Abstract
Human observers searched a background texture of 639 uniformly tilted bars, spanning 34°×46° in visual angle, for a bar with a unique tilt located at one of 28 locations, 15° from the display's centre. There were three critical, randomly interleaved, trial conditions: monocular, with all bars presented to the same single eye; and dichoptic congruent (DC) or incongruent (DI), involving an ocular singleton that was either the target bar (DC) or a background bar on the opposite lateral side from the target (DI). When the target bar was tilted 50° from the background bars, reaction times to report whether it was in the left or right half of the display were shorter in the DC condition, longer in the DI condition, than that in the monocular condition, whether or not subjects were aware or informed of the different dichoptic conditions. In another experiment, when the target bar was tilted 20° from the horizontal background bars, and with all bars having the same (uniform) or different (non-uniform) luminance values and being masked after 200 milliseconds, observers' reports of the target's tilt direction from horizontal were more accurate in DC than the other conditions, irrespective of the luminance condition. With the same stimuli, but without a tilt singleton, the same observers were at chance reporting the existence of an ocular singleton when the bars had non-uniform luminances. These findings suggest that the ocular singleton acted as a valid or invalid exogenous cue to the target. Further experiments showed that set size effects in a difficult visual search under monocular presentation can be dramatically diminished in the DC presentation. Since primary visual cortex (V1) has substantially more monocular cells (for eye origin information) than other visual areas, and is least associated with awareness, our findings support the hypothesis that V1 acts as a bottom-up saliency map (Li 2002, Trends Cognitive Sciences, 6(1):9–16).
Gatsby Charitable Foundation for funding.