Popout by unique eye of origin: A fingerprint of the role of primary visual cortex in bottom-up saliency

Except for one class in primary visual cortex (V1), all visual cortical neurons are binocular. Therefore, information about the eye of origin of inputs is unavailable in any visual area except V1. This study reports that the visual location of an ocularly unique input (e.g., an item in the left eye among background items in the right eye) or an ocular contrast (where the input eye changes) is salient pre-attentively. This supports the hypothesis that V1 creates a bottom-up saliency map (Li, Trends. Cog. Sci. 6:9-16 (2002)). In a display containing many uniformly tilted bars except for one uniquely tilted, moderately salient, target bar, human subjects were asked to report quickly whether the target was in the left or right half of the display. Subjects were unaware that the stimulus was presented in one of the following conditions randomly interleaved within a session: binocular, when all bars were presented binocularly; monocular, when all bars were presented to a single eye; and dichoptically congruent (DC), when the target bar was presented in a different eye from the background bars. Each subject's mean response time (RT) to the DC stimuli was significantly shorter than that to the monocular stimuli, and was typically shorter than, or occasionally comparable to, that to the binocular stimuli, suggesting that unique eye origin can speed up attentional shift to the orientation singleton. In subsequent experimental sessions, added to the randomly interleaved conditions above was another, dichoptically incongruent (DI), condition, when the target bar was presented in the same eye as the background bars, except that a single background bar, on the opposite half of the display to the target, was presented in the other eye. The subjects were informed of the different stimulus appearance and/or configuration conditions, and that they should respond only to the location of the orientation singleton and not of any other location that might attract their attention. Each subject's mean RT to the DI stimuli was significantly longer than that to the DC stimuli, suggesting that the unique eye origin can automatically distract attention away from a target spatially separate from itself. Qualitatively the same results were observed for an analogous orientation segmentation task in which subjects were asked to report quickly the location of a border between two textures of uniformly tilted bars differing in their tilt. Thus, unique eye of origin or ocular contrast is salient, attracting attention automatically to facilitate, or interfere with, the attentional shift to the task relevant location which coincides with,or is away from, the dichoptically salient location, respectively