Postural requirements and progression velocity in young walkers

This article describes developmental changes in gait velocity and relates these changes to gait parameters that index postural stability (step width and lateral acceleration) and two components of velocity (cadence and step length). Five children were observed longitudinally over a 2-year period after onset of independent walking. Their range of speed increased threefold in the first 6 months of independent walking and then remained constant. In contrast, step width decreased approximately twofold. Whereas in adults, cadence and step length contribute approximately equally to speed, when infants first begin to walk independently, increase in velocity is due mostly to increased step length. After 5 months of independent walking, the pattern reverses, and increase in velocity is due primarily to increased cadence. The pattern remains constant over the next 18 months. From a developmental point of view, the data lead us to interpret early walking (the first 5 months) as a process of integration of postural constraints into the dynamic necessities of gait movement. A second phase, beginning after 4 to 5 months of independent walking, is considered to be a tuning phase characterized by a more precise adjustment of the gait parameters.     

Effects of Pointing Rate and Availability of Visual Feedback on Visual and Proprioceptive Components of Prism Adaptation

While looking through laterally displacing prisms, subjects pointed 60 times straight ahead of their nose at a rate of one complete movement every 2 or 3 s, with visual feedback available early in the pointing movement or delayed until the end of the movement. Sagittal pointing was paced such that movement speed covaried with pointing rate. Aftereffect measures (obtained after every 10 pointing trials) showed that when the limb became visible early in a pointing movement, proprioceptive adaptation was greater than visual, but when visual feedback was delayed until the end of the movement, the reverse was true. This effect occurred only with the 3-s pointing rate, however. With the 2-s pointing rate, adaptation was predominately proprioceptive in nature, regardless of feedback availability. Independent of the availability of visual feedback, visual adaptation developed more quickly with 3-s pointing, whereas proprioceptive adaptation developed more rapidly with 2-s pointing. These results are discussed in terms of a model of perceptual-motor organization in which the direction of coordinative (guidance) linkage between eye-head (visual) and hand-head (proprioceptive) systems (and consequently the locus of discordance registration and adaptive recalibration) is determined jointly by pointing rate and feedback availability. An additional effect of pointing rate is to determine the rate of discordant inputs. Maximal adaptive recalibration occurs when the input (pointing) rate matches the time constant of the adaptive encoder in the guided system.     

Discriminability between alternatives in a switching-key concurrent schedule

Six pigeons were trained to discriminate between two intensities of white light in a symbolic matching-to-sample procedure. These stimuli were then used to signal which schedule was available on the main key in a switching-key concurrent schedule. The concurrent schedules led to a symbolic matching-to-sample phase in which the subject identified the concurrent schedule to which it last responded before a reinforcer could be obtained. The concurrent schedules were varied across conditions. Discriminability, measured during the symbolic matching-to-sample performance, was high throughout and did not differ across the two procedures. Performance in the concurrent schedules was like that typically obtained using these schedules. Delays were then arranged between completion of the concurrent schedules and presentations of the symbolic matching-to-sample phase. A series of conditions with an intervening delay of 10 s showed that both concurrent-schedule performance and symbolic matching-to-sample performance were affected by the delay in a similar way; that is, choice responding was closer to indifference.     

Effects of reinforcer delays on choice as a function of income level

Three rats earned their daily food ration by responding during individual trials either on a lever that delivered one food pellet immediately or on a second lever that delivered three pellets after a delay that was continuously adjusted to ensure substantial responding to both alternatives. Choice of the delayed reinforcer increased when the number of trials per session was reduced. This result suggests that models seeking closure on choice effects must include a parameter reflecting how preference changes with sessionwide income. Moreover, models positing that reinforcer probability and immediacy (1/delay) function equivalently in choice are called into question by the finding that probability and immediacy produce opposing effects when income level is changed.     

Cultural practices of Hispanics: implications for the prevention of AIDS

Hispanic Americans have one of the highest rates of HIV seroprevalence among all ethnics groups in the US, with high rates being especially noticeable among women and children. Were it known which cultural factors have the most influence on whether Hispanics engage in high-risk behavior for HIV transmission, prevention interventions could be targeted accordingly. To that end, this study was mounted to identify which Hispanic cultural factors relate to condom use. 117 males and 73 females aged 17-56 years of mean age 25.67 were surveyed in Washoe county, Nevada. These mostly young adults had recently immigrated to the western US. It was initially posited that fate orientation, male- female relationships, family relationships, machismo behavior, and religion would have equal influence with respect to condom use. Analysis found that condom use was largely associated with and determined by men who are the principal buyers of condoms. A machismo attitude toward protecting women by using condoms was also associated with condom use. Neither fate orientation with respect to AIDS, nor religion were important determinants of condom use, even though 86.5% of the respondents were Catholic. The degree to which respondents adhered to traditional Hispanic cultural values was influenced by the degree of education and acculturation. On the basis of these findings, the authors suggest targeting AIDS prevention messages to males, while emphasizing the protection of women through condom use. They also suggest that both education and acculturation levels be assessed before implementing prevention programs.     

The legal consensus about foregoing life-sustaining treatment: its status and its prospects

The legal consensus that has evolved through adjudication and legislation since the Karen Quinlan case in 1976 is founded on the premise that there is a bright line between passive euthanasia and active euthanasia. Indeed, the term passive euthanasia is often eschewed in favor of less emotionally-laden terminology such as "forgoing life-sustaining treatment" or "terminating life support" so as to further sever any possible connection with active euthanasia. Legal approval has been bestowed upon passive euthanasia under certain circumstances while active euthanasia is routinely condemned. This consensus was put to a test in 1990 when the United States Supreme Court ruled on the Cruzan case. However, the Court's narrow decision did not upset the consensus, and in the most significant appellate decisions handed down by state courts since Cruzan, there has been a reaffirmation--and possibly even an extension--of the consensus. Two other threats to the legal consensus about forgoing life-sustaining treatment have begun to manifest themselves: the increasing pressure for mercy killing and "futility" cases. Both of these challenge the fundamental premises on which the consensus is grounded.     

库恩的可否证性模式

库恩和拉卡托斯都不同意把理论的评价看作是理论的可证实性或理论的可否证性这一传统观点。他们的批判分为两个题目:首先,一种科学理论并非只能凭经验的结果来检验;其次,人们不会在没有现成可替代的理论时就摈弃一种理论。在拉卡托斯的术语里,要么被接受,要么被摈弃的是理论的研究纲领而不仅仅是理论本身。进一步说,某一特定的范式或研究纲领的摈弃并不就是简单地被取代,一种替代范式在另一种范式或研究纲领被摈弃之前必须发生效用。最近,人们对拉卡托斯夸大第二点进行了批判。F.M.亚克罗德认为,在科学史上有一些情况是,一项研究被简单地摈弃了,而且的确是被合理地摈弃了,尽管还没有替代者。在本文中,我     

甘岑的自然演绎演算

迄今为止,我们所主要讨论的逻辑演算,即用公理方法处理的狭谓词演算,标志着把数学方法运用于逻辑获得了显著的成功。这种演算的形式上的简易是令人称道的,从纯粹数学的观点看,它可以同公理化的射影几何及抽象群论这一类数学观念的完满体现的学科相媲美。此外,作为应用于研究演绎系