Self-Optimization of Walking in Nondisabled Children and Children With Spastic Hemiplegic Cerebral Palsy

Children voluntarily adopt a frequency and movement pattern for walking. The force-driven harmonic oscillator (FDHO) model was used in this study for accurate prediction of the preferred walking frequency of nondisabled children and children with spastic hemiplegic cerebral palsy. Four potential optimality criteria with which the preferred walking pattern was forced to comply were examined: minimization of physiological costs, maximization of mechanical energy conservation, minimization of asymmetry in lower limb movements and minimization of variability of interlimb and intralimb coordination. Age and gender-matched nondisabled children (n = 6) and children with spastic hemiplegic cerebral palsy (n = 6) were tested under six frequency conditions of walking at a constant speed on a treadmill. For the nondisabled children, the results indicated that their preferred walking frequency could be accurately predicted by the FDHO model. They freely adopted a walking pattern that minimized physiological costs, asymmetry, and variability of inter- and intralimb coordination. For the children with spastic hemiplegic cerebral palsy, the prediction of preferred overground walking frequency required that the FDHO model be modified to account for muscle mass and leg length discrepancies between limbs and increased stiffness. Most of the children achieved the same optimality goals as the nondisabled when walking at the preferred frequency. However, the children were found to use different mechanisms to attain these goals: for example, a steeper increase observed in physiological cost at higher frequencies; a lowered center of gravity of the body, which allowed for angular symmetry; and greater variability of between-joint coordination in the nonaffected limb and less variability in the affected limb.     

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the rnistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interresponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This "peripheral" hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.     

Residence time and choice in concurrent foraging schedules

Five pigeons were trained on a concurrent-schedule analogue of the “some patches are empty” procedure. Two concurrently available alternatives were arranged on a single response key and were signaled by red and green keylights. A subject could travel between these alternatives by responding on a second yellow “switching” key. Following a changeover to a patch, there was a probability (p) that a single reinforcer would be available on that alternative for a response after a time determined by the value of λ, a probability of reinforcement per second. The overall scheduling of reinforcers on the two alternatives was arranged nonindependently, and the available alternative was switched after each reinforcer. In Part 1 of the experiment, the probabilities of reinforcement, ρ_red and ρ_green, were equal on the two alternatives, and the arranged arrival rates of reinforcers, λ_red and λ_green, were varied across conditions. In Part 2, the reinforcer arrival times were arranged to be equal, and the reinforcer probabilities were varied across conditions. In Part 3, both parameters were varied. The results replicated those seen in studies that have investigated time allocation in a single patch: Both response and time allocation to an alternative increased with decreasing values of λ and with increasing values of ρ, and residence times were consistently greater than those that would maximize obtained reinforcer rates. Furthermore, both response- and time-allocation ratios undermatched mean reinforcer-arrival time and reinforcer-frequency ratios.     

对人的尊严的挑战——试论P.辛格的优先权——实利主义

一个社会对其成员重视和承认的程度,首先表现为这个社会如何对待其成员中的弱者和受歧视的人。而在每个社会遭受歧视的人中,无疑也包括那些由于先天或后天的缘故患有精神上或肉体上的疾病而需要特别照顾的人。1989年以来,首先是这一社会群体的代言人,对他们的遭遇提出了强烈的抗议。而他们的抗议,又引发了德国舆论界对澳大利亚哲学家彼得·辛格的关注。特别是辛格关于可以对残疾人(既包括先天不足的婴儿,也包括老年人)施行强制性安乐死的论述,点燃了论战的烽火,更激起了一部分人的强烈抗议。残疾人的代盲人指出,辛格的“实用伦理学”不仅是对“残疾人”,而且从根本上讲是对人的错误认识。