Neural mechanisms of visual singleton detection: Evidence from human electrophysiology

It is sometimes necessary to search for visual objects of potential interest that are underspecified (e.g., any illegal item in a suitcase). The search for such an object can be accomplished easily if it possesses a unique feature that makes it stand out from its surrounding. In this case, observers can simply search for the most salient item in the environment (singleton detection). Surprisingly, the neuro-cognitive processes involved in singleton detection are still poorly understood. The overarching aims of this thesis were to reveal neuro-cognitive processes involved in singleton detection using event-related potentials (ERPs) and to address specific questions about the role of attention in singleton-detection tasks. Experiment 1 reexamined the claim that attentional processes associated with an ERP component called the N2pc are absent in singleton detection. The results revealed several ERP components, including the N2pc and a newly discovered component that tracked the time course of singleton detection (the singleton detection positivity; SDP). It was concluded that singleton detection involves some of the same attentional processes as those required for feature-based search. Experiment 2 employed a go/no-go variant of the singleton-detection task to determine whether the attentional processes observed in singleton detection are triggered automatically, as some researchers believe. ERP indices of singleton detection (SDP) and attentional selection (N2pc) were markedly reduced or absent on no-go trials, demonstrating that rapid assessment of task relevancy can prevent salience-driven capture of attention in the singleton-detection task.