Anticipatory modulation of precision grip force with variations in limb velocity of a curvilinear movement

The authors examined the relationship between peak velocity of a discrete horizontal elbow flexion movement in which the hand path was curvilinear and premovement modulation of precision grip force. The velocity of the movements of 7 participants was varied from maximal velocity to a velocity that required several seconds to reach a target. An object instrumented with force transducers for the forefinger and thumb measured precision grip force. There was a positively accelerating quadratic relationship between grip force change before movement and peak velocity of the ensuing limb movement. On some low-velocity trials, premovement grip force modulation reflected a net decrease. In contrast, high-velocity trials were preceded by net increases in grip force. Using cluster analysis, the authors classified grip forces in low-velocity movements as an empirically distinct set of entities from grip forces in high-velocity movements. The cluster of high-value grip forces suggested an anticipatory strategy that allowed participants a large safety margin in grip force to avoid object slip on movement initiation. The cluster of low-value grip forces at movement initiation suggested a second anticipatory strategy in which participants changed grip force very little, perhaps to increase the ability of proprioceptors in the hand to sense force changes. Those findings suggest that modulation of grip force before initiation of movements in which the hand path is curvilinear may be governed by two distinct velocity-dependent anticipatory strategies.     

The Neuropsychological Bases of Category Learning

There is a long history of research into how people learn new categories of objects or events. Recent advances have led to new insights about the neuropsychological basis of this critically important cognitive process. In particular, there is now good evidence that the frontal cortex and basal ganglia contribute to category learning, that medial temporal lobe structures make a more minor contribution, and that categorization rules are not represented in visual cortex. There is also strong evidence that normal category learning is mediated by at least two separate systems. A recent neuropsychological theory of category learning that is consistent with these data is described.     

Impulse Characteristics in Rapid Movement

Three untested assumptions of the impulse-variability model were examined in two experiments utilizing rapid, uni-planar limb movements. Experiment 1 varied movement distance (A) and movement time (MT) in a rapid-timing paradigm where the subject moved a lever through a certain distance in a certain time. Experiment 2 varied A in a reversal response where the S made a rapid elbow flexion and extension in a given MT. Displacement recordings were made on every trial. KR (knowledge of results) about MT was given after every trial. The results can be summarized as follows: (a) As predicted by the model, variations in impulse size and velocity were directly related to the impulses size; (b) There was no correlation between the accelerative and decelerative impulse durations recorded during the reversal response supporting the notion that the impulses might be independent; (c) Negative correlations (–.20 to –.50) were demonstrated between peak acceleration and impulse duration for both experiments, counter to the predictions of the model; and (d) Counter to the predictions of the model, timing error (VEt) increased as A decreased for rapid-timing responses. When the correlational results are taken into account, the model has the capacity to account for curvilinear relationships between relative timing error and movement speed. Overall, the results suggest that the impulse variability model requires some restructuring before it can be considered a viable model for the control of rapid limb movements.     

The Role of Rights in Practical Reasoning: ``Rights' versus ``Needs'

This paper considers the proposal, associated with the CriticalLegal Studies movement (CLS) that the language of rights shouldbe replaced with the language of needs. It argues that thelanguage of needs is no less contestable, and has an even lesssecure relation to the idea of social duty than the idea ofrights. The paper rejects the notion that rights are usuallynegative claims on others – claims to their forbearance –and argues that rights can be understood perfectly well as adiscourse in which affirmative claims are articulated. Moreover,rights are naturally associated with the idea of a moral system– a well-thought-through set of demands, in which potentialconflicts have been addressed and resolved. The concept ofneed does not have such systemic implications.     

A behavior genetic investigation of adolescent motherhood and offspring mental health problems

The present study examines the relations between adolescent motherhood and children's behavior, substance use, and internalizing problems in a sample of 1,368 children of 712 female twins from Australia. Adolescent motherhood remained significantly associated with all mental health problems, even when using a quasiexperimental design capable of controlling for genetic and environmental confounds. However, the relation between adolescent motherhood and offspring behavior problems and substance use was partially confounded by family background variables that influence both generations. The results are consistent with a causal relation between adolescent motherhood and offspring mental health problems, and they highlight the usefulness of behavior genetic designs when examining putative environmental risks for the development of psychopathology. The generalizability of these results to the United States, which has a higher adolescent birth rate, is discussed.     

Spatial Error Detection and Assimilation Effects in Rapid Single and Bimanual Aiming Movements

In 2 experiments, spatial error detection capability and movement accuracy were investigated in both single and bimanual rapid aiming movements. In both experiments, right-handed college-age participants (N = 40 [Experiment 1]; N = 24 [Experiment 2]) used light, aluminum levers to make quick single and dual reversal movements in the sagittal plane in a time to reversal of 210 ms to either the same or different target locations involving identical (Experiment 1) or mirror-image (Experiment 2) movements. In Experiment 1, the shorter-distance limb overshot the target by 15-23&percent; when paired with a limb traveling at least 20 degrees farther, but no spatial assimilations were shown when movements differed by 20 degrees or less. In Experiment 2, the shorter-distance limb overshot 22-29&percent; when paired with a limb traveling 20 degrees farther, but spatial assimilations were not mitigated when both limbs moved to the same target position. Participants underestimated movement amplitude in all dual conditions but particularly when spatial assimilations were noted. Correlations between actual and estimated errors decreased from single to dual trials in both experiments. The findings suggest that spatial assimilations are caused by bimanual differences in movement amplitude, regardless of movement direction, and that individuals have greater difficulty identifying errors in simultaneous actions, especially when spatial assimilations are present, than identifying errors in single-limb actions.     

Treatment outcomes of Vietnam veterans with Posttraumatic Stress Disorder.

This study examined changes on the Millon Clinical Multiaxial Inventory for 45 Vietnam veterans who completed a specialized inpatient treatment program. The average length of stay for these veterans was 140 days. Patients' scores on the Millon decreased on 12 of the 20 scales and increased on 8, and their Posttraumatic Stress Disorder-related symptoms of anxiety and dysthymia decreased significantly. Posttraumatic Stress Disorder-related character styles, schizoid, avoidant, and passive-aggressive, also showed significant decreases.     

Generalization of error detection across motor tasks by men and women

One of the cornerstones of the human motor learning process is the ability to self-detect and self-correct movement errors. However, despite their importance, relatively little research has been done on these topics. One unanswered question is whether error detection is a general ability or one specific to the task to be learned. To investigate this issue, 66 college-age participants (49 women and 17 men) performed four motor learning tasks: an anticipation-timing task, a slow arm-positioning task, a rapid arm-movement task (400-msec. goal), and a tone-duration production task (400-msec. goal). 50 practice trials were provided on each task, 35 with knowledge of results (KR) and 15 without KR. Participants verbally estimated error on all trials before KR was given, except for the slow positioning task on which overall error in performance was the measure of error detection. Error detection was developed for each task but transfer of this ability only occurred when two tasks shared the same movement pattern. Men performed better on anticipation-timing than women, but men and women detected errors equally well on all tasks.     

The effects of concurrent task interference on category learning: evidence for multiple category learning systems

Participants learned simple and complex category structures under typical single-task conditions and when performing a simultaneous numerical Stroop task. In the simple categorization tasks, each set of contrasting categories was separated by a unidimensional explicit rule, whereas the complex tasks required integrating information from three stimulus dimensions and resulted in implicit rules that were difficult to verbalize. The concurrent Stroop task dramatically impaired learning of the simple explicit rules, but did not significantly delay learning of the complex implicit rules. These results support the hypothesis that category learning is mediated by multiple learning systems.