Psychomotor decline can be described by discontinuities in response trajectories

Force exerted in the isometric button-press response in a choice reaction time experiment was measured in order to study discontinuities in the response trajectory. Groups of young and elderly subjects were compared on performance with four choice reaction time tasks of varying difficulty. An additional comparison (with one task only) was made with a group of brain-damaged subjects. Velocity changes within the response trajectory were used to locate its discontinuities. The principal finding was that response discontinuities were least frequent in young subjects, intermediate in elderly, and most frequent in brain-damaged. Also, in the comparison of young and elderly across four stimulus conditions, the number of discontinuities was greater with more difficult than with easier tasks. Psychomotor decline was interpreted as an increase in the number of response discontinuities.