Scaled position integration areas: accounting for Weber’s law for separation

Perhaps the most basic fact about human visual encoding of relative spatial position is that the length or separation discrimination threshold increases with the mean length or separation being judged. In this study, the cause of this increase was investigated by measuring the effect of a parallel flanking line on the perceived separation of a pair of target lines. A standard separation discrimination paradigm was used with the flanking line placed outside the target pair. Perceived target separation was increased by the presence of the flanking line whenever the distance to the flanking line was less than the mean target separation. Modeling this effect as the product of a Gaussian weighting function times the distance to the flanking line, we inferred the size of the position integration area. The increase in the position integration area with increasing separation was found to be sufficient to account for the concomitant increase in separation discrimination thresholds, i.e., for Weber’s law for separation.