Podokinetic circular vection: characteristics and interaction with optokinetic circular vection.

Stabilising horizontal body orientation in space without sight on a rotating platform by holding to a stationary structure and circular 'treadmill' stepping in the opposite direction can elicit an illusion of self-turning in space (Bles and Kapteyn in Agressologie 18:325-328, 1977). Because this illusion is analogous to the well-known illusion of optokinetic circular vection (oCV), we call it 'podokinetic circular vection' (pCV) here. Previous studies using eccentric stepping on a path tangential to the rotation found that pCV was always contraversive relative to platform rotation. In contrast, when our subjects stepped at the centre of rotation about their vertical axis, we observed an inverted, ipsiversive pCV as a reproducible trait in many of our subjects. This ipCV occurred at the same latency as the pCV of subjects reporting the actually expected contraversive direction, but had lower gain. In contrast to pCV, the nystagmus accompanying circular treadmill stepping had the same direction in all individuals (slow phase in the direction of platform motion). The direction of an individual's pCV predicted the characteristics of the CV resulting from combined opto- and podokinetic stimulation (circular treadmill stepping while viewing a pattern rotating together with the platform): in individuals with contraversive pCV, latency shortened and both gain and felt naturalness increased in comparison with pure oCV, whereas the opposite (longer latency, reduced gain and naturalness) occurred in individuals with ipCV. Taken together, the reproducibility of ipCV, the constant direction of nystagmus and the fact that pCV direction predicts the outcome of combined stimulation suggest that ipCV is an individual trait of many subjects during compensatory stepping at the centre of rotation. A hypothetical model is presented of how ipCV possibly could arise.

Optokinetic circular vection: a test of visual-vestibular conflict models of vection nascensy.

The propensity to experience circular vection (the illusory perception of self-turning evoked by a rotating scene, CV) as reflected by its onset latency exhibits considerable interindividual variation. Models of CV nascensy have linked this delay to the time it takes the visual-vestibular conflict to disappear. One line of these "conflict models" (Zacharias and Young in Exp Brain Res 41:159-171, 1981) predicts that, across individuals, CV latency (CVL) correlates positively with the vestibular time constant (TC) and negatively with the vestibular motion detection threshold (vTHR). A second type of models (Mergner et al. in Arch Ital Biol 138:139-166, 2000) predicts only an increase in CVL with TC. We here examine which of these predictions can be experimentally substantiated. Also, we ask whether the relative weight W O of the optokinetic contribution to the perception of real self-turning could also be a factor influencing CVL. We conducted 5 experiments in 29 subjects measuring: (1) CVL, (2) the TCs of velocity perception and of accompanying nystagmus during rotation in darkness and (3) likewise for displacement perception, (4) vTHR, and (5) W O as revealed by discordant visual-vestibular stimulation. CVL correlated with the nystagmus TC recorded during velocity estimation but with none of the other vestibular TCs nor with vTHR. Confirming earlier findings, CVL shortened with rising scene velocity. Finally, CVL correlated inversely with W O: the larger an individual's optokinetic weight, the shorter was his CVL. Taken together, our data favour the second type of models which invoke an antagonism between CV inhibition by the optokinetic-vestibular conflict and disinhibition by optokinetic stimulation. Idiosyncratic factors appear to strongly modulate the balance between inhibition and disinhibition, thus increasing CVL variability and obscuring the expected relation between CVL and TC.

Overcoming obstacles to late presentation for HIV infection in Europe.

OBJECTIVE: The central goal of the HIV in Europe Initiative is to promote testing and treatment throughout Europe and Central Asia in order to decrease the number of people living with HIV presenting late for care. This article summarizes the results from the HIV in Europe 2009 Conference and the early results of the projects set up by the initiative, and discusses their implications for the future. METHODS: In November 2009, 100 key stakeholders from 25 countries met in Stockholm at the HIV in Europe Conference. The focus was to address five key issues that contribute to the barriers to testing identified in 2007 at an innovative HIV conference. The conference discussed barriers to testing and other reasons for late presentation and outlined concrete recommendations to address the problem. RESULTS: An early result of the initiative has been stimulation of the process of reaching a consensus definition of what is meant by a 'late presenter', with this definition to be implemented at the European level. Steps are being taken to advocate for appropriate health policies and surveillance data related to HIV throughout Europe. Also, the initiative has set up projects related to the barriers to testing, i.e. criminalization law, stigmatization and lack of offering of testing for people presenting with certain indicator diseases. CONCLUSIONS: The final results of ongoing projects will be published and widely disseminated in 2010 and beyond. The HIV in Europe Initiative will continue to reinforce collaboration, advocacy and networking activities in the field throughout Europe.

Eye-head coordination in moderately affected Huntington's Disease patients: do head movements facilitate gaze shifts?

In addition to many other symptoms, Huntington's Disease (HD) also causes an impairment of oculomotor functions. In particular, saccadic eye movements become progressively slower and more difficult to initiate; ultimately, patients are forced to recur to large head thrusts as means to initiate gaze shifts. We wondered whether, as a precursor of this condition, head movements would facilitate gaze shifts already in early stages of the disease. We studied horizontal head movements and eye-head coordination in 29 early stage HD patients (Ps) and 24 age matched controls (Cs). Subjects tracked random horizontal steps of visual or auditory targets while their heads were either stabilised (saccade amplitudes

Large longitude libration of Mercury reveals a molten core.

Observations of radar speckle patterns tied to the rotation of Mercury establish that the planet occupies a Cassini state with obliquity of 2.11 +/- 0.1 arc minutes. The measurements show that the planet exhibits librations in longitude that are forced at the 88-day orbital period, as predicted by theory. The large amplitude of the oscillations, 35.8 +/- 2 arc seconds, together with the Mariner 10 determination of the gravitational harmonic coefficient C22, indicates that the mantle of Mercury is decoupled from a core that is at least partially molten.

Dynamical configuration of binary near-Earth asteroid (66391) 1999 KW4.

Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primary's proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.

Asteroid 1950 DA's encounter with Earth in 2880: physical limits of collision probability prediction.

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.

Binary asteroids in the near-Earth object population.

Radar images of near-Earth asteroid 2000 DP107 show that it is composed of an approximately 800-meter-diameter primary and an approximately 300-meter-diameter secondary revolving around their common center of mass. The orbital period of 1.755 +/- 0.007 days and semimajor axis of 2620 +/- 160 meters constrain the total mass of the system to 4.6 +/- 0.5 x 10(11) kilograms and the bulk density of the primary to 1.7 +/- 1.1 grams per cubic centimeter. This system and other binary near-Earth asteroids have spheroidal primaries spinning near the breakup point for strengthless bodies, suggesting that the binaries formed by spin-up and fission, probably as a result of tidal disruption during close planetary encounters. About 16% of near-Earth asteroids larger than 200 meters in diameter may be binary systems.

Sensor fusion by neural networks using spatially represented information.

A neural network model based on a lateral-inhibition-type feedback layer is analyzed with regard to its capabilities to fuse signals from two different sensors reporting the same event ("multisensory convergence"). The model consists of two processing stages. The input stage holds spatial representations of the sensor signals and transmits them to the second stage where they are fused. If the input signals differ, the model exhibits two different processing modes: with small differences it produces a weighted average of the input signals, whereas with large differences it enters a decision mode where one of the two signals is suppressed. The dynamics of the network can be described by a series of two first-order low-pass filters, whose bandwidth depends nonlinearly on the level of concordance of the input signals. The network reduces sensor noise by means of both its averaging and filtering properties. Hence noise suppression, too, depends on the level of concordance of the inputs. When the network's neurons have internal noise, sensor noise suppression is reduced but still effective as long as the input signals do not differ strongly. The possibility of extending the scheme to three and more inputs is discussed.

Health Canada commits to strengthening efforts with respect to injection drug use and HIV/AIDS.

On 31 August 2001, over 20 months after the release of the Canadian HIV/AIDS Legal Network's report on Injection Drug Use and HIV/AIDS: Legal and Ethical Issues, Health Canada responded to the report by making a commitment to both "strengthening and expanding efforts with respect to injection drug use."

Vestibular perception of self-rotation in different postures: a comparison between sitting and standing subjects.

We investigated whether posture - either seated (S) or upright standing (O, orthostatic) - affects the vestibular perceptions of angular velocity (V) and displacement (D) in the horizontal plane. We also examined whether the two perceptions are equivalent, that is, whether perceived displacement can be viewed as the time integral of perceived velocity. Sinusoidal stimuli were delivered to subjects sitting on a Barany chair or standing on a turning platform. Frequencies ranged from 0.028 Hz to 0.45 Hz, peak-to-peak amplitudes from 11.3 degrees to 180 degrees, and peak velocities from 4 degrees/s to 64 degrees/s. Perceptions were measured by retrospective magnitude estimation in relation to a standard stimulus (STD) of 0.11 Hz, 45 degrees, 16 degrees/s. For D-estimates, two different moduli were assigned to the STD: Either "45 degrees" (allowing subjects to use the familiar degree scale, which can easily be related to the body scheme) or "10" (which bears no relation to an accustomed scale). For V-estimations the modulus was always "10" (there is no "natural" velocity scale). D-estimates exhibited only a marginal, non-significant dependence on posture (S larger than O); they were highly veridical (linear function of stimulus amplitude, gain close to 1) when subjects used the degree scale but had a reduced gain (approximately 0.76) with a modulus of 10. V-estimates, on the other hand, varied with posture (S significantly larger than O), particularly upon presentation of large stimuli; also, they deviated increasingly from veracity as stimulus magnitude increased (saturating function). Finally, posture had no effect upon the vestibular detection threshold. The frequency response of D-estimates, tested with stimuli of constant amplitude and varying frequency, was bimodal at low frequencies: stimuli were either not detected at all or were veridically estimated, on average (with a large scatter, though). The frequency response of V-estimates, tested with stimuli of constant peak velocity, exhibited a continuous increase with stimulation frequency. We conclude that published quantifications of vestibular self-motion perception, collected mostly with sitting subjects, are likely to be applicable also to the more natural situation of standing subjects provided they are based on displacement indications; in contrast, velocity indications appear to be modulated by posture. The different susceptibility of displacement and velocity estimates to posture and their incongruent frequency characteristics suggest that perceived displacement does not, or does not always, equal the time integral of perceived velocity. The persistence of nearly veridical displacement estimates at low frequencies suggests the intervention of cognitive processes.

Benefit of echocontrast-enhanced transcranial color-coded duplex ultrasound in the assessment of intracranial collateral pathways.

BACKGROUND AND PURPOSE: Although clinically important, proper assessment of intracranial arterial collateral pathways by transcranial color-coded duplex sonography (TCCD) in patients with internal carotid artery (ICA) high-grade stenosis or occlusion is occasionally made difficult by an insufficient temporal bone window, an unfavorable insonation angle, or low flow velocity or volume. In these cases, echocontrast could be helpful to increase the diagnostic confidence or to make the diagnosis at all. METHODS: We investigated 50 temporal windows of 44 patients with ipsilateral high-grade (>/=70%) ICA stenosis or occlusion and insufficient native transtemporal insonation conditions before and after the application of the echo enhancer Levovist with an infusion pump. RESULTS: Compared with the precontrast scans, echocontrast allowed for more segments to be evaluated by pulsed Doppler sonography (P<0. 0001) and for longer lumen segments to be displayed on color mode (P<0.0001). Also, collateral flow via the anterior and posterior communicating artery could be demonstrated in 25 and 32 scans, respectively, compared with only 1 demonstration of each collateral pathway before the application of contrast medium (both P<0.0001). Similarly, with the help of contrast medium, flow velocity in the middle cerebral artery could be measured in 45 cases compared with only 26 cases before contrast was applied (P<0.0001). CONCLUSIONS: In patients with poor precontrast visualization of intracranial arteries, echocontrast-enhanced TCCD is very helpful in the assessment of intracranial collateral pathways recruited downstream to ICA stenoses and occlusions.

Rapid HIV screening at the point of care: legal and ethical issues.

In March 2000, the Canadian HIV/AIDS Legal Network released a comprehensive report entitled Rapid HIV Screening at the Point of Care: Legal and Ethical Issues. The release of the report coincided with the issuing of the first license to sell rapid HIV test kits in Canada for use by health-care professionals at the "point of care," and received considerable media attention. The report makes 23 recommendations to federal and provincial/territorial health officials, health-care professionals and their professional associations and regulatory bodies, and test kit manufacturers, with a view to ensuring that the potential benefits of this testing technology are maximized and the potential harms are prevented or minimized. We reproduce here the executive summary of the report.

HIV/AIDS in prisons: more new developments.

Since our last review, there have been new developments in several jurisdictions in the area of HIV/AIDS and prisons. This column reviews some important developments in Canada and in the US: the results of the evaluation of HIV/AIDS harm-reduction measures in the Canadian federal prison system; another Canadian inmate's fight for methadone maintenance treatment in prison; an important article on health care in Canadian prisons which states that there is little evidence that the federal prison system "is making any serious effort to provide treatment to any drug users"; the US Supreme Court's refusal to review a lower court's ruling permitting Alabama prisons to segregate HIV-positive inmates; an important ruling stating that inmates are entitled to receive medications in jail without interruption; and research showing that women inmates in the US are 23 times more likely to have AIDS than women in the general population.

Echocontrast-enhanced ultrasound of extracranial internal carotid artery high-grade stenosis and occlusion.

BACKGROUND AND PURPOSE: Proper assessment of extracranial internal carotid artery high-grade stenosis and occlusion by extracranial color-coded duplex sonography (ECCD) is occasionally made difficult by shadowing, an unfavorable insonation angle, low flow velocity or volume, or a deep insonation depth. In these cases, echocontrast could be helpful to quantify the degree of stenosis and to diagnose occlusion. METHODS: We investigated 17 arteries with poor precontrast investigation conditions and suspected high-grade stenosis or occlusion by contrast-enhanced ECCD. RESULTS: Compared with the precontrast scans, echocontrast allowed for significantly more segments to be evaluated by pulsed Doppler sonography (P<0.001) and for longer lumen segments to be displayed on color mode (P<0.001). Because it was now possible to place the sample volume right into the jet of the stenosis, the maximal flow velocity registered increased in all patients with stenosis. CONCLUSIONS: Echocontrast-enhanced ECCD of the carotid arteries is helpful for stenosis classification in a small group of preselected patients with poor original examination conditions.

Estimation of self-turning in the dark: comparison between active and passive rotation.

The present work compares passive and active rotations in darkness with the aim of characterizing the contribution of efferent and proprioceptive information to the perception of angular displacement. The perception of angular displacements was measured in 12 naive subjects (Ss), who either stood on a rotating platform (passive mode, P) or actively turned about their vertical axis by stepping around "on the spot" on a stationary platform (active mode, A). Rotations consisted of short acceleration epochs followed by constant velocity periods of 18.5, 37, and 55 degrees /s, with angular displacements ranging from 30 degrees to 810 degrees (presented in a randomized order); in the case of active turning, Ss had learned to approximately produce any of these three velocity levels on command. Ss indicated perceived displacement either verbally (verbal estimation mode, E), or by stopping their rotation when self-displacement appeared to match the magnitude specified by the experimenter (targeting, T). The resulting four conditions (PE, PT, AE, AT) were administered blockwise. In none of the four conditions was there a systematic dependence of perception on turning velocity. Therefore, the results were pooled across velocities, and the Ss' performance was summarized in the form of estimation curves showing median estimates as a function of physical displacement. There were several differences between the passive and active modes: AE- and AT-estimation curves were linear, close to veracity, and fairly similar to each other. In contrast, the PE-curve was curved rightwardly ("saturation"), with small displacements being overestimated and large ones underestimated, whereas the PT-curve was linear and indicated a pronounced overestimation of large displacements. Moreover, both the random and the systematic errors (measures of individual consistency and correctness of individual calibration, respectively) were significantly smaller in the active than in the passive modes. The observed independence of Ss' perception from turning velocity also during passive rotation suggests that the perceptual time constant was significantly longer than 16 s (a value cited as typical for vestibular perception), being possibly "enhanced" by contextual implications and by expectations of the Ss. The clear improvement of perceptual performance in the active mode testifies to the importance of the efferent and proprioceptive signals arising during active motion. On the assumption that these signals are about as "noisy" as the vestibular ones, the smaller errors during active turning could result from their combination with the vestibular signal. Alternatively, they could also be intrinsically less noisy than the vestibular signal and simply replace the latter during active motion. In the context of these alternatives (which are not exhaustive), the general problem of sensory fusion is discussed, that is, by which mechanisms are signals from different sensory sources combined to obtain a unified representation of the self's orientation.

Podokinetic after-rotation does not depend on sensory conflict.

Humans who have been stepping for 10 min or more about their vertical axis on a counterrotating platform while fixating on a stationary visual scene continue to circle in the same direction when they attempt, thereafter, to step on firm ground in darkness without turning ("podokinetic after-rotation": PKAR). In the present report, we investigate whether PKAR is due to: (1) a sensory reinterpretation triggered by the conflict between the visual signal of stationarity and the somatosensory message of feet-on-platform rotation, or (2) an adaptation of the somatosensory afferents to prolonged unilateral stimulation irrespective of visual stimulation. Subjects (Ss) circled for 10 min about their vertical axis on an either stationary or counterrotating platform while they were either in darkness, or exposed to an optokinetic stimulus, or to a "head-fixed" stationary pattern. Thereafter, Ss first stood motionless in darkness for 30 s, allowing vestibular after-effects to decay, and then tried (still without vision) to step in place on the stationary platform without turning while their body rotation was recorded by a potentiometer coupled to the head. All conditions involving podomotor activity without concomitant optokinetic stimulation evoked similar PKAR. With optokinetic stimulation, PKAR became larger, apparently because it was summed with an optokinetically induced after-rotation (oPKAR). This oPKAR could be demonstrated in isolation when Ss were passively rotated in front of the OKN-pattern instead of actively circling. PKAR could not be "dumped"; it reappeared after 30 s of straight stepping under visual control. We suggest that PKAR is caused by adaptation of the somatosensory channel and not by a sensory conflict.

Topography of the lunar poles from radar interferometry: a survey of cold trap locations.

Detailed topographic maps of the lunar poles have been obtained by Earth-based radar interferometry with the 3.5-centimeter wavelength Goldstone Solar System Radar. The interferometer provided maps 300 kilometers by 1000 kilometers of both polar regions at 150-meter spatial resolution and 50-meter height resolution. Using ray tracing, these digital elevation models were used to locate regions that are in permanent shadow from solar illumination and may harbor ice deposits. Estimates of the total extent of shadowed areas poleward of 87.5 degrees latitude are 1030 and 2550 square kilometers for the north and south poles, respectively.

Eye-head coordination in labyrinthine-defective humans.

Eye-head coordination during saccadic gaze shifts normally relies on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibulo-ocular reflex (VOR) stabilizes postsaccadic gaze while the head movement is still going on. Acute bilateral loss of vestibular function is known to cause overshoot of gaze saccades and postsaccadic instability. We asked how patients suffering from chronic vestibular loss adapt to this situation. Eye and head movements were recorded from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fixed in primary position, (2) free to move, or (3) preadjusted to different head-to-target offsets (to provoke head movements of different amplitudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accuracy, in terms of the mean of the signed gaze error, was better in patients than in normals. However, unlike in normals, the errors of patients exhibited a large scatter and included many overshoots. These overshoots cannot be attributed to the loss of VSR because they also occurred when the head was not moving and were diminished when large head movements were provoked. Patients' postsaccadic stability was, on average, almost as good as that of normals, but the individual responses again showed a large scatter. Also, there were many cases of inappropriate postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was already on target. Performance in patients was affected only marginally when large head movements were provoked. Except for the larger lag of the head upon the eye, the temporal coupling of eye and head movements in patients was similar to that in normals. Our findings show that patients with chronic vestibular loss regain the ability to make functionally appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding of the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space signal. Moreover, this variability also permeates their gaze performance in the absence of head movements.