Not simply two sides of the same coin: Target enhancement and distractor suppression draw on independent neural mechanisms

Enhancement and suppression have traditionally been described as a shared process; enhancing relevant information subsequently suppresses irrelevant information, but there may also exist separable mechanisms. Recent evidence indicates that target enhancement and distractor suppression can be measured separately, using both behavioral and physiological approaches. It is unclear, however, whether the underlying mechanisms are also separable. We used an individual differences approach to investigate whether enhancement and suppression strengths vary independently across individuals, which would provide strong evidence supporting the existence of separate mechanisms. This approach, however, requires dissociable, internally reliable individual-difference measures. We found reliable enhancement measures for both endogenous cueing effects (Experiment 1) and statistical learning effects (Experiment 2) for target location. Measures of suppression in Experiments 1 and 2 were unreliable. In Experiment 3, we encouraged a single strategy ("singleton-search mode") by changing the colors and shapes of targets and distractors across trials. We found reliable individual differences in reaction time measures for enhancement and suppression. Critically, these reliable measures did not covary. Experiments 4 and 5 largely replicated the results of Experiment 3. Again, individual-difference measures of enhancement and suppression strength did not covary. These results show a clear dissociation: While enhancement yielded reliable individual differences across task manipulations, suppression measures were usually unreliable and sensitive to task parameters. When individual-difference measures of both enhancement and suppression were reliable, they did not covary. These results cannot be explained solely by a shared-mechanism account of enhancement and suppression, and instead support the hypothesis that independent enhancement and suppression mechanisms exist.