Investigation of visual space using an exocentric pointing task

Classically, it has been assumed that visual space can be represented by a metric. This means that the distance between points and the angle between l ines can be uniquely defined. However, this assumption has never been teste d. Also, measurements outdoors, where monocular cues are abundant, conflict with this model. This paper reports on two experiments in which the struct ure of visual space was investigated, using an exocentric pointing task. In the first experiment, we measured the influence of the separation between pointer and target and of the orientation of the stimuli with respect to th e observer. This was done both monocularly and binocularly. It was found th at the deviation of the pointer settings depended linearly on the orientati on, indicating that visual space is anisotropic. The deviations for configu rations that were symmetrical in the median plane were approximately the sa me, indicating that left/right symmetry was maintained. The results for mon ocular and binocular conditions were very different, which indicates that s tereopsis was an important cue. In both conditions, there were large deviat ions from the veridical. In the second experiment, the relative distance of the pointer and the target with respect td the observer was varied in both the monocular and the binocular conditions. The relative distance turned o ut to be the main parameter for the ranges used (1-5 m). Any distance funct ion must have an expanding and a compressing part in order to describe the data. In the binocular case, the results were much more consistent than in the monocular case and had a smaller standard deviation. Nevertheless, the systematic mispointings remained large. It can therefore be concluded that stereopsis improves space perception but does not improve veridicality.