Electrophysiological correlates of detecting a visual target and detecting its absence: the role of feature dimensions.

Electrophysiological measurements were taken from observers performing a visual search within a single feature dimension, and between multiple dimensions. The N2pc to selected singleton target stimuli (rightward tilted lines) was increased when targets varied between feature dimensions, compared to the N2pc to the same rightward tilted line targets, in a condition in which targets varied only between feature values within the same dimension. The anterior and posterior N2 were not reliably modulated, but P3(b) amplitude was higher for singleton present trials that varied between dimensions than for those that varied within. The ERP elicited by singleton absent trials showed reduced P3 amplitude in the between-dimension condition. The electrophysiological modulations were accompanied by increased reaction times in the between-dimension condition, on both singleton present and absent trials. The results suggested that visual target detection is affected by early dimension-specific weighting of the current attentional task set. Furthermore, exhaustively searching multiple feature dimensions to determine the absence of a target incurs dimensional switching costs, possibly at a later stage.