Correcting slightly less simple movements

RESUMEN

Many studies have analysed how goal directed movements are corrected inresponse to changes in the properties of the target. However, only simplemovements to single targets have been used in those studies, so little isknown about movement corrections under more complex situations.Evidence from studies that ask for movements to several targets in sequencesuggests that whole sequences of movements are planned together. Planningrelated segments of a movement together makes it possible to optimise thewhole sequence, but it means that some parts are planned quite long inadvance, so that it is likely that they will have to be modified. In the presentstudy we examined how people respond to changes that occur while they aremoving to the first target of a sequence. Subjects moved a stylus across adigitising tablet. They moved from a specified starting point to two targetsin succession. The first of these targets was always at the same position butit could have one of two sizes. The second target could be in one of twodifferent positions and its size was different in each case. On some trials thefirst target changed size, and on some others the second target changed sizeand position, as soon as the subject started to move. When the size of thefirst target changed the subjects slowed down the first segment of theirmovements. Even the peak velocity, which was only about 150 ms after thechange in size, was lower. Beside this fast response to the change itself, thedwell time at the first target was also affected its duration increased after thechange. Changing the size and position of the second target did not influencethe first segment of the movement, but also increased the dwell time. Thedwell time was much longer for a small target, irrespective of its initial size.If subjects knew in advance which target could change, they moved fasterthan if they did not know which could change. Taken together, these resultssuggest that the whole sequence is treated as one action, which can becorrected if the properties of any of the targets change. The precise nature andtiming of the correction depends on how the change influences the task