner.
In the last quarter of the 20th century, the P1 started to be studied looking at what is called the P1 "effect" in the selective attention domain.
Van Voorhis and Hillyard found modulations in the P1 due to attention using the famous paradigm used by Eason, Harter, and White.
Participants were also instructed to either look for the target passively or press a button whenever the double flash occurs.
Ever since this experiment, the difference between the P1 amplitude when the participant is attending in the correct and incorrect visual field (or the P1 effect) has been extensively studied as part of selective attention.
[2][6] Roughly speaking, the C1 has a negative polarity if the stimuli is presented in the upper half of the visual field (when using a mastoid reference) but it has a positive polarity if the stimuli is presented in the lower half of the visual field.
[7] Its amplitude maximum is over the lateral occipital scalp, approximately right over the ventrolateral prestriate cortex, contralateral to the visual field in which the stimuli is presented.
These paradigms vary with type of stimuli used and time in between stimuli but the base paradigm mainly involved the participant attending to a specific part of the visual field while looking for a target in his or her entire visual field.
Participants are to look for a target that differs from the rest of the stimuli on some number of dimensions such as size, length, luminance, etc.
[8] Stimuli presented in the left half of the visual field will elicit more negativity over the rightward occipital and parietal channels.
Stimuli presented in the right half of the visual field will elicit a negativity over the leftward occipital and parietal channels.
[11] Luck, Hillyard, Mouloua, Woldorff, Clark and Hawkins proposed that the P1 effect is a reflection of a "cost of attention.
This decrement or suppression of the P1 represents the cost of having to stop attending to one area and shift the attention to the place where the target stimulus is located.
Instead, early aspects of visual processing (as reflected in the C1) seem to unfold in a manner that is unaffected by the allocation of attention over space.
Clark, Fan, and Hillyard using a paradigm whereby circular checkerboards were presented in different visual fields, localized the C1 to the striate cortex using a 2-dipole BESA approach.
[20] Di Russo, Martinez, and Hillyard used sinusoidally modulated black and white checkerboard circles in the four different hemifields (upper-right, upper-left, lower-right, and lower-left) to look at the location of the C1.
[6] Other papers using a combination of fMRI and BESA dipole modeling have also pointed to the P1 coming from the ventrolateral prestriate cortex.
[12][21] Further evidence that the P1 is located along the ventral pathway comes from a studies using both ERPs and positron emission tomography.