Mechanisms of displacement discrimination with a visual reference.

A nearby visual reference point facilitates displacement discrimination. For example, a nearby stationary point of light improves discriminating left or right displacement of a point by several fold. This reference effect interacts with the temporal characteristics of displacement. Discrimination thresholds rise with increases in the delay between the offset of the initial stimulus and the onset of the displaced stimulus. Moreover, the effect of a nearby reference point is larger with a long delay than with a short delay. This interaction was investigated using two nonexclusive hypotheses of the mediating mechanisms. A single-signal hypothesis specifies a single mediating mechanism that aggregates the effect of displacement and delay. A single-noise hypothesis specifies a single mediating mechanism that aggregates the effect of reference and delay. Each of these hypotheses predicts an equivalence property similar to the equivalent background principle of adaptation research. The equivalence required by the single-signal hypothesis was not satisfied, disconfirming the hypothesis. In contrast, the equivalence required by the single-noise hypothesis was satisfied. The equivalence was extended to delays where the reported judgment was of perceived position and not of perceived movement. The second result is compatible with displacement discrimination mechanisms that are the same function of displacement, whatever their other differences.