Search PubMed⌕ Search

Biomedical subjects

O Bock

Publications and source records attributed to O Bock.

At least 37 records · Page 2Linked to original sources
PubMed2000

The mechanisms of movement preparation: a precuing study.

It is well known that precues about the possible locations of upcoming targets reduce the manual reaction time. The present study investigates at which stage of the sensorimotor system such precues act. Several light dots were displayed as a precue; one of them became the target, and subjects had to produce a manual response to the target as fast as possible. Exp. A varied independently the number of precues and the area of space they occupy; we found that the reaction time of pointing movements depended on spatial extent, but not on the number of choices. The outcome was similar in Exp. B, where subjects produced stereotyped 'tapping' movements irrespective of target position. Taken together, both findings support the view that precues act mainly at a stage concerned with the internal representation of space, rather than with response selection or movement preparation. The effects of precues were preserved when subjects fixated throughout the precuing period (Exp. C), but not when precue and target positions were uncorrelated (Exp. D). These findings do not support the alternative interpretations, that precues act by guiding the eyes into the vicinity of targets, or by elevating the subjects' arousal level.

Adult↗
PubMed2000

Quantitative molecular analysis of laser-microdissected paraffin-embedded human tissues.

Laser microdissection enables the contamination-free isolation of morphologically defined pure cell populations from archival formalin-fixed paraffin-embedded tissue specimens. Cells isolated by this method have been characterized by a wide variety of qualitative molecular assays, e.g. loss of heterozygosity, point mutations, clonality and lineage origin. The recently introduced real-time PCR technology renders the reliable quantification of very small amounts of nucleic acids possible. Several groups including our own showed that this technique can be successfully applied for the quantification of DNA and RNA isolated from microdissected archival tissue sections, even after immunohistochemical staining. The exact analysis of quantitative changes of nucleic acids during the course of pathological alterations has thus become possible. In many situations these quantitative changes can be expected to be more important than qualitative changes. The new technology for the quantification of structural genomic alterations and changes in the gene expression pattern in conjunction with microdissection have equipped morphologists with a powerful tool to study reactive and neoplastic changes of tissues.

Cell Separation↗
PubMed2000

Is dual-task performance necessarily impaired in space?

Recent single-subject experiments in space have reported impaired dual-task performance that could result from either a direct effect of microgravity on the central nervous system or from the multistressor environment. We sought to distinguish between these hypotheses using 6 astronauts in the 16-day NASA Neurolab mission, testing them at intervals with a dual task consisting of primary pursuit tracking without vision of the hand and secondary reaction time (RT). The participants were highly trained, instructed to maintain a fixed attention strategy, and restrained in the apparatus. The results showed that absolute and variable tracking error, as well as correct RT and the standard deviation of RT, were unimpaired. However, RT errors became more variable, an effect attributed to a decrease in strategic control. We conclude that the impairments observed in previous dual-task space experiments can be attributed to stressors rather than to microgravity and that performance deficits are probably not a necessary concomitant of space flight if attention is paid to task design and astronaut training.

Adult↗
PubMed2000

A review of cognitive and perceptual-motor performance in space.

Although a consensus developed early in the space program that complex operations could be performed successfully in space, it appears that humans are slower there than on Earth. Recent experiments aimed at determining the cause of this slowing suggest that central cognitive operations are unaffected but that perceptual-motor performance is degraded under certain circumstances. Two hypotheses have been proposed to account for this degradation-the direct effects of microgravity on the central nervous system and the non-specific effects of multiple stressors. Present evidence can be interpreted as supporting either hypothesis and further experiments are required to settle the question. This issue has practical implications since the countermeasures needed to ameliorate or prevent performance deficits will differ according to which hypothesis is correct. Understanding and ameliorating performance deficits will help ensure safe operations aboard the International Space Station and during a mission to Mars.

Cognition Disorders↗
PubMed1999

Reprogramming of grip aperture in a double-step virtual grasping paradigm.

The present study investigated the control of manual prehension movements in humans. Subjects grasped luminous virtual discs with the thumb and index finger, and we recorded the instantaneous grip aperture, defined as the 3-D distance between the thumb and index finger. Target size could remain constant (single-step trials) or unexpectedly change shortly after target appearance (double-step trials). In single-step responses, grip aperture varied throughout the movement in a consistent fashion. Double-step responses exhibited distinct corrective modifications, which followed the target change with a latency similar to the normal reaction time. This suggests that visual size information has a fast and continuous access to the processes involved in grip formation. The grip-aperture profiles of single-step responses had a different shape when the target called for an increase than when it called for a decrease in the initial finger distance. The same asymmetry was observed for aperture corrections in double-step trials. These findings indicate that increases and decreases of grip aperture are controlled through separate processes, engaged equally by the appearance and by the size change of a target. Corrections of grip aperture in double-step trials had a higher peak velocity and reached their maximum as well as their final value earlier than the aperture profiles of single-step trials. Nevertheless, the total duration of double-step trials was prolonged. These response characteristics did not fit with either of the three corrective strategies previously proposed for double-step pointing movements, which could indicate that grasping and pointing movements are controlled by different mechanisms. However, more data are needed to substantiate this view.

Algorithms↗
PubMed1999

Dependence of peripheral tremor on mechanical perturbations: a modeling study.

The present study scrutinizes the popular view that tremors of central origin but not those of peripheral origin are largely resistant to mechanical perturbations. We explore the effects of perturbations in a well-established model of peripheral tremor and document that (a) tremor frequency can remain unchanged when spring or weight loads are added, (b) entrainment by external drives can be limited to drives of similar frequency, and (c) resetting of tremor phase by torque pulses can remain fractional. This resistance to mechanical perturbations arises in the model because peripheral neuromuscular dynamics act as a limit-cycle oscillator which, by its very nature, will absorb moderate changes to signals and parameters. We conclude from our study that resistance to mechanical perturbations is not an exclusive property of central tremors, but rather may also be found in peripheral tremors. Other criteria are therefore needed to distinguish between different origins of tremor.

Models, Biological↗
PubMed1999

Load dependence of simulated central tremor.

Previous studies have argued that tremors of central versus peripheral origin can be distinguished based on their load dependence: the frequency of peripheral tremor decreases when a weight is added to the tremulous limb, while the frequency of central tremors remains unchanged. The present study scrutinizes the latter statement. We simulated central tremor using a simple network of coupled neural oscillators, which receives proprioceptive feedback from the motor periphery. The network produced a self-sustained, stable oscillation. When the gain of proprioceptive feedback was high, oscillation frequency decreased in the presence of an inertial load. When the gain was low, the oscillation frequency was load independent. We conclude that load dependence is not an exclusive property of peripheral tremors but may be found in tremors of central origin as well. Therefore, the load test is not sufficient to reject a central tremor origin.

Central Nervous System↗
PubMed1998

Placebo-controlled multicenter study of oral alendronate in postmenopausal osteoporotic women. FOSIT-Study-Group. Fosamax International Trial.

OBJECTIVES: To evaluate effects on bone mineral density (BMD), safety, and tolerability of a single daily dose of alendronate (10 mg), administered for 1 year to postmenopausal women with osteoporosis. METHODS: This interim analysis includes the first approximately 20% of patients to complete treatment in a large, placebo-controlled study (the Fosamax International Trial (Fosit)), which enrolled 1908 patients from 34 countries. Patients < or = 85-year-old with osteoporosis (lumbar spinal BMD > or = 2 S.D. below mean for mature premenopausal Caucasian women) were randomly assigned to treatment with alendronate or placebo once daily in the morning; all patients received supplemental calcium (500 mg/day). Dual-Energy X-ray Absorptiometry (DXA) was used to measure BMD in spine and proximal femur. RESULTS: A total of 297 patients had BMD data available for analysis. Patients treated with alendronate showed progressive increase of BMD during treatment. At 12 months, mean BMD had increased significantly (P < 0.001) at the lumbar spine (5.6%), trochanter (3.6%), and femoral neck (2.6%) in the alendronate group. Increases in BMD were significantly (P < 0.001) greater than in the placebo group at all sites. Among 442 patients assessed for safety, there were no statistically or clinically significant differences between treatment groups in the incidence of adverse events, including upper gastrointestinal adverse events, or laboratory abnormalities. CONCLUSIONS: Results of this multinational study show that oral alendronate, administered as 10 mg once daily for 1 year, is generally well tolerated and produces significant, progressive increases in BMD at the lumbar spine and proximal femur of postmenopausal women with osteoporosis.

Administration, Oral↗
PubMed1998

Problems of sensorimotor coordination in weightlessness.

Previous studies about human sensorimotor coordination in space are inconclusive: it was reported that subjects in weightlessness point too high or too low, too fast or at normal speed, with increased or with normal variability; and that their tracking performance is degraded or normal. A better understanding of human performance in space would be desirable not only from the basic science perspective, but also for operational reasons. We propose a conceptual framework to explain the reported diversity, and to point out avenues for future research. We argue that exposure to weightlessness produces sensorimotor discordance, to which subjects gradually adapt through processes similar to those involved in earthbound adaptation. These processes require substantial information-processing resources in the brain, which may not be easily available during the hectic pace of a space mission. Within this framework, it is not surprising that previous data on sensorimotor performance in space were incongruent, as demand and availability of resources may have differed between missions, or even between subjects. We therefore propose that future work should control resource demand and availability, and study their effects on sensorimotor performance before and during space missions, in order to deconfound their effects from the immediate effects of gravity. A suitable hardware for such research is presented.

Adaptation, Physiological↗
PubMed1998

Control of isometric force in hypergravity.

BACKGROUND: Previous work suggests that proprioceptive signals are degraded in hypergravity (hyper-G). We therefore, expected that production of finely graded force is disturbed as well. METHODS: Subjects produced isometric force with their thumb and index finger upon verbal instruction, before, during and after exposure to +1.5 Gz and +3 Gz. Produced force was orthogonal to the direction of gravity. RESULTS: In hyper-G, responses to a given target value were significantly higher (by about 400 pond) than in normal gravity, while the modulation of produced force with target force didn't change. The results in +1.5 Gz and +3 Gz were quantitatively similar, and a positive aftereffect was found. CONCLUSIONS: Subjects underestimate by a constant amount the force they produce in hyper-G. Our results are reminiscent of similar findings with pointing and grasping movements in hyper-G.

Adult↗
PubMed1997

Effects of a tilted visual background on human sensory-motor coordination.

The present study investigated the effects of a tilted visual background on perceived hand orientation, and on the execution of aimed arm movements. Subjects were seated in a room tilted about their mid-sagittal axis to the left or right. They were asked to indicate the gravitational vertical or the body midline by rotating their supported or free, unseen hand about the longitudinal forearm axis. They were further asked to draw vertical lines with their unseen arm, and to point with the hand at visual targets. Our results indicate that if the hand is stationary, tilted environments induce an illusory hand and body tilt in the opposite direction; the effects on the hand is substantially smaller than that on the body. We found no evidence for illusory hand tilt with line drawing, and pointing movements were not noticeably modified by background tilt. We concluded that the latter two tasks provide dynamic cues about hand orientation, which remain veridical in tilted environments, and can be utilized for fast corrections of motor commands.

Adult↗
PubMed1997

Visuo-motor adaptation: evidence for a distributed amplitude control system.

We investigated the constraints for visuo-motor adaptation in human pointing movements. Subjects pointed at sequentially presented visual targets while visual feedback about their finger position was either absent (pre- and post-period), or was manipulated such as to require a gradual reduction of response amplitude (per-period). We found that response amplitudes were smaller during the post- than during the pre-period, which documents the existence of adaptation to distorted visual feedback. We further found that adaptation can transfer fully to untrained amplitudes (Exp. 1), although the amount of transfer may be reduced if trained and untrained amplitudes are substantially different (Exp. 2). However, selective adaptation of one amplitude but not another can also be yielded if the paradigm explicitly asks for it (Exp. 3), and if the two amplitudes differ by more than about 10 cm (Exp. 4). We conclude from these findings that the adapted mechanism consists of amplitude-specific elements, tuned to amplitude spans of some 10 cm.

Adaptation, Physiological↗
PubMed1996

Performance of a simple aiming task in hypergravity: I. overall accuracy.

BACKGROUND: Visuo-motor performance is known to be affected by exposure to hyper-gravity (hyper-G), but the underlying mechanisms remain to be determined; the present study investigated the role of target mislocalization. METHOD: Subjects pointed before, during and after exposure to hyper-G at targets without seeing their hand. Target positions were displayed: a) throughout each pointing response; b) before response onset; or c) in normal gravity prior to a set of movements. RESULTS AND CONCLUSIONS: For all display conditions, subjects pointed higher in hyper-G than in normal gravity from the first movement on. We attribute the discrepancy between this finding and previous results (8, 12) to different movement strategies. The effects of hyper-G on pointing performance were small, but sustained when targets were displayed before or throughout each movement, but they were large and transient when targets were memorized in normal-G. We conclude that too-high pointing in hyper-G cannot be simply explained by the "elevator illusion," and propose a tentative interpretation based on known perceptual deficits.

Adult↗
PubMed1996

Performance of a simple aiming task in hypergravity: II. detailed response characteristics.

BACKGROUND: Literature proposes three hypotheses for impaired movement execution in hyper-G. The present study attempted to discriminate between these hypotheses by comparing kinematic characteristics and final accuracy of pointing movements in different gravity levels. METHOD: Subjects pointed without seeing their hand at targets presented before, during and after exposure to hyper-G. RESULTS: After factoring out movement amplitude, peak vertical velocity and the skewness of velocity profiles tended to increase, while movement duration tended to decrease with increasing G-level. Further, final response position was slightly less modulated by target position in hyper-G than in normal-G. CONCLUSION: Although not all findings reached statistical significance, the observed pattern of results corroborates the hypothesis (2) that the motor system re-interprets hyper-G as increased arm weight.

Adult↗
PubMed1996

Grasping of virtual objects in changed gravity.

BACKGROUND: Little is known about the effects of changed gravity on the execution of grasping movements, even though such movements play an important role in normal motor behavior of humans. HYPOTHESIS: The formation of an adequate grip aperture is impaired in changed gravity. METHOD: During parabolic flight, five subjects grasped mirror-viewed virtual targets with their thumb and index finger. From their video-taped responses, we determined grip aperture as the distance between the two fingertips. RESULTS: In changed gravity, the final grip aperture was about 15% smaller than in normal gravity, and the peak grip aperture was about 30% less modulated by target size. Both findings were similar in hyper-G and in micro-G. CONCLUSIONS: We conclude that (virtual) grasping in changed gravity is affected by a deterioration of visual and/or proprioceptive signals, or by the increased computational burden of controlling movements in unusual force environments.

Adult↗
PubMed1995

On the role of extraretinal signals for saccade generation.

We investigated the accuracy of sequential saccadic eye movements, executed without visual feed-back. We found evidence that the final error of one saccade is corrected during the next, which supports the existence of extraretinal inputs to the saccadic generator. The corrections, however, were incomplete, which suggests that extraretinal signals are only partially effective.

Eye Movements↗
PubMed1994

Scaling of joint torque during planar arm movements.

The present study scrutinized the "Motor Program" concept for aimed arm movements. Human subjects pointed at visual targets in a horizontal plane, with movements of varying starting positions, amplitudes and directions. We recorded movement kinematics and subsequently calculated the shoulder and elbow joint torque profiles. Our results indicate that the shape of torque profiles is rather uniform across movements and joints. We defined the size of those profiles by six "landmark variables", which could be subsequently reduced to three factors using factor analysis: one factor represented torque magnitude and two represented different aspects of torque timing. Additional analyses indicated that total torque duration is an important controlled signal. Our findings conform with the view that movements are executed by playing back scaled versions of prototypical joint torque profiles.

Adult↗