SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
A thyratron is an important component of a medical linear accelerator. The working life of a thyratron has been found to be dependent on its physical orientation in the accelerator. This observation is a result of maintaining accelerators for over 15 years.
The ability to localize a virtual sound source in the horizontal plane was evaluated under varying levels of sustained (+Gz) acceleration. Participants were required to judge the locations of spatialized noise bursts in the horizontal plane (elevation 0 degrees) during exposure to 1.0, 1.5, 2.5, 4.0, 5.5, and 7.0 +Gz. The experiment was conducted at the U.S. Air Force Research Laboratory's Dynamic Environment Simulator, a three-axis centrifuge. No significant increases in localization error were found between 1.0 and 5.5 +Gz; however, a significant increase did occur at the 7.0 +Gz level. In addition, the percentage of front/back confusions did not vary as a function of +Gz level. Collectively, these results indicate that the ability to localize virtual sound sources is well maintained at various levels of sustained acceleration. Actual or potential applications include the incorporation of spatial audio displays into the human-computer interface for vehicles that are operated in acceleration environments.
The human limiting values for sudden accelerations we have determined can be used to evaluate specific physical conditions that cause problems in maintaining postural balance. A comparison between the data obtained in the laboratory and the situations occurring during public transport by tram, bus, and metro revealed that both the initial impetus ("jerk") and the level of acceleration found in practice were sufficient to ensure that none of the individuals measured in the laboratory would have been able to retain their balance without extra support. The study suggests that limiting the initial jerk component of the acceleration would help considerably in alleviating the problem.
The otolith-function study is remarkably behind the semicanal-function study. In the present paper, we introduced briefly our on-going studies on eye movements including nystagmic elicitation during lateral (Gy) linear acceleration with step and sinusoidal modes using a sled-type accelerator. The eye movements were recorded by EOGs (DC) from subjects who looked at an imaginary target of their straight ahead in darkness during G-loading up to 0.5 G. Corresponding to the +Gy and -Gy segments, nystagmus and/or deviation in eye position were frequently induced in some subjects, but none or slightly in the other. The nystagmus changed the beating direction dependently on the Gy direction, while the eye-deviation could be either direction of compensatory or anticompensatory. In half of subjects, nystagmus elicitation was absent or low at 0.3 G, while it tended to increase above 0.3 G. The nystagmic elicitation was similar to each other between the both modes of acceleration, and directional preponderance (DP) was observed in some subjects. There was no correlation between the DP and the nystagmic slow-phase velocity. Functional meanings of these findings were discussed.
OBJECTIVE: To develop an instrument that could be sandwiched between the hoof and shoe of horses and that would reliably measure vertical ground reaction forces and three-dimensional acceleration at the walk, trot, and canter. ANIMALS: 5 clinically sound Thoroughbreds. PROCEDURES: The recording instrument (weight, 350 g) consisted of 2 metal plates, 2 bolts, 4 load cells, and 3 accelerometers. It was mounted to the hoof with a glue-on shoe and devised to support as much load exerted by a limb as possible. The load cells and accelerometers were wired to a 16-channel transmitter, and transmitted signals were received and amplified with a telemetry receiver. RESULTS: The recording instrument could measure in real time the 4 components of the ground reaction force or their resultant force along with acceleration in 3 dimensions as horses walked, trotted, or cantered on a treadmill. Patterns of force-time curves recorded for consecutive strides were similar to each other and to those previously reported, using a force plate. CONCLUSIONS AND CLINICAL RELEVANCE: The recording instrument developed for use in the present study allowed us to record vertical ground reaction force and acceleration in 3 dimensions in horses at the walk, trot, and canter.
The magnitudes of the hand-transmitted vibration, 297 hand-held tools and workpieces were measured at workplaces. The tools were used mainly in construction and manufacturing industries. The current standard for the measurement of hand-transmitted vibration has been based on the frequency-weighted acceleration of dominant axis method. According to the proposal of ISO/CD5349-1, the results of measurements (462 points) were recalculated by the method of the frequency-weighted acceleration sum. There is 32% of difference in average value between the acceleration sum and the dominant axis method.
When the body/head motion is sensed by otolith organs, they respond not only to the resultant acceleration of the motion but also to the gravitational force. We investigated the influence of the gravity vector on the otolithic-ocular reflex caused by motion in normal subjects. The sled type linear accelerator, moving back and forth with a frequency of 0.25 Hz and an amplitude of 2 m, generated right-left linear acceleration with a maximum magnitude of 0.5 g. We tested every subject under seven different postures: 1) 135 degrees forward tilted (F.T.), 2) 90 degrees F.T., 3) 45 degrees F.T., 4) upright sitting, 5) 45 degrees backward tilted (B.T.), 6) 90 degrees B.T., and 7) 135 degrees B.T. Horizontal eye movements with nystagmic pattern were elicited by these stimulations and were recorded by EOG. The eye movement data were analyzed by using a computer with the following procedure. After extraction of saccadic components from the data, the remaining parts were connected smoothly. The waves reconstructed in this manner, considered to be primary otolithic-ocular responses, were processed with the FFT method for calculating the amplitude of the component at 0.25 Hz. As a result, the responses were larger in the forward tilted postures than in the backward tilted postures.
Periodical stimulation of the otoliths during linear horizontal displacement leads not only to motion and displacement perception but also to the "Hilltop Illusion", which is the perception of being transported not on an earth-horizontal path, but over a small hill. This phenomenon is explained by the partial reinterpretation of the linear acceleration to a tilt of the gravitational vertical. The dynamics of the illusion and its role in the perception of spatial orientation were investigated for eight frequencies. The subjects were exposed to sinusoidal linear acceleration and had to dynamically adjust a luminous line display to their subjective vertical. This served as a quantitative indicator of the amount of perceived tilt with respect to gravity. The high frequency part (0.1-0.33 Hz) of the experiment was performed on a conventional linear acceleration device (sled), the low frequency part (0.0083-0.05 Hz) on the sled centrifuge (a rotating sled). The amplitude ratio of the settings of the subjective vertical was found to be similar to that of a lowpass with a 15 s time constant. However, the phase shifts never exceeded a 40 lag, which together with other experimental results led to the conclusion that a non-linear, possibly predictive type of information processing must be at work.
When an object's motion is influenced by gravity, as in the rise and fall of a thrown ball, the vertical component of acceleration is roughly constant at 9.8 m/sec2. In principle, an observer could use this information to estimate the absolute size and distance of the object (Saxberg, 1987a; Watson, Banks, von Hofsten, & Royden, 1992). In five experiments, we examined people's ability to utilize the size and distance information provided by gravitational acceleration. Observers viewed computer simulations of an object rising and falling on a trajectory aligned with the gravitational vector. The simulated objects were balls of different diameters presented across a wide range of simulated distances. Observers were asked to identify the ball that was presented and to estimate its distance. The results showed that observers were much more sensitive to average velocity than to the gravitational acceleration pattern. Likewise, verticality of the motion and visibility of the trajectory's apex had negligible effects on the accuracy of size and distance judgments.
Thresholds for the detection of constant acceleration and deceleration of a discrete object moving along horizontal and vertical axes were studied. A staircase methodology was used to determine thresholds for three average velocities (0.7, 1.2, and 1.7 deg/sec). Thresholds, expressed as the proportion of velocity change, did not differ significantly among the average velocities; thus, a consistent Weber-like fraction is suggested by the data. Furthermore, there was an interaction between the axis of motion (horizontal or vertical) and the sign of the velocity change (acceleration or deceleration): accelerations were easier to detect along the vertical axis, decelerations along the horizontal axis.
BACKGROUND: During positive acceleration, there is a diminished flow of blood to all regions above the heart. This is manifested by the commonly described loss of peripheral vision, greyout and blackout, which have been investigated extensively. The ability to select appropriate scanning patterns and to efficiently process visual information is one of the important determinants of scan effectiveness. This study investigates the performance of the smooth pursuit system under sustained +3 Gz before any signs of loss of vision. METHODS: Eleven subjects with no known oculomotor and vestibular anomalies participated in the study. Horizontal and vertical pursuit at amplitudes of 10 and 20 degrees were investigated in each of the subjects over 4 separate days. During each test session, pursuit targets of a predictable sine wave, oscillating at 0.2, 0.4, 0.8, 1.2 and 1.6 Hz were presented to the subjects in a random order. Horizontal and vertical eye movements were recorded using the El-Mar eye tracking system. The subjects were tested in 4 trials: 1) at 1 G before exposure to increased acceleration; 2) during sustained +3 Gz; 3) immediately after the +3 Gz exposure; and 4) 5 min after the +3 Gz exposure. RESULTS: Breakdown in smooth pursuit in response to horizontal and vertical sinusoidal stimuli during +3 Gz is indicated by a statistically significant decrease in gain and an increase in phase lag (p < 0.01). This is most obvious when the stimulus frequency is greater than 0.4 Hz. Qualitatively, the pursuit response during acceleration was ataxic and disorganized in appearance. CONCLUSION: It is postulated that degradation of pursuit gain and phase could be due to central hypoxia, and that the increase of G loading on the vestibular system could affect the neural integration of the pursuit signal in the vestibular nuclei with its direct output to the oculomotor system.
Relapse of chronic myeloid leukemia (CML) in chronic phase after allogeneic stem cell transplantation (SCT) can be successfully treated by donor lymphocyte infusion (DLI). However, relapse of accelerated phase CML, blast crisis, or acute leukemia after allogeneic SCT are resistant to DLI in the majority of cases. In vitro-selected and expanded leukemia-reactive T-cell lines may be more effective in inducing an antileukemic response in vivo. To treat a patient with accelerated phase CML after allogeneic SCT, leukemia-reactive cytotoxic T-lymphocyte (CTL) lines were generated from her HLA-identical donor. Using a modification of a limiting dilution assay, T cells were isolated from the donor, selected based on their ability to inhibit the in vitro growth of CML progenitor cells, and subsequently expanded in vitro to generate CTL lines. Three CTL lines were generated that lysed the leukemic cells from the patient and inhibited the growth of leukemic progenitor cells. The CTL did not react with lymphocytes from donor or recipient and did not affect donor hematopoietic progenitor cells. The 3 leukemia-reactive CTL lines were infused at 5-week intervals at a cumulative dose of 3.2 x 10(9) CTL. Shortly after the third infusion, complete eradication of the leukemic cells was observed, as shown by cytogenetic analysis, fluorescence in situ hybridization, molecular analysis of BCR/ABL-mRNA, and chimerism studies. These results show that in vitro cultured leukemia-reactive CTL lines selected on their ability to inhibit the proliferation of leukemic progenitor cells in vitro can be successfully applied to treat accelerated phase CML after allogeneic SCT.
The results of experimental studies of the effect of +Gz acceleration on the transmissive properties of the man-operator involved in the tracking system are discussed. With an increase of the acceleration value and the exposure time the law of distribution of tracking errors made by the operator varies from the normal to the uniform pattern. The estimates of the quality of tracking of a complex signal upon an exposure of the operator to accelerations of different values are given.
When fighter aircrafts change directions, the pilots are subject to a centrifuge force. This force can be enough to induce the pilot loss of consciousness (LOC). The physiological cause of this LOC is a brain hypoxia which results from the increase of the hydrostatic pressure. Under gradual onset rate + Gz acceleration (GOR+Gz), the brain hypoxia occurs with visual prodromes familiar to fighter pilots. In this case, they feel a reduction of the peripheral vision (Grey-out), followed by a loss of the central vision (Black-out). The useful time of consciousness can be prolonged by using anti-G suit. Under rapid onset rate +Gz acceleration, a new symptomatology of the LOC, characterized by the absence of a grey or black-out, has been observed. The objective of this theoretical study is a better understanding of mechanisms which cause fighter pilot inflight LOC. A physical model has been developed. It allows to take into account the influence of the rate of change in acceleration on the cerebrovascular system behavior.
The objective of the test program was to study the effect of parametric changes in helmet inertial properties on the biodynamic response of human volunteers subjected to +Gz impact accelerations. Test data was used to drive a computer model (DYNAMAN) to estimate the loads and torques in the neck during impact. Currently, only seven of eleven test cells with variations in the inertial properties of the helmet along the x-axis of the head have been analyzed. Preliminary data analysis indicates that the biodynamic response of the head under the tested conditions is slightly more sensitive to the moment of inertia of the helmet than its weight alone even though both variables showed a general trend for the head accelerations (linear and angular) to increase. It has been shown that the model can give good estimates of the compression loads in the neck, but that the torque estimates will be low, possibly by a factor of three. Further refinements of the neck joint parameters in the model will be required in order to increase the motion of the head segment during impact acceleration and will be done prior to completing the remaining test cell analysis. Finally, all the test data will be evaluated to determine if the current interim head criteria require modification.
OBJECTIVE: To study the relationship between cardiac arrhythmias and autonomic nervous regulation in pilots under +Gz acceleration. METHOD: Dynamic ECG during +Gz exposures in 36 orthostatic intolerance pilots and 62 healthy pilots were analysed and compared. RESULT: The orthostatic intolerance pilots had obviously lower +Gz tolerance and more cardiac arrhythmias. The cardiac arrhythmias could affect +Gz tolerance. CONCLUSION: The cardiac arrhythmias under +Gz acceleration could be taken as an index of evaluating cardiovascular compensatory function and warning against acceleration (+Gz) induced loss of consciousness (G-LOC).
The Space Acceleration Measurement System (SAMS) has been developed by NASA to monitor the microgravity acceleration environment aboard the space shuttle. The amount of data collected by a SAMS unit during a shuttle mission is in the several gigabytes range. Adaptive Resonance Theory 2-A (ART2-A), an unsupervised neural network, has been used to cluster these data and to develop cause and effect relationships among disturbances and the acceleration environment. Using input patterns formed on the basis of power spectral densities (psd), data collected from two missions, STS-050 and STS-057, have been clustered.
The effects of positive acceleration on the CNS were studied by applying negative pressure to the lower body (LBNP) of unanesthetized rabbits. The LBNP which can simulate well the effects of positive acceleration induced high voltage slow bursts with amplitude 600-800 microV and frequency of 2-3 Hz in the EEG of the rabbits, which seemed to be quite similar to those observed by Adey et al. in the EEG of the centrifuged monkeys and cats. Because the physiological effects of LBNP as well as positive acceleration had been greatly attributed to disturbance of circulation in the CNS, much effort was made in this experiment to study the mechanisms triggering the slow bursts especially in relation to changes in hemodynamic cardiovascular measurements. By LBNP, both blood pressure and blood flow fell markedly and cerebral circulation was obstructed, but these changes were recovered to some extent approximately 10 seconds after the application of LBNP. These changes fell again until the release of LBNP, when cerebral blood flow was profoundly increased. The slow bursts seemed to be induced only in such periods when cerebral blood flow was profoundly increased after a failure of cardiovascular compensatory mechanisms such as a decrease of heart rate with irregular cardiac rhythm had occurred by repeated application of a greater LBNP. These results suggest that the necessary important change inducing the slow bursts is a sudden rush of blood to the anoxic CNS and is not only an anoxic condition of the CNS during LBNP, and the bursts may possibly be triggered by mechanical distortion of the neuronal network by a sudden fluid movement within the anoxic brain or change in permeability of blood vessels and the glial cells which might greatly modulate neuronal excitability by such sudden return of blood to the anoxic CNS.