SEARCH · PubMed Health

Results for “Acceleration”

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.

At least 199 records · Page 11Linked to original sources

PubMed1993

Elevated sympathetic nerve activity in patients with accelerated essential hypertension.

To determine if an abnormality exists in the sympathetic nervous system of patients with accelerated hypertension, we recorded muscle sympathetic nerve activity (MSNA) from the tibial nerve by microneurography in eight benign essential hypertensives and seven accelerated essential hypertensives. Basal MSNA, plasma renin activity, and plasma angiotensin II levels were significantly higher in accelerated hypertensives than in benign hypertensives (P < 0.05). To clarify the relationship between the renin-angiotensin axis and sympathetic nervous system in the accelerated hypertensives, we measured the MSNA after 7 d of oral administration of captopril (75 mg/d) for antihypertensive treatment in the benign hypertensives and accelerated hypertensives. After administering captopril, the arterial pressure decreased significantly in the benign hypertensives and accelerated hypertensives with decreases in plasma angiotensin II levels, and the decreases in arterial pressure were greater in the accelerated hypertensive than in the benign hypertensives. After captopril administration, the MSNA decreased significantly in the accelerated hypertensives but did not change in the benign hypertensives. Thus, in accelerated hypertensives, sympathetic tone is elevated, and the elevated sympathetic tone is closely related to the activated renin-angiotensin axis tone.

Adult↗

PubMed2006

rhBMP-2/calcium phosphate matrix accelerates osteotomy-site healing in a nonhuman primate model at multiple treatment times and concentrations.

BACKGROUND: While recombinant human bone morphogenetic protein-2 (rhBMP-2) administered in a calcium phosphate cement accelerates osteotomy-site healing in animal models when administered three hours after surgery, definitive fracture treatment is often delayed. The present study evaluated the ability of rhBMP-2, administered in a new particulating calcium phosphate matrix, to accelerate nonhuman primate fibular osteotomy-site healing following treatment at multiple treatment times and concentrations. METHODS: The ability of 1.5-mg/mL rhBMP-2/calcium phosphate matrix to accelerate osteotomy-site healing when administered three hours, one day, one week, or two weeks after surgery was first evaluated with use of bilateral proximal and distal fibular osteotomy sites in adult male monkeys. In a second study, the healing of osteotomy sites that had been treated with the administration of calcium phosphate matrix alone and with different concentrations of rhBMP-2/calcium phosphate matrix (0.5 mg/mL, 1.5 mg/mL, or 4.5 mg/mL) seven days after surgery was compared with that of contralateral, untreated osteotomy sites. In a third study, the histologic progression of osteotomy-site healing following treatment with 1.5-mg/mL rhBMP-2/calcium phosphate matrix or calcium phosphate matrix alone, administered three hours or one week after surgery to the osteotomy site, was assessed at multiple time points for as long as twenty-four months after surgery. RESULTS: Radiographs demonstrated increased callus area and more rapid healing in response to 1.5-mg/mL rhBMP-2/calcium phosphate matrix administered over the range of treatment times after surgery as compared with the findings of previous reports on untreated osteotomy sites. Bone formation appeared at the osteotomy sites sooner following treatment at one and two weeks as compared with the findings at the earlier time-points. Scintigraphic imaging at one day and one week after surgery showed prolonged retention of rhBMP-2 at the osteotomy site following an initial burst release. In the second study, radiographic, peripheral quantitative computed tomographic, biomechanical, and microscopic evaluation demonstrated that administration of 1.5 and 4.5-mg/mL rhBMP-2/calcium phosphate matrix one week after surgery accelerated osteotomy-site healing by 40% to 50% compared with the findings in untreated controls. The magnitude of acceleration was less in response to 0.5-mg/mL rhBMP-2/calcium phosphate matrix, and calcium phosphate matrix alone did not accelerate osteotomy-site healing. Histological evaluation indicated that an increased cellular infiltrate and increased direct bone formation contributed to the accelerated osteotomy-site healing following administration of rhBMP-2/calcium phosphate matrix at one week compared with three hours after surgery. CONCLUSIONS: A single percutaneous injection of rhBMP-2/calcium phosphate matrix accelerated healing in nonhuman primate fibular osteotomy sites over a wide range of treatment times. Efficacy was optimized in association with the administration of 1.5-mg/mL rhBMP-2/calcium phosphate matrix. Delaying treatment for one week further accelerated healing because of an increase in the number of responding cells and an increase in direct bone formation.

Animals↗

PubMed2004

Gradation of Neck Muscle Responses and Head/Neck Kinematics to Acceleration and Speed Change in Rear-end Collisions.

Recent epidemiological evidence shows that the potential for whiplash injury varies with both the average acceleration and speed change of a rear-end collision. The goal of this study was to examine the gradation of neck muscle responses and the head and neck kinematics to rear-end collision pulses in which the acceleration and speed change were independently varied. Thirty subjects (15F, 15M) underwent 36 consecutive rear-end collisions consisting of three different average accelerations (ā = 0.5, 0.9 and 1.3 g) and three different speed changes (Deltav = 0.25, 0.50 and 0.75 m/s). Onset and amplitude of the sternocleidomastoid (SCM) and cervical paraspinal (PARA) muscle responses were measured using surface electromyography. Kinematic measures included linear and angular accelerations and displacements of the head and torso. The results showed that the amplitude of the muscle and kinematics responses was graded to both collision acceleration and speed change. The magnitude of early peaks in the head/neck kinematics correlated more strongly with collision acceleration (r(2) = 0.63 to 0.69), whereas the magnitude of later kinematic peaks correlated more strongly with collision speed change (r(2) = 0.59 to 0.95). Onset of the SCM muscle response correlated only weakly with collision acceleration and speed change (r(2) </= 0.07). A collision pulse descriptor obtained by multiplying average acceleration and speed change (āDeltav) yielded the strongest and most consistent correlations with neck muscle (r(2) = 0.48 to 0.58) and head/neck kinematic responses (r(2) = 0.78 to 0.94). This measure of collision severity is also consistent with the recent epidemiological evidence that whiplash symptom intensity and duration increases with both average acceleration and speed change.

Journal Article↗

PubMedDate not supplied

[Determining factors of acceleration in sustained monomorphic ventricular tachycardias in response to bursts of stimuli].

To analyse the determinants of acceleration of sustained monomorphic ventricular tachycardias in response to bursts of rapid ventricular pacing, we studied 46 consecutive patients with 90 distinct ventricular tachycardias during which one or more burst of rapid pacing were delivered. Tachycardia acceleration was observed in 11 tachycardias in 8 patients. The highest incidence of acceleration was observed in patients with left ventricular dysfunction of non coronary origin. There was a non significant trend towards lower values of left ventricular ejection fraction in patients with acceleration. There were no significant differences between ventricular tachycardias with of without acceleration in respect to: clinical presentation, QRS morphology, tachycardia cycle length and treatment with antiarrhythmic drugs. The shortest cycle of bursts of rapid pacing, was lower in tachycardias with acceleration than in those without it (229 +/- 57 ms vs 283 +/- 67 ms; p = 0.006) and tachycardias with acceleration showed a lower relation between burst cycle length and tachycardia cycle length (69 +/- 9% vs 84 +/- 8%; p < 0.001). The negative predictive value of acceleration was 75% for bursts of rapid pacing with a cycle length > or = 250 ms, and 96% for values of the relation between the burst cycle length and the tachycardia cycle length > or = 70%. This parameter show a very high discriminating value with of without antiarrhythmic drugs effect. Acceleration of sustained monomorphic ventricular tachycardias in response to bursts of rapid pacing depends on, the pacing rate and the relation between pacing rate and tachycardia rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed2004

Change of voice characteristics during +3 Gz acceleration.

BACKGROUND: This study was performed to test the feasibility of an experimental approach for assessing voice changes during exposure to increased +Gz acceleration. Such changes are probably due to mechanical alterations of the structures involved in voice production. This may be relevant to automatic speech analysis for flight control. Because voice control by means of auditory feedback may compensate for acceleration effects, the investigations included conditions with masked auditory feedback. METHOD: Four male subjects read standard speech test material while seated in a human centrifuge both during sustained +3 Gz acceleration and in a reference condition. Both test runs were repeated with masking of the auditory feedback by a white noise presented via headphones. Microphone and acceleration signals were recorded on a PC-based dynamic signal acquisition board. Long-time average spectra (LTAS), fundamental frequency (F0), and the frequency of the first, second, and third formant (F1, F2, F3) of the vowels /a/, /o/, and /i/ were extracted from the microphone signal for the different conditions. RESULTS: LTAS clearly differed between the masking conditions, but not between reference and +3 Gz conditions. F0 clearly rose with auditory masking and showed a small increase under +3 Gz acceleration. Several effects of +3 Gz acceleration on formant frequencies were found, all of rather small magnitude. Increased acceleration lowered F2 for the vowel /i/. A decrease of F3 was observed for vowels /i/ and /o/. CONCLUSION: This pilot study has shown the feasibility of an experimental approach to assessing voice changes during exposure to increased +Gz acceleration. Exposure to +3 Gz showed small effects on FO and several formant frequencies. A definitive forthcoming study should assess the significance of these effects by investigating a greater number of subjects during exposure to acceleration higher than +3 Gz.

Adult↗

PubMed2005

The University of Tennessee's accelerated family medicine residency program 1992-2002: an 11-year report.

BACKGROUND AND OBJECTIVES: In 1992, with the approval of the American Board of Family Medicine (ABFM) (formerly known as the American Board of Family Practice), we established an accelerated residency program (ARP) involving five residency programs at the University of Tennessee (UT). An accredited resident can complete medical school and residency in a combined total of 6 years. This paper is a report of our experience with the ARP. Our objective was to determine if accelerated residents performed as well as or better than non-accelerated residents. METHODS: Students are selected for the ARP on the basis of academic achievement, life experience, interviews, and commitment to family medicine. For the accelerated residents, we tracked outcomes measures, including medical school grade point average (GPA), US Medical Licensing Examination (USMLE) scores, ABFM In-training Examination scores, and board certification status. RESULTS: From 1992 to 2002, 47 students entered the ARP at five UT residency programs. Five students did not complete the program. The average entering GPA was 3.17, and the average USMLE Step I score was 207. The accelerated residents, on average, performed better on ABFM In-training Examinations in the first and third years of residency than the non-accelerated residents did. Accelerated residents have a 100% ABFM certification rate. A total of 76% practice in Tennessee, and 65% began practice in a rural county. CONCLUSIONS: The UT ARP has been an effective means for allowing medical students to complete their family medicine training in 6 years. Accelerated residents have performed as well as or better than non-accelerated residents on standardized testing.

Academic Medical Centers↗

PubMed1986

Percent acceleration time: a new method of fetal assessment.

The percent acceleration time (PAT) is a fetal heart rate parameter, which may be derived from the nonstress test (NST). It is based on measurement of the duration of individual movement-associated fetal heart rate accelerations (MAFAs), obtained during a nonstress test, and is calculated by the formula: (formula, see text) The authors studied 148 fetuses undergoing four or more serial nonstress tests who were delivered within one week of their last test. Percent acceleration time of fetuses with normal outcomes had a mean value of 15.5 +/- 10.0 (SD)%, correlated well with the frequency and amplitude of movement-associated fetal heart rate accelerations but was independent of gestational age and mean baseline fetal heart rate. No normal fetus had a percent acceleration time of less than 5% on its last nonstress test, while only one abnormal fetus had a percent acceleration time that exceeded the mean percent acceleration time of the normal group. All abnormal fetuses had a significant progressive decline in percent acceleration time (mean: 50.6%). Percent acceleration time values were more sensitive in identifying fetuses with perinatal compromise than were conventional criteria for nonstress test reactivity, and may provide a useful alternative to assessment of fetal status when other quantitative methods are unavailable (continuous ultrasound visualization) or less reliable (maternal perception, tokodynamometry, palpation).

Female↗

PubMed1996

Variation in artificial heart acceleration and sound production with prosthetic valve selection in vitro.

In an attempt to explore methods to reduce total artificial heart (TAH) acceleration and sound production, in vitro measurements of TAH acceleration and sound were made when using a variety of prosthetic valves in a test ventricle. A miniature, uniaxial, high fidelity accelerometer was glued to the housing of a UTAH-100 left ventricle adjacent to the inflow and outflow ports and parallel to the axis of diaphragm excursion. A miniature, high fidelity contact microphone was glued to the opposite side of the ventricular housing between the inflow and outflow ports. Data was collected over a range of heart rates, ventricular filling volumes and control modes while using tilting disc valves (TDV) and polymer trileaflet valves (PTV). For both valve types, the peak systolic acceleration impulse was lower (approximately equal to 50%) when the ventricle was fully rather than partially filled and the peak diastolic acceleration impulse was lower (approximately equal to 50%) when the ventricle was fully rather than partially ejected. The magnitude of the acceleration with PTVs was approximately equal to 20x less than the TDVs (0.5 to 2.2 g vs. 10 to 49g). The magnitude of the sound production was also considerably less with the PTVs (28 to 49 db vs. 50 to 64 db). Diastolic acceleration and sound production was approximately twice the systolic value for the TDVs; the reserse was true for PTV sound production. These data demonstrate the substantial reduction in TAH impulse acceleration and sound production by selecting PTVs over TDSs. With PTVs, the TAH impulse acceleration is the same as the natural heart. Consequently, this ability to lower TAH acceleration and sound production to the level of the natural heart may lead to a reduction in component wear, patient discomfort and other undesirable consequences of TAH implantation.

Aorta↗

PubMedDate not supplied

[Heart rate accelerations in the fetus during fetal blood sampling sub partu].

Fetal heart rate acceleration is often observed when fetal blood sampling (FBS) is performed during labour. The aim of this study was to examine the influence of the occurrence of a fetal heart rate acceleration on the fetal condition immediately after the FBS incision during labour. 117 pregnant patients with a total of 210 FBS were included in the study. Particular attention was given as to whether an acceleration > or = 15 bpm and > or = 15 sec occurred immediately after the FBS incision. So 2 groups were formed: reactive (159 cases with acceleration) and non-reactive (51 cases without acceleration). No FBS acceleration was observed in any case with acidosis during labour. Therefore the occurrence of an FBS acceleration can be regarded as a reliable predictor of a fetal pH value of > or = 7.20 during labour. The test sensitivity amounts to 77% and the specificity and the positive predictive value are 100%. If an acceleration occurred immediately after the FBS incision, in 6% of the cases (negative predictive value) an acidosis with a pH value of < 7.20 was registered. The cesarean section rate increases by more than double from 13% to 33% when no FBS acceleration occurs.

Acidosis, Respiratory↗

PubMedDate not supplied

Effects of acceleration on the accuracy of MR phase velocity measurements.

Acceleration in blood flow can affect the accuracy of phase velocity measurements. Convective acceleration is due to changes in flow geometry and is independent of the time-varying acceleration caused by flow pulsatility. To analyze the effects of convective acceleration on flow velocity measurements, phase velocity measurements were obtained in steady laminar flow in the convergent segment of a 90%, hourglass-shaped stenosis phantom at a Reynolds number of 1,500. Measurements at the stenosis indicated that convective acceleration caused the measured values of average cross-sectional velocity to deviate as much as 37% from the theoretical values. The magnitude of the error could be accounted for by including the convective acceleration term in the phase shift equation. Convective acceleration effects should not be ignored in flow velocity measurements through stenoses, even when time-dependent acceleration due to flow pulsatility can be neglected.

Acceleration↗

PubMedDate not supplied

Accelerator-based epithermal neutron sources for boron neutron capture therapy of brain tumors.

This paper reviews the development of low-energy light ion accelerator-based neutron sources (ABNSs) for the treatment of brain tumors through an intact scalp and skull using boron neutron capture therapy (BNCT). A major advantage of an ABNS for BNCT over reactor-based neutron sources is the potential for siting within a hospital. Consequently, light-ion accelerators that are injectors to larger machines in high-energy physics facilities are not considered. An ABNS for BNCT is composed of: (1) the accelerator hardware for producing a high current charged particle beam, (2) an appropriate neutron-producing target and target heat removal system (HRS), and (3) a moderator/reflector assembly to render the flux energy spectrum of neutrons produced in the target suitable for patient irradiation. As a consequence of the efforts of researchers throughout the world, progress has been made on the design, manufacture, and testing of these three major components. Although an ABNS facility has not yet been built that has optimally assembled these three components, the feasibility of clinically useful ABNSs has been clearly established. Both electrostatic and radio frequency linear accelerators of reasonable cost (approximately 1.5 M dollars) appear to be capable of producing charged particle beams, with combinations of accelerated particle energy (a few MeV) and beam currents (approximately 10 mA) that are suitable for a hospital-based ABNS for BNCT. The specific accelerator performance requirements depend upon the charged particle reaction by which neutrons are produced in the target and the clinical requirements for neutron field quality and intensity. The accelerator performance requirements are more demanding for beryllium than for lithium as a target. However, beryllium targets are more easily cooled. The accelerator performance requirements are also more demanding for greater neutron field quality and intensity. Target HRSs that are based on submerged-jet impingement and the use of microchannels have emerged as viable target cooling options. Neutron fields for reactor-based neutron sources provide an obvious basis of comparison for ABNS field quality. This paper compares Monte Carlo calculations of neutron field quality for an ABNS and an idealized standard reactor neutron field (ISRNF). The comparison shows that with lithium as a target, an ABNS can create a neutron field with a field quality that is significantly better (by a factor of approximately 1.2, as judged by the relative biological effectiveness (RBE)-dose that can be delivered to a tumor at a depth of 6cm) than that for the ISRNF. Also, for a beam current of 10 mA, the treatment time is calculated to be reasonable (approximately 30 min) for the boron concentrations that have been assumed.

Beryllium↗

PubMed2004

Non-linear eye movements during visual-vestibular interaction under body oscillation with step-mode lateral linear acceleration.

We investigated visual-vestibular interactions in normal humans, where a constant speed of optokinetic stimulation was combined with whole body oscillation of lateral linear acceleration (10 m stroke). The acceleration mode was not sinusoidal, but rectangular (step). The pure optokinetic reflex (reference OKR) and the OKR under combined stimulation (combined OKR) were induced by a random-dot pattern projected onto a hemispherical dome-screen affixed to a chair on a linear accelerator. The translational vestibulo-ocular reflex (tVOR) was determined separately in the dark during acceleration-step oscillation. Since the tVOR was masked by the OKR under combined stimulation, the interaction was assessed as changes in combined-OKR velocity at two segments of opposing acceleration; in other words, tVOR directions identical to (agonistic) and opposite to (antagonistic) the OKR direction. When a moderate optokinetic stimulus-speed of 40 deg/s was combined with a moderate acceleration of 0.3 G (3.0 m/s2) as in Experiment 1 (N=10), the combined-OKR velocity always increased during the agonistic condition, and the motion of the visual pattern was perceived as slow and clear in this segment. On the other hand, during the antagonistic condition, the combined-OKR velocity either remained unchanged or increased moderately, and the motion of the visual pattern was sensed as fast and unclear. Notably, in most subjects, the velocity difference in combined-OKR between the agonistic and antagonistic conditions was around the value of the tVOR velocity. In five of the ten subjects who completed an additional test session with the acceleration level increased from 0.3 to 0.5 G (4.9 m/s2), similar findings were maintained individually, suggesting independent behavior of tVOR. Therefore, we hypothesized that the interaction could be direction-selective; in other words, both tVOR and OKR are additive during the agonistic condition, but tVOR is suppressed during the antagonistic condition. To extend this hypothesis further, another group of subjects was exposed to three different optokinetic-stimulus speeds of 20, 40 and 60 deg/s combined with an acceleration of 0.3 G (Experiment 2, N=15). Combined stimulation tended to optimize the combined-OKR velocity around the given optokinetic stimulus-speed, especially in those cases where the reference-OKR velocity deviated significantly from the stimulus speed. Changes in combined-OKR velocity were small at 20 deg/s, and were likely to be linear across the agonistic and antagonistic conditions. With increasing optokinetic stimulus-speeds, the direction-selective asymmetry hypothesized above was maintained in more than half of the subjects, while in the other subjects the combined-OKR velocity difference increased remarkably, probably due to an enhancement of the OKR gain. We conclude that tVOR suppression during the antagonistic stimulus-condition and non-linearity in the tVOR-OKR interaction are characteristic of the otolith system, even under moderate-stimulus environments, in contrast to the linear eye-movement interaction in the semicircular canal system.

Acceleration↗

PubMed2001

Reciprocal angular acceleration of the ankle and hip joints during quiet standing in humans.

Human quiet standing is often modeled as a single inverted pendulum rotating around the ankle joint, under the assumption that movement around the hip joint is quite small. However, several recent studies have shown that movement around the hip joint can play a significant role in the efficient maintenance of the center of body mass (COM) above the support area. The aim of this study was to investigate how coordination between the hip and ankle joints is controlled during human quiet standing. Subjects stood quietly for 30 s with their eyes either opened (EO) or closed (EC), and we measured subtle angular displacements around the ankle (thetaa) and hip (thetah) joints using three highly sensitive CCD laser displacement sensors. Reliable data were obtained for both angular displacement and angular velocity (the first derivative of the angular displacement). Further, measurement error was not predominant, even among the angular acceleration data, which were obtained by taking the second derivative of the angular displacement. The angular displacement, velocity, and acceleration of the hip were found to be significantly greater (P<0.001) than those of the ankle, confirming that hip-joint motion cannot be ignored, even during quiet standing. We also found that a consistent reciprocal relationship exists between the angular accelerations of the hip and ankle joints, namely positive or negative angular acceleration of ankle joint is compensated for by oppositely directed angular acceleration of the hip joint. Principal component analysis revealed that this relationship can be expressed as: thetah=gammathetaa with gamma=-3.15+/-1.24 and gamma=-3.12+/-1.46 (mean +/-SD) for EO and EC, respectively, where theta is the angular acceleration. There was no significant difference in the values of y for EO and EC, and these values were in agreement with the theoretical value calculated assuming the acceleration of COM was zero. On the other hand, such a consistent relationship was never observed for angular displacement itself. These results suggest that the angular motions around the hip and ankle joints are not to keep the COM at a constant position, but rather to minimize acceleration of the COM.

Acceleration↗

PubMed2005

Coordination of head and trunk accelerations during walking.

The purpose of this study was to investigate the relationship between oscillatory dynamics of the head and trunk in each plane of motion during walking. Head and trunk accelerations of ten healthy subjects (age: 23 +/- 4 years) were measured in the vertical (VT), anterior-posterior (AP) and mediolateral (ML) directions using a pair of tri-axial accelerometers. All subjects performed five walking trials along a level 20 m walkway at their preferred gait velocity (1.30 +/- 0.15 m s(-1)). Acceleration data were analysed using power spectral, harmonic and regularity measures. Results indicated that: (1) oscillations of the head were smoother, with a greater proportion of power at lower frequencies than oscillations of the trunk, (2) differences in power spectral properties between the head and trunk were most pronounced in the ML direction, (3) coupling between VT-AP, VT-ML, and AP-ML accelerations were greater for the head than trunk, and (4) for both segments, the weakest coupling was observed for AP-ML acceleration relations. Overall, the results of this study suggest that accelerations of the head are significantly attenuated, and more tightly controlled, compared to accelerations of the lower trunk. This attenuation process was particularly evident for the ML direction, whereby head accelerations showed the greatest differences compared to ML accelerations at the trunk.

Acceleration↗

PubMed1995

Doppler determined aortic acceleration after dipyridamole in the prediction of coronary artery disease.

Change in the acceleration of aortic blood flow with stress testing is reported to reflect the presence of myocardial ischaemia. We studied its clinical usefulness when compared with dipyridamole thallium scintigraphy in 101 patients, of whom 64 had coronary angiography. Maximum aortic acceleration increased after dipyridamole (P < 0.0001), although no correlation existed between the aortic acceleration and evidence of thallium perfusion abnormalities. For the patients who had angiography, the increase in aortic acceleration was similar for those with no significant coronary stenoses, single vessel or multi-vessel disease. Compared with coronary angiography, Doppler measurement of maximum aortic acceleration had a sensitivity of 92% and a specificity of 37% for the detection of coronary artery disease. When patients with previous myocardial infarction or left ventricular dysfunction were excluded, there was still no relationship between the maximum aortic acceleration and the presence of coronary artery disease. We conclude that changes in the acceleration of aortic blood flow after dipyridamole stressing do not predict the presence or severity of coronary artery disease as measured from perfusion defects at thallium scintigraphy or by coronary angiography. We have observed a wide variability of aortic maximum acceleration in the evaluation of myocardial ischaemia, which we feel introduces serious limitations to its use in routine clinical practice.

Acceleration↗

PubMed1996

Acceleration-sensitivity threshold of Physarum.

Free-living cells show distinct gravisensitivities and often use the gravity ('g') vector for their spatial orientation. The rhythmic contractions of the ameboid Myxomycete (acellular slime mold) Physarum polycephalum are a sensitive parameter which can be modified by external stimuli. Space experiments and ground-based 0 x g simulation studies established that the contraction period transiently decreases after a transition from 1 x g to 0 x g with a back-regulating process starting after 30 min. For determination of the threshold of acceleration sensitivity, a slow-rotating centrifuge microscope (NIZEMI--Niedergeschwindigkeits-Zentrifugenmikroskop) was used, providing in space accelerations from 0 x g to 1.5 x g. A stepwise acceleration increase revealed that the lowest acceleration level capable of inducing a response was 0.1 x g. The response to the acceleration increase was an increase in contraction period, in contrast to a stimulus deprivation, which led to a period decrease. The time schedule of the acceleration responses and back-regulating process seems to be fixed, suggesting that every acceleration being above the threshold can induce a complete response-regulation process. The low acceleration-sensitivity threshold favors rather large and dense cell organelles as candidates for the gravity receptor in Physarum.

Acceleration↗

PubMed1996

Perception of direction of visual motion. II. Influence of linear body acceleration.

We investigated whether linear whole-body acceleration along the interaural y-axis influenced the concurrent perception of visual motion direction as has been shown for angular accelerations. A sled running on air bearings along a 7.5-m track was used to accelerate 18 subjects at two different linear accelerations. These young, healthy volunteers, aged 25.50 +/- 7.38 years, used a joystick to indicate whether or not they perceived visual motion to the left within a random-dot kinematogram continuously presented on a monitor moving with them. The percentage of coherently leftward moving pixels presented for a 640-ms period during acceleration was adjusted according to a Modified Binary Search (MOBS) procedure. Six conditions were tested, two acceleration levels of 1 and 2 m/s2 to both left and right with, at the higher acceleration, two different times of visual motion presentation. Conditions were sequenced by means of a 6 x 6 Latin square balanced for order and carry over. A MANOVA did not show any statistically significant effects either for the independent variables acceleration, velocity, and direction of motion of the sled or for their interactions. The results obtained are in clear contrast to those obtained under rotatory stimulation. We conclude that the otolithic contribution to vestibular-visual motion processing is negligible.

Acceleration↗

PubMed1999

Humans use internal models to estimate gravity and linear acceleration.

Because sensory systems often provide ambiguous information, neural processes must exist to resolve these ambiguities. It is likely that similar neural processes are used by different sensory systems. For example, many tasks require neural processing to distinguish linear acceleration from gravity, but Einstein's equivalence principle states that all linear accelerometers must measure both linear acceleration and gravity. Here we investigate whether the brain uses internal models, defined as neural systems that mimic physical principles, to help estimate linear acceleration and gravity. Internal models may be used in motor contro, sensorimotor integration and sensory processing, but direct experimental evidence for such models is limited. To determine how humans process ambiguous gravity and linear acceleration cues, subjects were tilted after being rotated at a constant velocity about an Earth-vertical axis. We show that the eye movements evoked by this post-rotational tilt include a response component that compensates for the estimated linear acceleration even when no actual linear acceleration occurs. These measured responses are consistent with our internal model predictions that the nervous system can develop a non-zero estimate of linear acceleration even when no true linear acceleration is present.

Acceleration↗