While many theories of attention highlight the importance of similarity between target and distractor items for selection, few studies have directly quantified the function underlying this relationship. Across two commonly used tasks—visual search and sustained attention—we investigated how target-distractor similarity impacts feature-based attentional selection, in particular asking whether stimulus-based or psychological similarity better explains performance. We found that both similarity measures were non-linearly related to task performance, although psychological similarity explained a big portion of the non-linearities observed in the data, suggesting that measures of psychological similarity are more appropriate when studying effects of target-distractor similarities. Importantly, we found comparable patterns of performance in both visual search and sustained feature-based attention tasks, with performance (RTs and d’, respectively) plateauing at medium target-distractor distances and exponential functions capturing the relationship between stimulus-based and psychological similarity and performance well. In contrast, visual search efficiency, as measured by search slopes, was affected by only a narrow range of similarity levels (10-20°). These findings place novel constraints on models of selective attention and emphasize the importance of considering the similarity structure of the feature space. Broadly, the non-linear effects of similarity on attention are consistent with accounts that propose attention exaggerates the distance between competing representations, possibly through enhancement of off-tuned neurons.
Feature similarity is non-linearly related to attentional selection: evidence from visual search and sustained attention tasks