Abstract
Numerous everyday search tasks require humans to attentionally select and temporally store more than one object present in the visual environment. Recently, several studies sought to isolate the mechanisms underlying such subitizing tasks by means of electrophysiological measures, which led to a more fine-grained picture as to which processing stages are modulated by object numerosity. One critical limitation that all these studies share, however, is that they used stimulus designs in which the targets were exclusively defined by the same feature value. Accordingly, it remains an open issue whether these findings may generalize to search scenarios in which multiple targets are physically not identical. To systematically address this issue, we introduced three target context manipulations, in which multiple targets were randomly defined by (i) the same feature (sF), (ii) different features within the same dimension (dFsD), or (iii) different features across dimensions (dD). Our findings revealed that people's ability to enumerate multiple targets was remarkably influenced by inter-target relationships, with fastest responses for sF trials, slowest responses for dD trials, and responses were of intermediate speed for dFsD trials. Our electrophysiological analyses disclosed that one source of this response slowing was feature-based and originated from the stage of attentional selection (as indexed by PCN waves), whereas another source was dimension-based and associated with working memory processes (as indexed by P3b waves). Overall, our results point to a significant role of inter-target relationships in multiple object processing—a factor that seems to have been largely neglected in most subitizing studies.
Meeting abstract presented at VSS 2016