Abstract
In our visual search experiment, each item had two bars: one was tilted 45 degrees to the left from vertical for distractors and 45 degrees to the right for the target; the other is a horizontal or vertical bar centered at the same location. Each target or distractor is a rotated version of all other items. As the target had a uniquely oriented bar, it was typically the most salient item, both by the Feature Integration Theory (Treisman & Gelade, Cognitive Psychology 12:97–136, 1980), and the theory of the bottom up saliency map in V1 (Li, Trends in Cognitive Sciences, 6:9–16, 2002). The subjects were informed of this unique orientation, and were instructed to quickly report by button press whether the target was in the left or right half of the stimulus display. Reaction times (RTs) were measured and subjects' eye positions were tracked. We also measured the “reaction time of the eye” (RTE) defined as the first time that the eye position is close enough to the target. Typically, RT > RTE. Subjects reported that the target often “vanished” after they had initially detected it. Eyes were often seen to saccade to the target, then moved away or loitered around for a long time, before moving back to the target and the subject's button press. A control condition was designed by changing the uniquely oriented bar in the target to tilt 20 degrees to the right from vertical, so the target was no longer a rotated version of distractors. The gap between RT and RTE was significantly shorter in this control than that in the original condition, even though their RTs were comparable. The same result was found for other control conditions with comparable RTs. In the original condition, it is as if the eyes, driven by V1 through superior colliculus, locate the target by the bottom up saliency process of unique orientation pop out, while the top-down process of object recognition, presumably rotation invariant, intervenes with the fact that all items are identical objects.
This project is supported by RCUK and Gatsby Charitable Foundation