Does attention move or spread during mental curve tracing?

There are two theories that attempt to explain how attention is deployed when lines are traced. Initially, it was believed that a covert zoom lens moved along the line. Recent evidence has, however, suggested that attention spreads along the line, rather than moving along it, perhaps as part of an effortful object-parsing process. Three experiments tested the spreading and moving accounts of line tracing. Participants were presented with two intertwined lines and were required to trace one to find the correct target. On half the trials, a masked change occurred, most often near the top of the target line, that reversed the required response. If attention spreads along the line, the participants should have been able to notice the change whenever it occurred during the tracing process. However, the participants found it harder to spot the change if it occurred late in the tracing process. This suggests that resources were less frequently available to detect changes on portions of the line that had already been traced when the change occurred. The results argue against a spreading trace of attention that encompasses the whole line.