Illusory displacement of a moving trace with respect to the grid during oscilloscope motion

Observers report that a trace streak, which follows a sinusoidal path, moves vertically with respect to the oscilloscope’s grid when an oscilloscope is oscillated in the vertical plane. The vertical component of the trace streak motion with respect to the grid is illusory. This illusion is stable across a limited range of illumination and physical motion conditions. We hypothesize that this illusion is based on the manner in which the visual system calculates the vertical location of the grid and the trace: the trace location is determined on a moment-to-moment basis, whereas the grid tends to be seen in its average vertical position. The results of two experiments indicate that this hypothesis can account, at least partially, for the illusion. The illusion may have practical implications for pilots or navigators who track target blips on radar screens in moving aircraft.