Sustained and transient covert attention: A test for signal enhancement

How similar are sustained and transient covert attention? We know that the former is conceptually-driven and voluntary, whereas the latter is stimulus-driven and automatic. However, we do not know whether both types of attention are mediated by similar mechanisms. It has been shown that the Signal Enhancement Model can account for the effect of transient attention (Carrasco, Penpeci-Talgar, Eckstein, 2000). In this study, by using either an endogenous or exogenous precue, we investigated whether the same mechanism could mediate both sustained and transient by comparing their effects on contrast sensitivity across a range of spatial frequencies. Either an endogenous (a 150ms central pointer indicating target location followed by a 150ms ISI) or exogenous (a small circle peripherally flashed for 40ms adjacent to the target location followed by a 60ms ISI) precue was used to indicate the location of the target - a Gabor patch ranging in spatial frequency from 1-8 cpd. Five observers performed a 2AFC orientation discrimination task on a supra-threshold target tilted 4 to the right or left. The target appeared for 100ms at one of 8 possible locations at 4 eccentricity. To eliminate all variables that could be accounted for by an External Noise Reduction model, the target was supra-threshold and presented alone, without any distracters or masks. Contrast thresholds were obtained via an adaptive staircase procedure (QUEST). We compared the magnitude of contrast sensitivity enhancement brought about by both types of attention. According to External Noise Reduction, in the absence of any added external noise there should be no attentional effect on contrast sensitivity. However, given these conditions, we found that sustained covert attention enhanced contrast sensitivity. Moreover, the magnitude of the effect was similar to that of transient attention. These results provide evidence in support of the existence of Signal Enhancement for both types of covert attention.