A regular rhythm can make suppression unnecessary
In a typical attention experiment, a target and a distractor appear together, and the brain must pick one while pushing the other down. But that is not the only way selection works. When volunteers watched a fast stream of images and had to remember certain targets while ignoring salient distractors, their brain activity showed a clear target-selection signal but no sign of an active distractor-suppression signal [1]. In other words, the target was chosen without the distractors being actively suppressed.
The likely reason is timing. The stream had a clear and regular temporal structure, so the brain could use temporal expectation—predicting when the next target would arrive—to select efficiently [1]. This suggests that active suppression is most needed when target and distractor compete at the same moment, and less needed when the target's timing is predictable.
Anxiety and fast-moving distractors are two ways suppression breaks down
Suppression can fail when a person's emotional state tips the balance toward stimulus-driven attention. In people with high test anxiety, electrophysiological and behavioral measures showed that they were captured by irrelevant singleton distractors and failed to suppress them; the anxiety appeared to strengthen automatic, stimulus-driven attention and weaken goal-directed control [3].
The nature of the distractor matters too. Dynamic, moving salient distractors are difficult to actively suppress, although when dynamic stimuli are combined with color salience, suppression becomes possible [4]. That contrast suggests that suppression is not a single switch but depends on what the distractor looks like and which features make it stand out.
Suppression is stronger when you are about to move your eyes
Suppression also varies with how attention is deployed. When people prepared to make an eye movement (overt attention), letter identification at distractor locations was worse than during covert attention, where the eyes stay fixed—meaning distractor locations were suppressed more strongly before a saccade [5]. Target locations were equally well identified in both conditions, so the extra suppression was specific to distractors.
This effect was modulated by distance and hemifield: suppression at the distractor location tended to be stronger when the distractor was in the same hemifield as the target and when it was farther from the target [5]. So even when suppression is working, its strength depends on the spatial layout of the scene.
Sources used in this answer
Event-related Potentials Indicate Target Processing in the Absence of Distractor Suppression during Rapid Serial Visual Presentation.
During rapid serial visual presentation, target selection occurred without any electrophysiological signature of active distractor suppression, suggesting that a clear and regular temporal structure allows targets to be selected without suppressing distractors.
Distractor inhibition: Principles of operation during selective attention.
Inhibitory mechanisms in selective attention can be effective in some conditions and fail in others, with incompatible distractors demonstrating failures of selective attention.
Electrophysiological and behavioral evidence for the attention capture and suppression failure of irrelevant singleton in test anxiety
High test-anxious individuals showed attention capture by irrelevant singleton distractors and failed to suppress them, with test anxiety increasing stimulus-driven attention and weakening goal-directed control.
Attentional Suppression of Dynamic Salient Distractors and Its Transferability
Dynamic salient distractors are difficult to actively suppress, but when dynamic stimuli are combined with color salience, distractors can be suppressed; the study explores conditions such as jitter characteristics, training, and salience level.
Selective attention before saccades relies more on distractor suppression than covert attention
Distractor locations were suppressed more strongly before saccades than during covert attention, and this suppression was modulated by distance from the target and the hemifield in which target and distractors appeared.