New Paper: Combined Electrophysiological and Behavioral Evidence for the Suppression of Salient Distractors

Gaspelin, N., & Luck, S. J. (in press). Combined Electrophysiological and Behavioral Evidence for the Suppression of Salient Distractors. Journal of Cognitive Neuroscience.

Evidence that people can suppress salient-but-irrelevant color singletons has come from ERP studies and from behavioral studies.  The ERP studies find that, under appropriate conditions, singleton distractors will elicit a Pd component, a putative electrophysiological signature of suppression (discovered by Hickey, Di Lollo, and McDonald, 2009). The behavioral studies show that processing at the location of the singleton is suppressed below the level of nonsingleton distractors (reviewed by Gaspelin & Luck, 2018).  Are these electrophysiological and behavioral signatures of suppression actually related?

In the present study, Nick Gaspelin and I used an experimental paradigm in which it was possible to assess both the ERP and behavioral measures of suppression.  First, we were able to demonstrate that suppression of the salient singleton distractors was present according to both measures.  Second, we found that these two measures were correlated: participants who should a larger Pd also showed greater behavioral suppression.  

Correlations like these can be difficult to find (and believe).  First, both the ERP and behavioral measures can be noisy, which attenuates the strength of the correlation and reduces power.  Second, spurious correlations are easy to find when there are a lot of possible variables to correlate and relatively small Ns.  A typical ERP session is about 3 hours, so it's difficult to have the kinds of Ns that one might like in a correlational study.  To address these problems, we conducted two experiments.  The first was not well powered to detect a correlation (in part because we had no idea how large the correlation would be, making it difficult to assess the power). We did find a correlation, but we were skeptical because of the small N.  We then used the results of the first experiment to design a second experiment that was optimized and powered to detect the correlation, using an a priori analysis approach developed from the first experiment.  This gave us much more confidence that the correlation was real.

We also included a third experiment that was suggested by the alway-thoughtful John McDonald. As you can see from the image above, the Pd component was quite early in Experiments 1 and 2. Some authors have argued that an early contralateral positivity of this nature is not actually the suppression-related Pd component but instead reflects an automatic salience detection process.  To address this possibility, we simply made the salient singleton the target.  If the early positivity reflects an automatic salience detection process, then it should be present whether the singleton is a distractor or a target.  However, if it reflects a task-dependent suppression mechanism, then it should be eliminated when subjects are trying to focus attention onto the singleton. We found that most of this early positivity was eliminated when the singleton was the target. The very earliest part (before 150 ms) was still present when the singleton was the target, but most of the effect was present only when the singleton was a to-be-ignored distractor. In other words, the positivity was not driven by salience per se, but occurred primarily when the task required suppressing the singleton.  This demonstrates very clearly that the suppression-related Pd component can appear as early as 150 ms when elicited by a highly salient (but irrelevant) singleton.