Determination of target distance in a structured environment: Selection ofvisual information for action

In the literature relating to visuo-motor control, controversial data are f ound concerning the consequence of enriching the visual scene in the specif ication of the target's spatial coordinates. In this paper four experiments were carried out to unravel this issue. Based on spatio-temporal analysis of pointing movements carried out in an open loop condition, the effect of appending contextual elements in the vicinity of a visual target was invest igated, taking into account (1) their location in the visual field, (2) the extent of the movement, and (3) their presence during the planning and/or execution period of the movement. Taken as a whole, results showed that enr iching the visual scene gave rise to a decrease of perceptual underestimati on of distance (with no effect on the direction parameter), as otherwise ob served (dark environment). Though not deeply affecting reaction and movemen t time, this effect held whatever the target position, provided that the co ntextual elements were situated between the initial and terminal position o f the hand trajectory. The magnitude of the effect was, however, dependent upon the space conferred to the visual context. Furthermore, a higher spati al performance was observed when the latter was provided during the plannin g of execution period of the movement. Both effects combined when contextua l elements were provided during the entire movement, which suggests a conti nuous updating of target coordinates during the whole motor performance. Al together these findings underline a dynamic aspect of space perception, ori ginating, in part, in the functional use of contextual cues in the coding o f target distance. They also suggest that, provided the visual environment is structured, the retinal signal is widely used in the perception of targe t distance in visuo-manual tasks.