| Summary: | Many studies have analysed how goal directed movements are corrected in
 response to changes in the properties of the target. However, only simple
 movements to single targets have been used in those studies, so little is
 known about movement corrections under more complex situations.
 Evidence from studies that ask for movements to several targets in sequence
 suggests that whole sequences of movements are planned together. Planning
 related segments of a movement together makes it possible to optimise the
 whole sequence, but it means that some parts are planned quite long in
 advance, so that it is likely that they will have to be modified. In the present
 study we examined how people respond to changes that occur while they are
 moving to the first target of a sequence. Subjects moved a stylus across a
 digitising tablet. They moved from a specified starting point to two targets
 in succession. The first of these targets was always at the same position but
 it could have one of two sizes. The second target could be in one of two
 different positions and its size was different in each case. On some trials the
 first target changed size, and on some others the second target changed size
 and position, as soon as the subject started to move. When the size of the
 first target changed the subjects slowed down the first segment of their
 movements. Even the peak velocity, which was only about 150 ms after the
 change in size, was lower. Beside this fast response to the change itself, the
 dwell time at the first target was also affected: its duration increased after the
 change. Changing the size and position of the second target did not influence
 the first segment of the movement, but also increased the dwell time. The
 dwell time was much longer for a small target, irrespective of its initial size.
 If subjects knew in advance which target could change, they moved faster
 than if they did not know which could change. Taken together, these results
 suggest that the whole sequence is treated as one action, which can be
 corrected if the properties of any of the targets change. The precise nature and
 timing of the correction depends on how the change influences the task.
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