Assessing and reporting the accuracy of position measurements made with optical tracking systems

The use of optical tracking systems to record human movement is now widespread. Although such systems are convenient and potentially very accurate, they must be used carefully to ensure good data. This paper describes the procedures of calibration and reconstruction of position data in cartesian coordinates and suggests steps to maximize their accuracy. Procedures are proposed for characterizing the accuracy of measurement throughout the experimental workspace, and open discussion of the issue by the research community is invited.     

Relation between velocity and curvature in movement: equivalence and divergence between a power law and a minimum-jerk model

Unconstrained hand movements typically display a decrease in hand speed around highly curved sections of a trajectory. It has been suggested that this relation between tangential velocity and radius of curvature conforms to a one-third power law. We demonstrate that a one-third power law can be explained by models taking account of trajectory costs such as a minimum-jerk model. Data were analyzed from 6 subjects performing elliptical drawing movements of varying eccentricities. Conformity to the one-third power law in the average was obtained but is shown to be artifactual. It is demonstrated that asymmetric velocity profiles may result in consistent departures from a one-third power law but that such differences may be masked by inappropriate analysis procedures. We introduce a modification to the original minimum-jerk model by replacing the assumption of a Newtonian point-mass with a visco-elastic body. Simulations with the modified model identify a basis for asymmetry of velocity profiles and thereby predict departures from a one-third law commensurate with the empirical findings.     

Intermittent low-calorie regimen and booster sessions in the treatment of obesity

The present study investigated two new weight-control strategies: an intermittent low-calorie regimen and intermittent scheduling of booster sessions. A new approach to predicting patient weight loss, based on a pretreatment assessment with a highly-structured eating regimen, was also studied. Forty-eight obese patients were randomly assigned to either a Standard Behavioral weight-control condition or to an Intermittent Low-calorie Regimen (< 750 cal for 2 days/week) condition and to one of two maintenance schedules: a Spaced schedule in which the six booster sessions were held at monthly intervals, or a Massed schedule, in which four of the six meetings were held during the third month. Neither the intermittent low-calorie regimen nor the intermittent scheduling of booster sessions significantly affected weight loss. However, weight loss at 1 year was related to compliance to self-monitoring and to self-reported change in eating habits and exercise. In addition, weight loss at 1 year was related to weight loss during the initial pretreatment assessment period.     

The timing of interresponse intervals

Analogs of models of duration discrimination are here related to the timing of discrete motor responses. The measure of interest is the variability in duration of intervals collected in short interval reproduction tasks. For data from a Morse key-tapping task, it is shown that, taken separately, neither of the models described can completely account for the relation between the mean and the variance of the interresponse intervals.     

The height of handwriting

An experiment is described that examines the basis of the control of vertical amplitude (height) of handwriting. It is shown that changes in movement duration accompany changes in height whether the latter are associated with instructions to write larger than normal (between words) or because the letters to be written (e, l) differ in height (within words). The proportional differences in movement duration match the proportional differences in amplitude between words but not within words. It is concluded that two different time-based mechanisms are invoked for handwriting height control: interval adjustment for writing size changes between words and duration selection for letters differing in height within a word.     

Timing of movement phases of a repeated response

How can one determine the nature of timing control that exists among various distinct phases of movement in a repetitive activity? A statistical approach is suggested and is exemplified by reference to data on the timing of two phases of movement, arrival at and departure from the response plate, in repetitive finger tapping. Two contrasting models are presented, and the predictions for the covariation of the various intervals between the two movement phases are compared. Data on finger tapping support the model that assumes the initiation of each phase is centrally determined without reference to the time of occurrence of the immediately preceding phase.