Gaze capture by task-irrelevant, eye of origin, singletons even without awareness during visual search

The eye of origin of inputs is barely encoded in cortical areas beyond primary visual cortex. Thus human observers typically fail to perceive ocular distinctiveness - as when an item (an ocular or eye of origin singleton) is presented to one eye among a background of all other items presented to the other eye. Nevertheless, I recently showed (Zhaoping, 2008) that such singletons behave as exogeneous cues for attention. Visual search for an orientation singleton target bar among uniformly tilted distractor bars was easier (harder) if the target (or respectively a distractor) bar was also an ocular singleton. Using eye tracking (via electro-oculography or video tracking), I now confirm that this ocular singleton indeed automatically attracts gaze. Observers searched for an orientation singleton among hundreds of uniformly tilted distractor bars to quickly report whether the target was in the left or right half of the display spanning about 40x30 degrees. All bars were presented monocularly, and the gaze started at the center of the display at stimulus onset. If the ocular singleton was present, the first saccade after stimulus onset was typically directed to the lateral side of the display containing it, whether or not the ocular singleton was associated with the true (orientation-defined) target or with a distractor bar on the opposite lateral side from the target. In a second experiment using the greater accuracy of video eye tracking (albeit in a smaller display), observers had to quickly find and gaze at the orientation singleton target, an ocular singleton was present as a distractor bar in half the trials. The search display was masked once observers' gaze arrived at the target. Observers were often unable to report after mask onset whether they had seen the ocular singleton during search, even if they had directed their gaze to it