Spatial partitioning during visual search of a dyad

It has been shown recently that people can use visual feedback about the current gaze direction of a second searcher to effectively partition the search space (Zelinsky et al., VSS abstract 2005, p. 193). We investigated whether a partitioning of space during dual visual search can occur without explicit feedback and verbal communication. Pairs of participants were seated side-by-side and searched for a target letter (“T”) among a set of distractor letters (“L”), projected on a large projection screen (2.20 x 1.80 m), indicating target present or absent on their individual keyboard. The instructions were varied: In experiment 1 (N=20) neither collaboration nor competition was emphasized (neutral instruction condition, NIC). In experiment 2, participants (N=22) were instructed to collaborate in order to obtain an announced gratification for the best group performance (collaborative instruction condition, CIC). In both experiments participants wore hearing protections and communication was prohibited. In NIC, participants responded fastest when the target appeared at the opposite side of the screen's center. In CIC, however, reaction times were faster when target letters were presented on the same side of the participant, resulting in a linear relationship between horizontal target eccentricity and the prevalence of the faster response at a particular seating side. We conclude that attention became spatially biased in horizontal direction during dual visual search, as a result of the physical juxtaposition of the participants. These findings can be interpreted as evidence for an implicit spatial partitioning of search space which can occur even without explicit feedback and verbal communication