A SPATIAL GRADIENT OF ACCELERATION AND TEMPORAL EXTENSION UNDERLIES 3ILLUSIONS OF MOTION

If an object (or cue), is presented and shortly afterwards a line is d rawn with one end near to the object, motion away from the object loca tion is induced within the line. This line-motion illusion has best be en explained by postulating a facilitative spatial gradient that accel erates signal transmission most strongly near to the object, and less so with Increasing distance away from the object. This simple accelera tive-gradient model was tested in four experiments by either briefly p resenting the line, or replacing the line rendering with a dot moving at high velocity towards (or away from) the initial object location. O bservers first perceived motion away from the cue followed by motion t owards the cue (hence this new illusion is referred to as the ''two mo tion percepts'', or TMP illusion). The generality of the TMP illusion was investigated through the reports of forty-five inexperienced under graduates who were presented with TMP displays. Observers who were ask ed to pictorially reproduce their motion experience, drew a line expan ding away from the cue then contracting back towards it 85% of the tim e. Over 90% of individuals reported experiencing the illusion with a q uickly moving dot, The effects of several presentation parameters were investigated by the moving-dot method, and it was concluded that the accelerative-gradient model by itself was inadequate to explain TMP ph enomena. Two extended versions of the gradient model are proposed that place the locus of the TMP effects in properties of motion detection mechanisms or in temporal aspects of visual signal transmission.