Movement Stability Under Uncertain Internal Models of Dynamics

Sensory noise and feedback delay are potential sources of instability and variability for the on-line control of movement. It is commonly assumed that predictions based on internal models allow the CNS to anticipate the consequences of motor actions and protect the movements from uncertainty and instability. However, during motor learning and exposure to unknown dynamics, these predictions can be inaccurate. Therefore a distinct strategy is necessary to preserve movement stability. This study tests the hypothesis that in such situations, subjects adapt the speed and accuracy constraints on the movement, yielding a control policy that is less prone to undesirable variability in the outcome. This hypothesis was tested by asking subjects to hold a manipulandum in precision grip and to perform single-joint, discrete arm rotations during short-term exposure to weightlessness (0 g), where the internal models of the limb dynamics must be updated. Measurements of grip force adjustments indicated that the internal predictions were altered during early exposure to the 0 g condition. Indeed, the grip force/load force coupling reflected that the grip force was less finely tuned to the load-force variations at the beginning of the exposure to the novel gravitational condition. During this learning period, movements were slower with asymmetric velocity profiles and target undershooting. This effect was compared with theoretical results obtained in the context of optimal feedback control, where changing the movement objective can be directly tested by adjusting the cost parameters. The effect on the simulated movements quantitatively supported the hypothesis of a change in cost function during early exposure to a novel environment. The modified optimization criterion reduces the trial-to-trial variability in spite of the fact that noise affects the internal prediction. These observations support the idea that the CNS adjusts the movement objective to stabilize the movement when internal models are uncertain.

Clinical trials: the viewpoint of children with a chronic illness compared with healthy children

The views of over 200 children (11-16 years old, who were either healthy or attending paediatric medical clinics with a chronic illness) on issues relating to paediatric clinical trials were determined by a questionnaire. Children with a chronic illness had a better understanding of the concept of dosing of medicines (40% vs 23%) and placebo (34% vs 20%). They were more likely to support children's involvement in clinical trials (51% vs 37%). The altruistic nature of children in both groups was similar. It was of concern, however, that 57-63% children would participate as healthy volunteers in a cancer drug trial. Both groups were able to differentiate between the different types of illnesses where it is appropriate to study medicines in children (86-89% for illness where there is no treatment; 34-40% for 'me-too' drugs). Young people should be given more opportunities to be involved in decision-making regarding clinical trials of medicines.

Neck dissection: past and present

The presence of lymph node metastasis to the neck is accepted as one of the single most important adverse prognostic indicators of survival in squamous cell carcinoma of the head and neck. Neck dissection in its various forms is the standard surgical treatment for clinical and subclinical metastatic cancer to the neck. This paper is a review of the anatomy of the neck, history and specific types of neck dissection, indications, therapeutic options, and current challenges in the treatment of metastatic neck disease.

"They see you as a different thing": the experiences of men who have sex with men with healthcare workers in South African township communities

OBJECTIVES

To describe interactions between men who have sex with men (MSM) and healthcare workers (HCW) in peri-urban township communities in South Africa.

METHOD

Qualitative study using semistructured in-depth interviews and focus group discussions in the Gauteng province townships of Soweto and Mamelodi. 32 MSM were purposively sampled for in-depth interviews and 15 for focus group discussions. Topics explored included identity, sexuality, community life, use of health services and experiences of stigma and discrimination.

RESULTS

MSM felt their options for non-stigmatising sexual healthcare services were limited by homophobic verbal harassment by HCW. Gay-identified men sought out clinics with reputations for employing HCW who respected their privacy and their sexuality and challenged those HCW who mistreated them. Non-gay-identified MSM presented masculine, heterosexual identities when presenting for sexual health problems and avoided discussing their sexuality with HCW.

CONCLUSIONS

The strategies MSM employ to confront or avoid homophobia from HCW may not be conducive to sexual health promotion in this population. Interventions that increase the capacity of public sector HCW to provide appropriate sexual health services to MSM are urgently needed.

Population pharmacokinetics of tenofovir in HIV-1-infected pregnant women and their neonates (ANRS 12109)

Thirty-eight human immunodeficiency virus-1 (HIV-1)-infected pregnant women were administered tenofovir disoproxil fumarate (TDF; 300 mg)-emtricitabine (FTC; 200 mg) tablets: two at labor initiation and one daily for 7 days postpartum. Maternal, umbilical, and neonatal plasma tenofovir concentrations were measured by high-performance liquid chromatography and analyzed using a population approach. Data were described using a two-compartment model for the mother, an effect compartment linked to maternal circulation for cord, and a neonatal compartment disconnected after delivery. Absorption was greater for women delivering by caesarian section than for those delivering vaginally. The maternal 600 mg TDF administration before delivery produces the same concentrations as 300 mg administration in other adults. If the time elapsed between maternal administration and delivery is >or=12 h, two tablets of TDF-FTC should be readministered. Tenofovir showed good placental transfer (60%). Administering 13 mg/kg of TDF as soon as possible after birth should produce neonatal concentrations comparable with those observed in adults.

The relationship between HAART use and sexual activity among HIV-positive women of reproductive age in Brazil, South Africa, and Uganda

The purpose of this study was to determine whether current HAART use is associated with recent sexual intercourse among HIV-infected women (18-49 years) from Brazil, South Africa and Uganda. We conducted an analysis of survey data from a cross-sectional study, which enrolled 179 HIV-infected women receiving regular care from the Mbarara Hospital HIV Clinic in Uganda (n=85); the Perinatal HIV Research Unit in Soweto, South Africa (n=50); and the IPEC-Fiocruz cohort in Rio de Janeiro, Brazil (n=44). The primary outcome was sexual intercourse in the previous month. Secondary outcomes were protected sex and contraceptive use. We found that overall, 46% reported recent sexual intercourse. After adjusting for covariates, recent sexual intercourse was not associated with HAART use (AOR: 0.76; 95%CI: 0.34-1.72); however, it was significantly associated with being currently married, wanting to have more children and having higher HAART optimism. Among women reporting recent sexual intercourse (n=83), HAART users were significantly more likely to practice protected sex (crude OR: 3.64; 95%CI: 1.41-9.38) and non-significantly more likely to use contraceptive methods (crude OR: 2.15; 95%CI: 0.77-5.99). In summary, self-reported recent sexual intercourse is not more likely among women on HAART. Moreover, sexually active HAART users may be more likely to practice protected sex and use contraceptives.

Effect of remineralization/demineralization cycles on mineral profiles of Fuji IX Fast in vitro using electron probe microanalysis

BACKGROUND

There have been cononcerns about the dissolution of conventional glass ionomer cement (GIC) and its possible degradation when exposed to an acidic environment over time. The objective of this study was to investigate the effects of exposure of Fuji IX Fast to the simulated acidic aspects of the oral environment in terms of any change in the elemental composition of strontium (Sr), phosphorus (P), calcium (Ca) and fluorine (F) which resulted at the surface of this material.

METHODS

Sixty-five cylindrical block of Fuji IX Fast were prepared using split moulds. The demineralizing solution was an acetate buffered demineralizing solution at pH 403. The remineralizing solution was a buffered solution containing 1.5 mM Ca, 0.9 mM P and 10 ppm F at pH 7. The blocks of Fuji IX Fast were subjected either to two-day alternating cycles of remineralization and demineralization for up to 24 days (test); 6 two-day cycles of demineralizing or remineralizing solution separately, or deionized distilled water alone (controls) or were left untreated (base line control). Mineral profiles of Ca, P, Sr and F within 100 microm of the material surface were assessed following 8, 16 and 24 days of treatment (test); 4, 8 or 12 days (controls) or for baseline control samples, using electron probe microanalysis (EPMA).

RESULTS

There were significant changes in mineral profile in the test specimens in terms of Sr and Ca concentrations. A molecule for molecule exchange of these elements resulted between GIC and eluant solutions. Fluoride loss from the GIC occurredto the level comparable with uptake levels recorded in eluant solutions from previous studies. The ionic exchanges appeared to be the result of dissolution followed by an equilibrium-driven diffusion. These exchanges were superficial though substantial.

CONCLUSIONS

Simulated exposure of Fuji IX to the oral environment resulted in an exchange of Ca from the bathing solutions into Fuji IX to replace any Sr which was lost to the GIC. Fluorine loss from the GIC followed previously described patterns. The possible clinical significance of this exchange was discussed.

Clinical trials: the viewpoint of children

The views of 30 children (8-16 years old) attending paediatric medical clinics on paediatric clinical trials were determined by semi-structured interviews. Nineteen children recognised that there were risks involved with taking part in clinical trials. Risks concerned with being paid were recognised by all children.

Comparative uptake of fluoride ion into enamel from various topical fluorides in vitro

BACKGROUND

There are many forms of topical fluoride available today, making the decision as to which is most effective to manage the immediate caries risk problem at hand, very difficult. The objective of this project was to determine the concentration and pattern of fluoride ion uptake into enamel from a variety of categories of topical fluoride recently available in Australia.

METHODS

Extracted, intact molar teeth were sectioned to provide six plates of smooth surface enamel. Windows of enamel 2 x 6 mm were exposed to a variety of topical fluorides for periods simulating those used in vivo. Following drying, the slates of enamel were exposed to 2 ml of 0.1 M HCl as a chemical biopsy agent for incremental periods of time. The concentrations of fluoride ion in the biopsy solutions for both test and background (control) slates of enamel were determined directly using a fluoride combination selective electrode in conjunction with a high impedance pH meter. Cumulative amounts of fluoride were determined for each topical fluoride agent.

RESULTS

The concentrations of fluoride ion taken up into enamel were generally proportional to those present in each agent. However, those from APF gel greatly exceeded the amounts taken up from NaF gel. Also, the concentrations taken up from some of the highly concentrated metal fluorides were surprisingly low. Prior etching of enamel increased uptake and prolonged application of APF gel provided no extra benefit.

CONCLUSIONS

Some topical fluorides, e.g., APF gel, provided a greatly increased uptake and to a greater depth than other self-application products. However, the frequency of its use should be considered with caution where patients have glass-based restorations.

Enhancement of fluoride release from glass ionomer cement following a coating of silver fluoride

BACKGROUND

This study investigated the extent to which a coating of 10% silver fluoride (AgF) on discs of glass jonomer cements (GIGs) would enhance the release of fluoride ion into eluting solutions at varying pH.

MATERIALS AND METHODS

Forty discs each of Fuji LX, Fuji VII and of Vitrebond were prepared in a plastic mould. Twenty discs of each material were coated for 30 seconds with a 10% solution of AgF. Five discs each of coated and uncoated material were placed individually in 4m1 of differing eluant solutions. The eluant solutions comprised deionized distilled water (DDW) and three separate acetate buffered solutions at pH 7, pH 5 and pH 3. After 30 minutes the discs were removed and placed in five vials containing 4m1 of the various solutions for a further 30 minutes. This was repeated for further intervals of time up to 216 hours, and all eluant solutions were stored. Fluoride concentrations in the eluant solutions were estimated using a fluoride specific electrode, with TISAB IV as a metal ion complexing and ionic concentration adjustment agent. Cumulative fluoride release patterns were determined from the incremental data.

RESULTS

The coating of AgF greatly enhanced the level of fluoride ion release from all materials tested. Of the uncoated samples, Vitrehond released the greater concentrations of fluoride ion, followed by Fuji VII. However, cumulative levels of fluoride released from coated samples of the GICs almost matched those from coated Vitrebond.

CONCLUSIONS

It was concluded that a coating of 10% AgF on GICs and a resin modified GIC greatly enhanced the concentration of fluoride released from these materials. This finding might be applied to improving protection against recurrent caries, particularly in high caries risk patients, and in the atraumatic restorative technique (ART) of restoration placement.

The potential for dental plaque to protect against erosion using an in vivo-in vitro model--a pilot study

BACKGROUND

Tooth erosion is a problem for professional wine tasters (exogenous erosion from frequent exposure to wine acids) and for people with gastro oesophageal reflux disease (GORD) and bulimia who experience frequent reflux of gastric contents into the mouth (endogenous erosion from mainly HCl). The objective in this study was to determine whether plaque/pellicle could provide teeth with any protection from two common erosive acids, using an in vivo-in vitro technique.

METHODS

Tiles of human tooth enamel and root surfaces were prepared from six extracted, unerupted third molar teeth and sterilized. Mandibular stents were prepared for six volunteer subjects and the tiles bonded to the buccal flanges of these stents. They were worn initially for three days to permit a layer of pellicle and plaque to form over the tile surfaces, and for a further 10 days of experimentation. Following cleaning of the plaque/ pellicle layer from the tiles on the right side flange, all the tiles were submerged in either 0.06M HCl or white wine for an accumulated time of 600 and 1500 minutes, respectively. Depths of erosion were determined using light microscopy of sections of the enamel and root tiles. SEM of the lesion surfaces was carried out to investigate the nature of erosive damage and of plaque/pellicle remnants.

RESULTS

Retained plaque was found to significantly inhibit dental erosion on enamel, from contact with both HCl and wine, compared with that resulting following its removal. However, it was found to provide no significant protection on root surfaces. SEM analysis of the tile surfaces revealed marked etching of enamel on the cleaned surfaces, and considerable alteration to the appearance of remaining plaque and pellicle on most surfaces.

CONCLUSION

Within the limitations of numbers of specimens, dental plaque/pellicle provided a significant level of protection to tooth enamel against dental erosion from simulated gastric acids and from white wine, using an in vivo-in vitro model. It was unable to provide any significant protection to root surfaces from these erosive agents. Possible reasons for this difference are explored.

Effect of gravity on human spontaneous 10-Hz electroencephalographic oscillations during the arrest reaction

Electroencephalographic oscillations at 10 Hz (alpha and mu rhythms) are the most prominent rhythms observed in awake, relaxed (eye-closed) subjects. These oscillations may be considered as a marker of cortical inactivity or an index of the active inhibition of the sensory information. Different cortical sources may participate in the 10-Hz oscillation and appear to be modulated by the sensory context and functional demands. In microgravity, the marked reduction in multimodal graviceptive inputs to cortical networks participating in the representation of space could be expected to affect the 10-Hz activity. The effect of microgravity on this basic oscillation has heretofore not been studied quantitatively. Because the alpha rhythm has a functional role in the regulation of network properties of the visual areas, we hypothesised that the absence of gravity would affect its strength. Here, we report the results of an experiment conducted over the course of 3 space flights, in which we quantified the power of the 10-Hz activity in relation to the arrest reaction (i.e., in 2 distinct physiological states: eyes open and eyes closed). We observed that the power of the spontaneous 10-Hz oscillation recorded in the eyes-closed state in the parieto-occipital (alpha rhythm) and sensorimotor areas (mu rhythm) increased in the absence of gravity. The suppression coefficient during the arrest reaction and the related spectral perturbations produced by eye-opening/closure state transition also increased in on orbit. These results are discussed in terms of current theories on the source and the importance of the alpha rhythm for cognitive function.

Persistence of nevirapine-resistant HIV-1 in women after single-dose nevirapine therapy for prevention of maternal-to-fetal HIV-1 transmission

Single-dose nevirapine (sdNVP) for prevention of mother-to-child transmission of HIV-1 can select nevirapine (NVP)-resistant variants, but the frequency, duration, and clinical significance of this resistance is not well defined. We used a sensitive allele-specific PCR assay to assess the emergence and persistence of NVP-resistant variants in plasma samples from 22 women with HIV-1 subtype C infection who participated in a study of sdNVP for prevention of mother-to-child transmission of HIV-1. The women were categorized into three groups on the basis of detection of NVP resistance by standard genotype analysis. Group 1 (n = 6) had NVP resistance detected at 2 and 6 mo after sdNVP, but not at 12 mo. Group 2 (n = 9) had NVP resistance detected at 2 mo, but not 6 mo. Group 3 (n = 7) had no NVP resistance detected at any time point. Allele-specific PCR analysis for the two most common NVP resistance mutations (K103N and Y181C) detected NVP-resistant variants in most (16 of 21) samples that were negative for NVP resistance by standard genotype, at levels ranging from 0.1% to 20% 1 yr after treatment. The frequency of NVP-resistant mutations decreased over time, but persisted above predose levels for more than 1 yr in > or = 23% of the women. These findings highlight the urgent need for studies assessing the impact of sdNVP on the efficacy of subsequent antiretroviral therapy containing NVP or other nonnucleoside reverse transcriptase inhibitors.

Kinematic and dynamic processes for the control of pointing movements in humans revealed by short-term exposure to microgravity

The generation of accurate motor commands requires implicit knowledge of both limb and environmental dynamics. The action of gravity on moving limb segments must be taken into account within the motor command, and may affect the limb trajectory chosen to accomplish a given motor task. Exactly how the CNS deals with these gravitoinertial forces remains an open question. Does the CNS measure gravitational forces directly, or are they accommodated in the motor plan by way of internal models of physical laws? In this study five male subjects participated. We measured kinematic and dynamic parameters of upward and downward arm movements executed at two different speeds, in both normal Earth gravity and in the weightless conditions of parabolic flight. Exposure to microgravity affected velocity profiles for both directions and speeds. The shape of velocity profiles (the ratio of maximum to mean velocity) and movement duration both showed transient perturbations initially in microgravity, but returned to normal gravity values with practice in 0 x g. Differences in relative time to peak velocity between upward versus downward movements, persisted for all trial performed in weightlessness. These differences in kinematic profiles and in the torque profiles used to produce them, diminished, however, with practice in 0 x g. These findings lead to the conclusion that the CNS explicitly represents gravitational and inertial forces in the internal models used to generate and execute arm movements. Furthermore, the results suggest that the CNS adapts motor plans to novel environments on different time scales; dynamics adapt first to reproduce standard kinematics, and then kinematics patterns are adapted to optimize dynamics.