An equilibrium-point model for fast, single-joint movement: II. Similarity of single-joint isometric and isotonic descending commands

The model for isotonic movements introduced in the preceding article in this issue is used to account for isometric contractions. Isotonic movements and isometric contractions are analyzed as consequences of one motor program acting under different peripheral conditions. Differences in isotonic and isometric EMG patterns are analyzed theoretically. Computer simulation of the EMG patterns was performed both with and without the inclusion of possible effects of reciprocal inhibition. A series of experiments was performed to test the model. The subjects made fast isotonic movements that were unexpectedly blocked at the very beginning in some of the trials. The observed differences in the EMG patterns between blocked and unblocked trials corresponded to the model's predictions. The results suggest that these differences are due to the action of a tonic stretch reflex rather than to preprogrammed reactions. The experimental and simulation findings, and also the data from the literature, are discussed in the framework of the model and the dual-strategy hypothesis. They support the hypothesis that the motor control system uses one of a few standardized subprograms, specifying a small number of parameters to match a specific task.     

Peer telephone dyads: Putting conclusions on hold

Conclusion Through systematic study of intervention trials, which are as carefully crafted and as critically examined as the one reported here, we can gain new insights into ways of intentionally creating the interpersonal and environmental conditions conducive to the expression and acceptance of support. To plan appropriate and acceptable changes in the social ecology, we must develop a theory of the intervention, drawing on both the voluminous generative base of research on social support and on knowledge gained from examination of the particular forces that potentiate and limit exchanges of support in the networks of the intended beneficiaries. In conducting the intervention, careful recordings of the interpersonal transactions that take place and their bearing on the parties' perceptions of the evolving character of the relationship can bring the needed process dimension to emergent theory on social support. And by evaluating the impact of the intervention on a range of proximal and distal outcomes, we can discern more clearly how social support's mechanisms of action confer measurable benefits, particularly those prized by the participants themselves.     

Auditory basis of maternal attachment in ducklings (Anas platyrhynchos) under simulated naturalistic imprinting conditions

The presence of broodmates during the imprinting process results in peer imprinting that interrupts a visually mediated maternal attachment. We sought to determine the conditions in which group-trained mallard ducklings (Anas platyrhynchos) acquire a maternal attachment. At 48 hr of age, ducklings were allowed to follow a vocal, stuffed mallard hen individually or in groups of 4. Individual ducklings showed a preference for the silent, familiar mallard over an unfamiliar pintail. Ducklings trained in groups did not show a preference. When the mallard maternal call was present during testing, group-trained ducklings overwhelmingly responded to it regardless of whether it came from the familiar mallard or an unfamiliar pintail. Training ducklings in groups, which simulates the natural social context of imprinting, results in peer imprinting. Thus, early in development the young become visually imprinted to each other, and the maternal call mediates attachment to the mother.     

Participation in a bystander intervention experiment and subsequent everyday helping: ethical considerations

This study examines how participation in a bystander experiment involving deception affects later everyday helping. Both subjects who had formerly participated in a bystander intervention experiment and a group of matched controls who had not participated were interviewed in a survey either 6–10 months (early) or 11–20 months (late) after completion of the experiment. Half of the previous subjects were prompted to recall their experimental experience in the interview. After leaving the interview, subjects encountered a needy person. Mere participation in the prior experiment did not influence helping during the early period but enhanced helping after 10 months had elapsed. Participation plus recall inhibited helping during the early period but not later. Compared with controls, past participants helped more with the passage of time. The results suggest that participation in bystander intervention experiments induces both context-specific cognitions about possible inauthenticity of need and generalized beliefs about the value of helping. With the passage of time, the former fade while the latter become more salient. Ethical implications of these findings are discussed.     

Smoking following myocardial infarction: a critical review of the literature

This paper critically reviews the available research on the effects of smoking cessation following acute myocardial infarction (MI). Studies that have examined the rate of smoking cessation following an MI indicate that approximately 1/3 to 1/2 of the smokers who suffer from MI subsequently reduce or quit smoking. Furthermore, studies that have examined subsequent mortality and morbidity suggest that individuals who quit smoking following MI exhibit lower mortality and morbidity than those who continue to smoke. It is argued that past studies may have overestimated post-MI smoking cessation rates, and by failing to control for a priori differences between quitters and continuing smokers (e.g., MI severity) may have underestimated the negative effects of smoking following MI. Suggestions for future research are proposed.     

Effect of age and gender in the control of elbow flexion movements

In previous studies of rapid elbow movements in young healthy men, characteristic task-dependent changes in the patterns of muscle activation when movement speed or distance was varied have been reported. In the present study, the authors investigated whether age or gender is associated with changes in the patterns of muscle activity previously reported in young men. Arm movements of 10 healthy older and 10 healthy younger participants (5 men and 5 women in each group) were studied. Surface electromyograms (EMGs) from agonist (biceps) and antagonist (triceps) muscles, kinematic and kinetic parameters, as well as anthropometric and strength measures were recorded. All 4 groups of participants showed similar task- (distance or speed) dependent changes in biphasic EMG activity. Similar modulation of the initial rate of rise of the EMG, integrated agonist and antagonist EMG activity, as well as their relative timing were observed in all 4 groups. Those results suggest that older individuals of both genders retain the control strategies for elbow movements used by young individuals. Despite the qualitative similarities in the patterns of muscle activation, the men moved more quickly than the women, and younger participants moved more quickly than older participants. Those performance differences could not be explained in terms of differences in body size and strength alone.     

A test of torque-control and equilibrium-point models of motor control

Based upon predictions of equilibrium-point (EP) models of motor control, a recent article by Jaric et al. (1999) [Human Movement Sciences, 18, 49–66] reported that the peak velocity (PV) of an inertially loaded single degree-of-freedom elbow movement was not altered to a statistically distinguishable degree by the accuracy of the subject's expectation about the size of the load. They concluded that their results confirmed their EP models and were incompatible with what they described as torque-control models. A review of the literature and analysis of a more extensive set of data does not support that conclusion. To the contrary, if the actual load is heavier (or lighter) than expected, it is moved more slowly (or faster) than it would have been, had expectation matched reality. Torque-control models predict those consequences, EP models do not.     

Parietal control of attentional guidance: the significance of sensory, motivational and motor factors

The lateral intraparietal area (LIP), a portion of monkey posterior parietal cortex, has been implicated in spatial attention. We review recent evidence showing that LIP encodes a priority map of the external environment that specifies the momentary locus of attention and is activated in a variety of behavioral tasks. The priority map in LIP is shaped by task-specific motor, cognitive and motivational variables, the functional significance of which is not entirely understood. We suggest that these modulations represent teaching signals by which the brain learns to identify attentional priority of various stimuli based on the task-specific associations between these stimuli, the required action and expected outcome.