Spatial location and hyperacuity: flank position within the centre and surround zones.

Sensitivity to a horizontal displacement of a vertical line was measured in order to ascertain the influence of the location of parallel flanking lines on the apparent position of features in visual space. The first experiment confirmed that the introduction of the flank added a component to the apparent shift which was towards the flank for small separations (less than 3-4') and away from the flank with larger separations. The second experiment investigated the notion that apparent location is derived by collecting information only from zones adjacent to the target and limited in the vertical extent by the target's height i.e. information orthogonal to the target's main axis. This was done by placing a vertical flank at a horizontal distance from the target that would be clearly within one zone or the other and measuring the effect of a vertical separation between the two flank halves. In the surround zone the amount of repulsion obtained was not influenced by vertical separation of the flank halves, even when they were several minutes higher (or lower) than the target line. In the central zone attraction was only obtained when the vertical separation was small enough to provide some overlap of the lines in the horizontal direction. With larger separations substantial repulsion was obtained. We conclude that while the central 'attraction' zone may only use information distributed in the direction of the line's displacement, it does so only within a 3-4' range on either side of the target line. The surround zone is not similarly limited in the region over which it collects information to influence the apparent location of features.