Scanning the dynamics of reaching and grasping movements

Two experiments were conducted to determine whether multiple stable grasping patterns exist in reaching movements of adults. Grasping patterns were distinguished by the production of different relative phases of final hand closing, a parameter we define as the relative time of final hand closure (T_rfc). In Experiment 1, male and female subjects (N=6) produced rhythmical reaching movements by following an auditory metronome at a comfortable pace (1.1 Hz) and synchronizing the grasp of the object with each beat of the metronome. Subjects were told to reach a maximum aperture and begin the final hand closing to the object at a marker placed at different points along the movement trajectory, effectively changing the required value of T_rfc. The results showed a negative linear relationship between the required T_rfc and the constant error of T_rfc suggesting that the preferred pattern (i.e., with the lowest total variability (Henry, 1975)) was an attractor in the language of non-linear dynamics. In Experiment 2, subjects were provided a trajectory 'template' for their fingers to make it easier to produce the different required relative phases. However, the results supported Experiment 1 in that subjects were unable to consistently produce grasping patterns other than their preferred pattern. These results suggest that the principles of non-linear dynamics may be extended from bimanual coordination tasks (e.g., Kelso, 1981, 1984; Kelso, Buchanan and Wallace, 1991) to tasks with a functional goal, such as grasping an object.