Abstract
Large objects appear to move slower than small objects (Leibowitz, 1985). This size-speed illusion may account for collisions between motor vehicles and trains at level crossings. Recent research has shown that approaching computer generated trains appear to be moving more slowly than motorcars (Clark, Perrone, & Isler, 2013). What is currently not known is the underlying mechanism responsible for this illusion. One possible reason for the misperceived relative speed is that different patterns of eye movements occur for the different sized objects. The perspective image of an approaching train is quite different to that of a car, with the front significantly larger than the back. It has been shown that initial saccades made to two unequal-sized stimuli land between the stimuli, but tend to fall closer to the larger stimulus than the smaller stimulus (Findlay, 1982). The length of such objects also appears to dictate saccadic landing positions, with longer objects having a more 'off-centre' landing position (Vitu, 1991). We therefore tested observers' relative speed perception for two computer generated moving objects (short and long) moving obliquely in depth towards the eye and presented sequentially. Image (x,y) position and eye velocity were recorded as the observers judged the objects' relative speed. Saccadic behavior was different for the short and long objects; initial saccades were localised around the visual centroid ('center of gravity') of the objects and hence were further from the front of the larger, long objects than the smaller ones. Pursuit velocities were also different and were slower for the long object. When observers were forced to track a fixation point at the same frontal location on the short and long objects the size-speed illusion was eliminated. Therefore different fixation positions and pursuit speeds for the small and large approaching objects seem to account for the illusion.
Meeting abstract presented at VSS 2014