Abstract
A briefly presented bar is perceived as extended from a pre-cued position, a phenomenon known as illusory line motion. The present study demonstrated a different sort of illusory motion derived purely from top-down processing. After the brief presentation of a red arrow cue (leftward, rightward, or control stimulus) above the central fixation cross on the white screen, a black horizontal bar with approximately 24° in length was presented below the arrow cue. A horizontal center of the arrow cue was aligned with that of the bar. The durations for the arrow and the bar were 100 and 50 msec, respectively. The observer's task was to judge the direction in which the bar was perceived as extended (right or left, in a 2AFC paradigm). One of six ISIs (0, 100, 400, 700, 1000, and 1500 msec) was employed between the arrow cue and the bar on each trial. The results revealed that with a very short ISI observers perceived the illusory motion in the direction suggested by the arrow cue even though there was no motion signal in the horizontal direction. Meanwhile, with the increase in ISI, the probability of illusory motion in the arrow direction decreased. This kind of illusory motion is attributable to attentional shift; spatial attention might firstly engage at the arrow position and gradually shift along the arrow direction, and this produces latency difference between the position around the arrow cue and the edge of the bar in the side the cue indicated.