Mechanisms of displacement discrimination with and without perceived movement

A nearby visual reference point improves displacement discrimination. This effect occurs regardless of the delay between successive displays. The delay can be varied from short delays (50 ms), where observers report perceived movement, to long delays (2,000 ms), where observers report no movement. The similarity of reference effects for such different delay conditions was investigated by testing the independence of reference and delay effects. The critical independence property is whether the spatial variables (displacement and separation from the visual reference) combine independently of the temporal variable (delay). This kind of independence did not hold for displacement discrimination at delays of 50 ms and 2,000 ms. A further experiment tested for the independence property at delays of 500 ms and 4,000 ms, where observers reported no perceived movement. For these longer delays, the property was satisfied. These results are discussed using the general concept of a psychophysical mechanism. Such a mechanism combines the effects of several stimulus variables into a single mediating representation. By this analysis, at least two mechanisms must mediate the reference effect on displacement discrimination, one for delays of 200 ms and less and another for delays of 500 ms and more.