Abstract
Goal: Exogenous spatial attention can be automatically engaged by brief peripheral cues. However, because studies of exogenous attention use highly salient precues to reliably engage it, little is known about the cueing threshold (i.e. degree of salience required to engage attention). Does exogenous attention have an “all-or-nothing” effect above some critical cue salience, or does it increase gradually? We explore this question by assessing attentional effects on apparent contrast (Carrasco, Ling & Read, 2004; Fuller, Rodriguez & Carrasco, 2007) as a function of cue contrast.
Methods: We first obtained psychometric functions for a 2AFC cue localization task, at 7° eccentricity, 3° above the horizontal meridian, over a range of seven cue contrasts. In the main experiment, a brief single precue was presented at one of the same peripheral cue locations or at fixation (neutral), followed after a 110 ms SOA by two 4° Gabor stimuli at 7° eccentricity on the horizontal meridian. Observers reported the orientation of the stimulus that was higher in contrast. By assessing which stimulus observers perceived as higher in contrast, we obtained psychometric functions and their concomitant points of subjective equality (PSE). The magnitude of the attention effect was calculated for each of five cue contrasts as the difference between the PSEs with the peripheral and the neutral cues. We tested for cue bias in a separate session using 100% contrast post-cues (cues presented after the stimuli), instead of precues.
Results: Observers perfectly localize the cue at ∼8% cue contrast. The attentional effect on apparent stimulus contrast emerges only above this level of cue contrast, and increases continuously with cue contrast. There is no attentional effect with the post-cue, ruling out cue bias. Despite its automaticity, the magnitude of exogenous attention's benefit modulates over a considerable range of cue salience. It is not “all-or-nothing.