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Helen Clark, John Perrone; The long and short of it: Size influences saccadic and pursuit behavior for objects moving in depth.. Journal of Vision 2015;15(12):202. doi: 10.1167/15.12.202.
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A size-speed illusion has been demonstrated for trains and cars approaching from the side whereby the longer trains are judged to be moving slower than shorter cars (Clark, Perrone & Isler, AAP, 55, 2013). We have also shown that this size-speed effect could be caused by observers saccading to, and tracking a point further from the front of longer objects, compared to shorter objects (Clark & Perrone, VSS, 2014). However it is currently unknown which visual aspect of the approaching objects instigates the change in eye movement behavior. Findlay (Vis Res., 22, 1982) measured saccades made toward two static squares separated horizontally and presented in the near periphery. He found that saccadic landing position was determined by the gap between the two squares. These findings were based on stationary stimuli but may provide insights into our size-speed effect and explain why observers look further from the front of long objects moving in depth thus generating slower pursuit velocities compared to short objects. We therefore tested Findlay’s theories with motion-based stimuli. Eye position and velocity were recorded as the observers judged the relative speed of two sequentially presented pairs of lines approaching from the periphery. Each pair in the sequence varied in the gap separation between lines (5.8° vs 2.2° deg). Results showed gap spacing had a significant effect on saccadic behavior; saccades landed further from the front line for large gaps than for small gaps. This is consistent with the static Findlay effect and the saccadic behavior observed with trains and cars. We also tested pairs of lines with different amounts of linear perspective but no significant differences were found in average eye position. These findings suggest that it is the length of objects in motion, rather than their perspective characteristics that determines saccadic landing positions and hence the perceived speed of the object.
Meeting abstract presented at VSS 2015
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