Eli Brenner, Jeroen B.J. Smeets; Smooth pursuit and manual interception. Journal of Vision 2009;9(8):1139. doi: 10.1167/9.8.1139.
Why do people generally pursue objects with their eyes while they move to intercept them? To find out we compared interception in two tasks with the same spatial and temporal requirements (1cm and 100ms) but in which different eye movements would maximize the critical spatial resolution. People had to either hit a target that was moving behind a line at the moment that it passed a gap in the line, without hitting the line, or to hit a target moving in front of a line into a gap in the line. The average pursuit gain during the last 200ms before the hit was 0.2 when hitting through the gap and 0.7 when hitting into the gap. The standard deviation in the hand's position as it passed the (fixated) gap was 3mm when hitting through the gap. The standard deviation in the hand's position relative to the (pursued) target was 6mm when hitting the target into the gap. The difference can be interpreted as a standard deviation in matching the timing of the hand to that of the target of 24ms. The average variability in the target's position at the moment of the hit was almost 8mm (40ms) when hitting through the gap, indicating that people are less good at timing interception on the basis of the retinal image of the target approaching the fovea than on the basis of the pursued target's position. The corresponding variability was similar when hitting into the gap, in which case the gap's image moved across the retina. In both tasks we found large systematic errors in the target's position with respect to the gap at the time of the hit. These errors were consistent with subjects relying heavily on previous trials for timing their hit. Thus, making appropriate eye movements is important for interception.