OBJECT-BASED VISUAL-ATTENTION IN LUMINANCE INCREMENT DETECTION

A cued reaction time task was used to test the hypothesis that there i s an 'object-based' component to shifts of attention mediating the det ection of luminance increment targets. The test stimulus consisted of two intersecting triangles forming a 'Star of David'. In two experimen ts, the cue was a brief brightening of one triangle. The target (a bri ght green dot) appeared on one of the triangles after a delay of 100, 200 or 500 msec. In one experiment, the target was more likely to appe ar on the cued triangle. In a second experiment, there was no continge ncy between cue and target. In both cases, reaction times to targets w hich appeared outside (but not inside) the cued triangle were more tha n 10 msec longer than other targets, but only al the shorter cue-targe t delays. This indicates that the attentional system which regulates l uminance increment detection cannot select the cued triangle. It appea rs that the attentional spotlight can be briefly deformed into a trian gular shape, and that it is the rapid, fast-decaying and reflexive exo genous system, rather than the slower acting, persistent and voluntary endogenous system, that mediates this effect. A third experiment usin g a central, symbolic cue showed no significant cue-validity effects, indicating no contribution from the endogenous system. It is concluded that tasks requiring only stimulus detection cannot unequivocally dis criminate between spatial and object-based components of attention. (C ) 1997 Elsevier Science Ltd.