Sequence preparation is not always associated with a reaction time cost.

Area of Science:

• Motor Neuroscience
• Cognitive Psychology
• Human Movement Science

Background:

• Reaction time (RT) increases with longer movement sequences, traditionally attributed to greater sequence preparation.
• However, ambiguity in selecting the first movement target also influences RT, complicating interpretation.
• The precise contribution of sequence preparation versus target ambiguity to RT remains unclear.

Purpose of the Study:

• To disentangle the effects of sequence preparation and first-target ambiguity on reaction time.
• To determine if preparing multiple movements incurs additional temporal costs.
• To investigate the influence of spatial target arrangement on movement initiation.

Main Methods:

• Developed a paradigm to independently manipulate the number of displayed targets, number of targets to acquire, and spatial arrangement.
• Measured reaction time (RT) and kinematic data during reaching movements.
• Varied first-target ambiguity by altering the spatial configuration and number of potential initial targets.

Main Results:

• RT increased with the number of sequence elements, but this effect was entirely explained by first-target ambiguity.
• When first-target ambiguity was eliminated, RT remained constant regardless of the number of displayed targets.
• Kinematic analysis confirmed that sequences were prepared even when RT did not increase.

Conclusions:

• Sequence preparation for reaching movements does not inherently increase reaction time.
• Observed increases in RT for longer sequences are attributable to the ambiguity in selecting the initial target.
• Preparing multiple reaches incurs no additional temporal cost beyond that of single reach preparation when target ambiguity is controlled.