The N2pc marks the moment of picking a target, not blocking a distractor
The N2pc is a brief shift in electrical activity recorded at electrodes over the posterior scalp, and it shows up when a target sits on one side of the display and a distractor on the other. Because it is larger over the hemisphere opposite the target, researchers read it as a sign that attention has locked onto that object [1][5]. In a rapid stream of images where there is a target but no competing distractor to suppress, a wave resembling the N2pc still appears, which fits the idea that this component tracks target processing on its own [2].
That matters for timing. If the N2pc were really a suppression signal in disguise, it should behave like one when distractors are present. Instead, studies that manipulate the target and distractor separately find the N2pc following the target: it is strong when the target is in the lower visual field and actually flips polarity when the target is in the upper field [1]. The same manipulation leaves the PD behaving in the opposite way, which is hard to explain if both components were measuring one shared mechanism [1].
The PD may be selection pointed somewhere else
The PD is a positive-going deflection often seen over the hemisphere opposite a distractor, and it has been widely read as the brain actively suppressing that distractor. A direct test of this idea found the PD appeared only when target and distractor could land on opposite sides of fixation; when that trial context was absent, so was the PD [4]. The authors interpret this as a strategy: participants learn to aim attention at the side opposite the distractor, and the resulting wave is really an N2pc to that opposite location rather than a suppression signal [4].
This is a claim about interpretation, not a settled fact, and it changes what the timing question means. If the PD is redirected target selection, then the brain is not first suppressing and then selecting; it is selecting away from the distractor. That would make the two components two faces of selection rather than a suppression-then-selection sequence [4].
Stress and display layout pull the two components apart
One way to test whether target selection and distractor suppression are separate is to disturb one and see if the other moves. Under acute stress, the N2pc to a lateral target was significantly smaller in the stress group than in controls, while the PD showed no significant difference [3]. That pattern is consistent with stress impairing target enhancement while leaving distractor suppression relatively intact, which supports treating them as distinct processes rather than a single wave [3].
Display geometry does something similar. When targets and distractors were moved above or below the horizontal midline, the N2pc and PD responded in opposite directions, and the N2pc even reversed polarity for upper-field targets [1]. Notably, these electrical changes did not show up as differences in search efficiency, so the components are not simply a readout of how fast or well people search [1]. The timing story is therefore about which neural process a wave reflects, not a stopwatch that predicts performance.
Why the timing question is not fully settled
The core disagreement is whether the N2pc and PD come from one mechanism or two. The elevation study concludes that its results fit a dissociation between target selection and distractor suppression, tied to how the visual cortex maps the upper and lower fields [1]. The PD study concludes the opposite for its own paradigm, arguing that what looks like suppression is selection of the opposite location [4]. Both are plausible readings of their data, and the difference may depend on the task: whether a distractor is a singleton to be ignored, whether negative cues are given, and whether target and distractor can appear on opposite sides.
What the evidence does support is a practical answer to the timing question. The N2pc is a reasonably reliable index of when target selection is happening, and it can appear without any distractor to suppress [2][5]. The PD is the better candidate for a suppression-related signal, but its meaning is contested and may shift with task context [4]. Until paradigms are compared directly, the safest reading is that target selection has a clear electrical signature, while the timing of genuine distractor suppression remains an open question rather than a measured sequence.
Sources used in this answer
Vertical Elevation as a Key Factor for the Neural Distinction of Target Selection and Distractor Suppression in Visual Search
N2pc and PD responded in opposite directions to vertical elevation of targets and distractors, with N2pc robust for lower-field targets and reversed for upper-field targets, while PD was larger for upper-field distractors and absent for lower-field ones, without affecting search efficiency.
Event-related potentials indicate target processing in the absence of distractor suppression during rapid serial visual presentation
A wave resembling the N2pc appeared during rapid serial visual presentation in the absence of evidence for active distractor suppression, consistent with the N2pc reflecting target selection.
Acute stress impairs target enhancement rather than distractor suppression in attention selection: evidence from the N2pc and PD
Acute stress reduced the N2pc to lateral targets but did not significantly change the PD, suggesting stress impairs target enhancement rather than distractor suppression.
The PD Reflects Selection of Nontarget Locations, Not Distractor Suppression
The PD occurred only when target and distractor could appear on opposite sides of fixation, suggesting it reflects selection of the opposite location rather than suppression of the distractor.
Competitive interaction degrades target selection: An ERP study
The N2pc is treated as reflecting target selection processes, with differences in the component potentially reflecting target processing and/or distractor suppression indexed by the Pd.