MECHANISMS FOR DISTANCE REPRODUCTION IN PERCEPTUAL AND MOTOR-TASKS

Two experiments were carried out: the control experiment and the doubl ing-distance experiment. In the control experiment subjects were prese nted with two visual stimuli whose distance was randomly varied. Subje cts were required to reproduce the interstimulus remembered distance i n two conditions. In one condition (reproduction by pointing) they poi nted to a virtual position in space. In the other condition (visual re production) they matched the distance by using two other visual stimul i. In the doubling-distance experiment, distances between the two rand omly presented stimuli were half of the distances used in the control experiment. Subjects were required to reproduce the double of the pres ented distance. As in the control experiment, reproduction was execute d in two conditions: reproduction by pointing and visual reproduction. In both experiments variable and constant errors were measured. Point ing kinematics were also analysed. The results of the control experime nt showed that subjects underestimated distance in reproduction by poi nting, whereas they overestimated distance in visual reproduction. Var iable errors increased with increasing distance, whereas they were not influenced by the type of reproduction. In the doubling-distance expe riment, subjects generally overestimated distance by the same amount i n both conditions. However, overestimation decreased with distance dur ing reproduction kinematics varied The results of the duction by point ing. between the two experiments. control experiment confirm the hypot hesis that perceptual judgement and visuo-motor transformation are two separate processes during which the same object attributes are indepe ndently analysed. However, the results of the doubling-distance experi ment suggest that perceptual judgement and visuo-motor transformation use the same mechanisms when object attributes are deduced by mental e laboration.