Privileged visual processing of the straight-ahead direction in humans.
Résumé
At any moment, the objects we face are endowed with a special behavioral status, either as potential obstacles during navigation or as optimal targets for visually guided actions. Yet, the gaze frequently jumps from one location to another when exploring the visual surroundings, so that objects located straight-ahead are often seen from the corner of the eyes. In the present study, we tested the hypothesis that peripheral vision might nevertheless ensure a privileged processing of these behaviorally important objects. Human subjects were asked to respond as fast as possible to the appearance of visual objects in their peripheral field of view while gazing successively in different directions. The visual objects formed similar images on the retina and differed only with respect to their egocentric location: either straight-ahead or eccentric with respect to the head/body midline. We found that straight-ahead objects elicit consistently shorter behavioral responses than eccentric objects (median difference of at least 10 ms). Additional experiments indicate that neither binocular visual cues nor full attentional resources play a fundamental role in this mechanism, and that it cannot be resumed to a simple preference for objects contralateral to the direction of gaze. These results are in agreement with recent electrophysiological findings showing that the early integration of gaze-related signals in the visual cortex of macaque monkeys lead to a higher neuronal sensitivity to the straight-ahead direction.