Edie Stevenson

University of Colorado at Boulder, Boulder, Colorado, United States

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Publications (2)0 Total impact

  • Stephen A. Wallace, Edie Stevenson, Alison Spear
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    ABSTRACT: 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 (Trfc). 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 Trfc. The results showed a negative linear relationship between the required Trfc and the constant error of Trfc 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.
    Human Movement Science - HUM MOVEMENT SCI. 01/1994; 13(2):255-289.
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    ABSTRACT: The purpose of this experiment was to examine the type of strategy subjects used in catching an object approaching on a non-intersection trajectory at different, but predictable, constant velocities. A small treadmill mounted on a table top supported a target dowel to be ‘caught’ by the subject's hand, which rested at the end of the treadmill. The treadmill produced six constant velocity approaches administered in blocks of trials in ascending order. The object and the subject's finger movements were kinematically examined using a three-dimensional motion analysis system. The results indicated that a type of perceptual-motor coupling between the value of the optical variable tau (τ) at initiation of the catching movement and the duration of the catching movement was being used, rather than a consistent τ strategy. In addition, a relatively constant ratio of the value of τ at initiation and τ at the start of the catching movement was observed across the six object velocities. These results show limitations in the use of a critical τ strategy, at least in this type of task. A functional relationship between the information in τ and how a movement is performed may allow for greater flexibility in the coordination of interceptive acts.
    Human Movement Science. 01/1992;