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

Abstract

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 target alone was presented in the absence of a local post-mask. We used a peripheral 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 target (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was presented alone in one of eight possible locations equidistant from fixation. Contrast sensitivity was consistently higher for peripherally- than for neutrally-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 were presented with vertical and horizontal Gabor patches an external noise reduction signal detection model accounted for the cueing benefit in a discrimination task (Experiment 1). However, such a model could not account for this benefit when location uncertainty was reduced, either by: (a) Increasing overall performance level (Experiment 2); (b) increasing stimulus contrast 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 variables predicted by the external noise reduction model, these results support the signal enhancement model of attention.

Fig. 1

A schematic representation of a trial sequence. In half the blocks, the target was preceded by a peripheral cue (as shown here) and the rest of the blocks were preceded by a neutral cue (a circle in the center of the display; not shown here). In Experiments 1, 2, and 3, no local post-mask appeared; in Experiment 4, a local post-mask appeared.

Fig. 2

Contrast sensitivity as a function of spatial frequency in the neutral condition. The functions depict the average of observers sensitivities in (a) Experiment 1 (discrimination task, n=6); (b) Experiment 2a (discrimination task, n=5); (c) Experiment 2b (detection task, n=3); and (d) Experiment 4 (discrimination, n=6).

Fig. 3

Contrast sensitivity as a function of spatial frequency in both neutral and peripheral conditions for two observers in Experiment 1.

Fig. 4

To quantify the attentional effect on contrast sensitivity, we plotted the ratio of contrast sensitivity in the peripheral cued condition to the neutral cued condition (P/N). Values above 1 illustrate an attentional benefit. The functions depict both the averages of observers sensitivity ratios (dotted line) as well as those predicted by the SDT model (solid line) under the same conditions, for (a) Experiment 1 (discrimination task at 82% correct); (b) Experiment 2a (discrimination task at 90% correct); (c) Experiment 2b (detection task at 90% correct); (d) Experiment 4 (discrimination task with local post-mask at 82% correct. The SDT model lies along the line corresponding to a ratio of 1).

Fig. 5

Average percent correct (corrected for guessing) for the detection, localization, and discrimination tasks for (a) Experiment 3a (vertical versus horizontal stimuli; n=2); and (b) Experiment 3b (tilted stimuli; n=4).

Fig. 6

To quantify the attentional effect on contrast sensitivity, we plotted the ratio of percent correct in the peripheral cued condition to the neutral cued condition (P/N). Values above 1 illustrate an attentional benefit. The functions depict both the averages of observers ratios for (a) Experiment 3a (vertical versus horizontal stimuli; n=2); and (b) Experiment 3b (tilted stimuli; n=4).

Fig. 7

Contrast sensitivity as a function of spatial frequency in both neutral and peripheral conditions for two observers in Experiment 4.