Spatial covert attention increases contrast sensitivity across the CSF: support for signal enhancement

This study is the first to report the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies when a targe t alone was presented in the absence of a local post-mask. We used a periph eral precue (a small circle indicating the target location) to explore the effects of covert spatial attention on contrast sensitivity as assessed by orientation discrimination (Experiments 1-4), detection (Experiments 2 and 3) and localization (Experiment 3) tasks. In all four experiments the targe t (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was prese nted alone in one of eight possible locations equidistant from fixation. Co ntrast sensitivity was consistently higher for peripherally- than for neutr ally-cued trials, even though we eliminated variables (distracters, global masks, local masks, and location uncertainty) that are known to contribute to an external noise reduction explanation of attention. When observers wer e presented with vertical and horizontal Gabor patches an external noise re duction signal detection model accounted for the cueing benefit in a discri mination task (Experiment 1). However, such a model could not account for t his benefit when location uncertainty was reduced, either by: (a) Increasin g overall performance level (Experiment 2); (b) increasing stimulus contras t to enable fine discriminations of slightly tilted suprathreshold stimuli (Experiment 3); and (c) presenting a local post-mask (Experiment 4). Given that attentional benefits occurred under conditions that exclude all variab les predicted by the external noise reduction model, these results support the signal enhancement model of attention. (C) 2000 Elsevier Science Ltd. A ll rights reserved.