Scale invariant features of differential spatial displacement discrimination.

For a configuration of three blobs, with Gaussian spatial contrast profiles, at threshold luminance contrast, differential spatial displacement discrimination thresholds were determined. The blobs were arranged one above the other, at equal spatial intervals. Thresholds were determined for displacements of the middle blob both orthogonal to and along the axis joining the outer two blobs. Thresholds for both tasks were obtained as a function of both the resolution and the separation of the blobs. The thresholds for both tasks increased linearly with decreasing resolution (increasing blur), for a constant ratio of the resolution parameter and the separation of the outer two blobs. At all levels of resolution there are two blob-separation regimes, in which different strategies are used to compute differential spatial displacements. Independent of the level of resolution, transition between those regions occurs when the separation of the outer two blobs is a constant multiple (approx. 25) of their blur parameter. We interpret these results as further evidence for a scale-invariant mechanism for differential spatial displacement computation.