Exogenous spatial attention shortens perceived depth

Although spatial attention has been found to alter the subjective appearance of visual stimuli in several perceptual dimensions, no research has explored whether exogenous spatial attention can affect depth perception, which is a fundamental dimension in perception that allows us to effectively interact with the environment. Here, we used an experimental paradigm adapted from Gobell and Carrasco (Psychological Science, 16[8], 644-651, 2005) to investigate this question. A peripheral cue preceding two line stimuli was used to direct exogenous attention to either location of the two lines. The two lines were separated by a certain relative disparity, and participants were asked to judge the perceived depth of two lines while attention was manipulated. We found that a farther stereoscopic depth at the attended location was perceived to be equally distant as a nearer depth at the unattended location. No such effect was found in a control experiment that employed a postcue paradigm, suggesting that our findings could not be attributed to response bias. Therefore, our study shows that exogenous spatial attention shortens perceived depth. The apparent change in stereoscopic depth may be regulated by a mechanism involving direct neural enhancement on those tuned to disparity, or be modulated by an attentional effect on apparent contrast. This finding shows that attention can change not only visual appearance but also the perceived spatial relation between an object and an observer.