responses during short-radius centrifugation...

Journal of Vestibular Research 24 (2014) 1–8 1DOI 10.3233/VES-130504IOS Press

Perception of verticality and cardiovascularresponses during short-radius centrifugation

Gilles Clémenta,∗, Quentin Delièreb and Pierre-François MigeottebaInternational Space University, Strasbourg, FrancebResearch Unit Vital Signs and Performance Monitoring, Royal Military Academy, Brussels, Belgium

Received 15 June 2013

Accepted 1 November 2013

Abstract.BACKGROUND: Artificial gravity using short-radius centrifugation has been proposed as an integrative countermeasure duringspaceflight.OBJECTIVE: To determine the rotation parameters of a short-radius centrifuge so that subjects rotating in the dark would feelas if they were standing upright.METHODS: Twelve subjects were lying supine in a nacelle on a 2.8 m-radius centrifuge with their head closer to the axis ofrotation and their feet pointing radially outwards. Subjects verbally reported body orientation for 26 combinations of centrifugerotation rate and nacelle pitch tilt. ECG and respiratory responses were also recorded.RESULTS: Five subjects felt like they were vertical when centrifugation elicited 1 g at their center of mass along their bodylongitudinal axis, whereas seven subjects felt they were vertical when they experienced about 1 g at ear level, regardless of thenacelle tilt angle. Heart rate variability varied with the subjects’ perception of verticality.CONCLUSIONS: These results suggest that one group of subject was relying principally on the otolith organs for the perceptionof verticality, whereas the other group was also relying on extravestibular somatosensory receptors. The crewmember’s perceptionof verticality might be a factor to take into account for the prescription for artificial gravity during spaceflight.

Keywords: Subjective vertical, otoliths, vestibular-autonomic reflex, artificial gravity

1. Introduction

Artificial gravity generated by centrifugation dur-ing spaceflight has been proposed as a multi-purposecountermeasure against bone, muscle, and cardiovas-cular deconditioning resulting from prolonged expo-sure to weightlessness. Continuous rotation of the en-tire spacecraft has considerable engineering, architec-tural, and financial implications. An alternative ap-proach being explored is to provide astronauts witha small spinning bed [26]. They would lie on their

∗Corresponding author: G. Clément, International Space Uni-versity, Parc d’Innovation, 1 rue Jean-Dominique Cassini, F-67400Illkirch-Graffenstaden, France. Tel.: +33 388 65 54 44; Fax: +33 38865 54 47; E-mail: [email protected].

back with their head near the center of rotation andtheir feet pointing radially outward (Fig. 1). Thus theirlower body could be loaded for a specific period oftime each day in approximately the same way as undernormal Earth gravity. While not expected to be as effi-cient a solution from a physiological standpoint, giventhe acceleration gradient effects and intermittent expo-sure, short-radius centrifugation may prove effective.The engineering costs and design risks would certainlybe lower as compared to designing a rotating space-craft [2].

There are differences and similarities between beingcentrifuged on Earth and in orbit. On Earth, the grav-itational force adds to the centrifugal force vectoriallyand produces a net specific gravito-inertial force thatis tilted relative to the horizontal (Fig. 1). In weight-

ISSN 0957-4271/14/$27.50 c© 2014 – IOS Press and the authors. All rights reserved

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lessness only the centrifugal force is present. However,both on Earth and in weightlessness, the centrifugalforce increases with the distance from the axis of rota-tion, so there is a gravity gradient throughout the longi-tudinal body axis. The primary objective of this studywas to evaluate the effects of this gravity gradient onthe perception of verticality in subjects exposed to cen-trifugation on a short-radius centrifuge.

In absence of visual cues, the neurovestibular sys-tem, through the otolith organs and the somatosensoryreceptors, plays a major role in the subjective verti-cal. The otolith organs also play a role in the activa-tion of the sympathetic nervous system in response tochanges in posture [10,18,25]. Previous studies havedemonstrated that approximately 64% of Space Shut-tle astronauts experienced profound orthostatic intol-erance when standing after landing, possibly due toinadequate sympathetic activation after adaptation toweightlessness [1,6,24]. This is a source of concern forintermittent centrifugation in orbit, which will mimictransient return in Earth gravity, and for adequate im-mediate post-flight functioning in a gravitational envi-ronment.

It is reasonable to believe that, to be the most effi-cient in mimicking Earth gravity, centrifugation shouldgive the sensation to the subjects that they feel verti-cal like on Earth. Recent studies have shown that or-thostatic intolerance could occur when subjects had thevisual illusion of standing upright from a supine posi-tion, without actual changes in body tilt [22]. The sec-ondary objective of this study was therefore to evaluatewhether the mode of cardiac control during centrifuga-tion was also influenced by the subject’s perception ofverticality.

2. Material and methods

2.1. Subjects

Twelve healthy male subjects participated in thisstudy. On average, they were 33 years old (range26–42), 1.76-m tall (range 1.67–1.83), and weighed73.5 kg (range 57.5–87). All participants received acomprehensive clinical assessment and gave their in-formed consent prior to the beginning this study. Theexperiment took place in the MEDES Flight Clinic atthe CHU of Rangueil in Toulouse, France using the Eu-ropean Space Agency Short-Arm Human Centrifuge(SAHC). The experimental protocol had received theagreement from the local ethical committee.

2.2. Experiment protocol

Subjects were lying supine in a nacelle on a 2.8 m-radius centrifuge with their head closer to the axis ofrotation and their feet pointing radially outwards. Acanopy was placed over the upper part of the subject’sbody to ensure that the subject had no visual cue or anymoving reference. A headset was used by the subject tocommunicate with the operators outside the centrifugeroom and to suppress any auditory orientation cues. Afive-point harness restrained the subject’s upper bodyin the nacelle. A custom headrest was used to preventhead movements.

During each run, the centrifuge was accelerated at3◦/s2 until constant velocity was achieved. The rota-tion rate of the centrifuge could be adjusted from 15to 40 rpm and the nacelle could be tilted in pitch from0◦ (horizontal) to 45◦ (head up) by increments of 5◦.After 1 minute of constant velocity, the subjects wereprompted by the operator to verbally report their ori-entation relative to horizontal. After about 3 min, thecentrifuge was then decelerated at 3◦/s2 until full stop.A rest period of 1 min separated two centrifuge runs.

The subjects estimated the orientation of their bodyrelative to horizontal in Earth-centric coordinates usinga clock scale. For example, when the subjects judgedtheir body as Earth-horizontal (0◦), the response was“15 min past the hour”; when the subjects judged theirbody as Earth-vertical (90◦), the response was “30 minpast the hour”, etc. Generally the responses were madewith an attempted precision of 0.5–1 min [21]. Inagreement with previous studies [8], subjects reporteda single tilt for the whole body. Occasional failures torespond prompted a dialog between the operator andthe subject. In only one instance was one subject un-able to respond (missing data < 1.5%).

The subjects experienced a practice run with thecentrifuge during the clinical assessment. Approxi-mately two weeks after this practice run, the experi-ment took place in the lab during two visits separatedby a few days. During the first visit, the subjects werefirst placed in a horizontal position (nacelle tilt = 0◦).The rotation rate ranged from 27.1 to 27.5 rpm (de-pending on subject’s height), so that Gz at their cen-ter of mass was 1 g. Verbal reports of body tilt, aswell as ECG and respiratory responses were recorded.Then, a series of 12 centrifuge runs followed, wherevarious combinations of rotation rate and nacelle tiltwere used. During the second visit, 12 other combina-tions were used. During these 24 centrifuge runs, sixnacelle tilt angles (0◦, 5◦, 10◦, 15◦, 30◦, 45◦) and four

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Fig. 1. When rotating at constant velocity (ω) on a short-radius centrifuge, subjects on Earth experience a gravito-inertial force (GIF), i.e., theresultant of the centrifugal force (Fc) and the gravitational force (Fg). The projection of GIF on the body longitudinal axis is Gz. The magnitudeof Gz depends on the rotation rate (ω), the tilt of the nacelle relative to the horizontal (α), and the distance to the axis of rotation. Gz is the largestat the feet, and the smallest at the ear level where the otoliths are located. In this example, ω = 31.8 rpm; α = 25◦; Gz at the subject’s center ofmass (COM) = 1.8 g; Gz (feet) = 2.7 g, Gz (ear) = 1.15 g; Gz gravity gradient = 57%.

rotation rates per angle were used. During all of thesecentrifuge runs, the subjects reported the orientation oftheir body relative to horizontal. In the last centrifugerun, the rotation rate and nacelle tilt were set so that allsubjects reported the perception of being vertical, andECG and respiratory movements were again recordedfor 4 minutes.

During a pilot study, a digital weight scale wasplaced under the feet of one of the subjects to measurehis body weight during centrifugation in the supine po-sition that generated 1 g at his center of mass. We foundthat the body weight measured during centrifugationand the body weight measured when standing uprighton the scale on the floor in the lab were different byless than 1%. This indicates that the shearing force be-tween the back of the subject and the nacelle was smallenough to be neglected in this study.

2.3. Data analysis

For each combination of centrifuge rotation rate andnacelle tilt, the centrifugal force directed along the sub-ject’s body longitudinal axis (Gz) was calculated at thesubject’s feet, center of mass (COM), and ear level(where the otolith organs are located). The Gz gravitygradient was also calculated by taking the differencebetween Gz at the feet and Gz at the ear, divided by Gzat the feet, multiplied by 100 (Fig. 1).

ECG and respiratory responses were continuouslyrecorded (1 kHz and 200 Hz, respectively) by theLifeShirt monitoring system from VivoMetrics Inc.(California, USA). The QRS waves were automaticallydetected in Matlab (v. 7.8, The MathWorks) using theSignal Processing Toolbox and the BioSig Toolbox.The results were visually inspected to avoid improper

detection. A time series of occurrences of R peaks wasconstructed, and the RR-intervals (RRI) were com-puted as the time differences between two consecutiveR peaks.

The respiratory sinus arrhythmia (RSA), i.e. thehigh-frequency (0.15–0.4 Hz) component of heart ratevariability, is a cardiorespiratory interaction mediatedby the vagus nerve. RSA is considered a marker ofthe parasympathetic modulation of heart rate [9]. RSAwas computed over every three breathing cycles, ac-cording to the method described by Migeotte and Ver-bandt [13]. A mean RRI and a mean RSA were thencalculated for each subject over the 4-min centrifuga-tion run (one run when the subjects were horizontalduring the first visit, and one run when all subjects feltbeing vertical during the second visit).

Linear regression, multivariate analyses of variance(ANOVA) and t-tests were performed using SPSS v.2.0. A p value of less than 0.05 was considered statis-tically significant. Data are presented as mean ± SD.

3. Results

3.1. Perception of verticality

Figure 2 shows the perception of tilt relative to hor-izontal reported by all 12 subjects during the 26 cen-trifuge runs they were exposed to. There was a spec-trum with inter-individual differences in the percep-tion of tilt for the various g levels. Five subjects (COMgroup) had the sensation to be vertical, i.e. reported a90◦ tilt, when they were supine and exposed to nearly1 g at their center of mass. The seven other subjects(OTO group) reported being tilted head-up by only 45◦

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Fig. 2. A. Verbal reports of tilt in all 12 subjects as a function of the g-load along the body longitudinal axis at their center of mass (Gz) duringvarious combinations of centrifuge rotation rate and nacelle tilt, with 0◦ equal to being horizontal and 90◦ equal to being vertical. B. Same dataaveraged for 0.1-g bins, with least square fits. Some subjects (COM) had the sensation of being tilted when Gz was as low as 0.5 g and horizontalwhen Gz was 1 g and higher. The other subjects (OTO) felt horizontal when Gz was closer to 2 g and higher.

Fig. 3. A. Magnitude of centrifugal force along the body longitudinal axis (Gz) at the center of mass and nacelle tilt angle for which the subjectsin the COM and the OTO groups had the sensation of being vertical. B. Magnitude of centrifugal force along the body longitudinal axis (Gz) atthe ear level and nacelle tilt angle for which the subjects in the COM and the OTO groups had the sensation of being vertical. C. Gravity gradientalong the body axis as a function of nacelle tilt angle for which the subjects in the COM and the OTO groups had the sensation of being vertical.

in average when they were exposed at 1 g at their cen-ter of mass. All the OTO subjects reported being verti-cal for 2 g at their center of mass. Simple linear regres-sions fit to the individual data indicated that the regres-sion coefficients were significantly different from zeroin both groups. Student t-tests on the coefficients in-dicated that the slope of the linear regression was sig-nificantly different between the two groups of subjects[p = 0.029].

The subjects in the COM group had the sensation ofbeing vertical, just like standing upright on Earth, forthe following rotation rates and nacelle tilts: 27.7 ± 1

rpm at 0◦, 27.2 ± 1.3 rpm at 5◦, 25.7 rpm ± 0.8 at10◦, 24.6 ± 2.4 rpm at 15◦, 21.3 ± 2.3 rpm at 30◦, and18.1 ± 2.1 rpm at 45◦. These values corresponded toa Gz acceleration of approximately 1.10 g ± 0.03 attheir COM, regardless of the nacelle tilt angle (Fig. 3A,COM).

The subjects in the OTO group had the sensation ofbeing vertical for the following rotation rates and na-celle tilts: 39.3 ± 4.3 rpm at 0◦, 38.0 ± 1.5 rpm at 5◦,37.1 ± 3.1 rpm at 10◦, 33.4 ± 3.1 rpm at 15◦, 30.5± 3.3 rpm at 30◦, and 20.9 ± 2.4 rpm at 45◦. Thesesvalues corresponded to a Gz acceleration of approxi-

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Fig. 4. Time series of changes in R-R Intervals (RRI) and Respiratory Sinus Arrhythmia (RSA) during centrifugation generating 1.3 g (Gz) atthe COM (ω = 27.8 rpm, α = 15◦) in one typical subject.

mately 2.14 ± 0.44 g at the COM when the nacellewas horizontal and 1.20 ± 0.22 g at the COM whenthe nacelle was tilted at 45◦ (Fig. 3A, OTO). The dif-ference in Gz at the COM between the two groups ofsubjects when they had the perception of being verticalwas significant [p = 0.004].

Interestingly, the subjects in the OTO group had thesensation of being vertical when the centrifuge rota-tion rate generated approximately 1.05 ± 0.08 g at theear level, regardless of the nacelle tilt angle (Fig. 3B).The difference in Gz at the ear level between the twogroups of subjects when they had the perception of be-ing vertical was also significant [p = 0.004].

The Gz gravity gradient ranged from 75–25% acrossall the conditions tested, and was not significantly dif-ferent between both subject groups (Fig. 3C).

3.2. Cardiovascular responses

Transient changes in RRI and RSA during centrifu-gation are shown in Fig. 4. As expected the RRI de-creases during centrifugation, i.e. the heart rate in-creases as a result of the increased hydrostatic pres-sure gradients. RSA also decreases under increased g-load. These changes in RRI and RSA are similar tothose observed in previous studies [12,14] and indi-

cate a reciprocally coupled sympathetic activation andparasympathetic inhibition mode of cardiac control,Decreases in RRI and RSA were transient, returning topre-centrifugation levels within 1 min (Fig. 4).

Multiple ANOVAs were performed with subjects’perception of body tilt (horizontal or vertical) and sub-ject group (COM or OTO) as independent variables,and physiological parameters (RRI and RSA) as de-pendent variables. There was a main effect of the per-ception of body tilt on RRI and RSA [F (1, 10) =33.772; p < 0.001; η2 = 77.2% and F (1, 10) =19.823; p < 0.001; η2 = 66.5%, respectively]. Whentesting the interaction between the subject group andthe perception of body tilt, the p value was superior to0.05 [F(1,10) = 04.411; p = 0.062]. However, becausethe magnitude of this effect was relatively high (η2 =30.6%) this interaction cannot be neglected. A largernumber of subjects should presumably give rise to asignificant interaction effect.

RRI and RSA were significantly smaller [t(6) =2.46, p = 0.004; and t(6) = 2.45, p = 0.007; re-spectively] for the subjects of the OTO group betweenthe centrifuge runs where they perceived being verti-cal (90◦) compared to those runs where they perceivedbeing tilted in average by 32.1◦ (SD 16.3◦) relative tothe horizontal (Fig. 5). However, RRI and RSA were

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Fig. 5. Mean ± SD of RRI (A) and RSA (B) in subjects of both the COM and OTO groups as a function of their reported mean body tilt relative tothe horizontal (± SD). The RRI and RSA values were averaged over the 4-min centrifugation runs during which the nacelle was either horizontal(mean Gz at COM = 1.02 g) or tilted (mean Gz at COM = 1.15 g or 1.58 g, for the COM and OTO groups respectively).

not significantly different for the subjects of the COMgroup between the centrifuge runs where they felt ver-tical with no nacelle tilt, and those runs where theyfelt vertical with the nacelle tilted. These results in-dicate that RRI and RSA depend on g level. In otherwords, cardiovascular response is stronger in the OTOgroup because stimulation of body graviceptors is alsostronger.

We were unable to demonstrate a statistically sig-nificant correlation between the degree of change incardiovascular responses and the individual measuresof tilt perception or the Gz value at their COM forthe entire group of 12 subjects. Indeed, although RRIand RSA decreased when the magnitude of Gz at theirCOM increased in the subjects in the OTO group, thiswas not the case for the COM group. Also, RRI andRSA were not different in both groups when the sub-jects perceived being vertical, although the Gz at theirCOM was significantly different (1.15 g ± 0.14 g inthe COM group, vs. 1.58 g ± 0.31 g in the OTO group;t(4) = 2.78, p = 0.03)

4. Discussion

Our results demonstrate that short-radius centrifuga-tion of subjects in the lying down position can elicitthe perception of standing upright and transient auto-nomic responses. These changes are caused by changesin otolith or other body graviceptors, or barorecep-tors that may contribute to cardio-respiratory control.These results are consistent with other data in humans

and animals suggesting that graviceptive inputs con-tribute to the cardiovascular response to a change inposture [25].

Although the main source of gravitational and in-ertial information is certainly expected to be thelabyrinth, somatic graviceptors are thought to existwithin the trunk as well [15]. Ground-based studies us-ing tilt tables and centrifuges with healthy individuals,as well as labyrinthectomized, paraplegic, or nephrec-tomized subjects have investigated these extravestibu-lar mechanoreceptors [16]. Their results demonstratedthat the influence of the proprioceptive receptors thatmeasure forces acting on the limbs and/or the skin onthe vertical subjective during tilt or centrifugation isminimal. On the other hand, they suggested the exis-tence of two potential somatic graviceptive systems lo-cated at the height of the last ribs, i.e., near the body’scenter of mass. One is a “vascular” graviceptive sys-tem, which would be activated by fluid shift duringpostural changes via afferent fibers from peripheralsensors (e.g. baroreceptors) and vagal withdrawal orsympathetic activation. The other is a “truncal” grav-iceptive system, which would take advantage of thedense liquid-filled kidneys structures and their pressurereceptors. The results obtained with the labyrinthec-tomized subjects indicate that the effects of these so-matic gravity receptors on the z-axis component of thepostural control system are, on average, equal to oreven larger than that of the otoliths [16]. Recent mod-els of human spatial orientation also postulate the exis-tence of an internal estimate of gravity that determinesthe perception of self-orientation with respect to grav-itational and inertial forces [3,11].

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In our study, the perception of verticality and therelated cardiovascular responses of 7 out of 12 sub-jects (58.3%) during centrifugation were based on theGz force measured at ear level. A nearly 1 g vectorat the ear level elicited the sensation of being verti-cal, regardless of the higher force levels on the otherbody parts and the gravity gradient. These subjectswere therefore strongly otolith-dependent. The otherfive subjects (41.7%) were more sensitive to extraves-tibular mechanoreceptors situated near their center ofmass, i.e. they were more dependent on the somaticgraviceptors or the internal estimate of gravity. Thesesubjects felt that they were vertical when centrifuga-tion elicited 1.10 g at the center of mass, which corre-sponds to nearly 1 g at the heart level.

A previous study by Vaitl et al. [19] indicated thatchanges in shifts of blood volume into or out of the tho-racic cavity induced by lower body positive pressure(LBPP) or lower body negative pressure (LBNP) ex-erted on the lower body also led subjects to feel tiltedhead-up or head-down, respectively. When LBPP andLBNP were combined with centrifugation, the percep-tion of verticality was influenced by both the otolithsand the fluid distribution in such a way that both inter-act in a compensatory manner [20]. It is reasonable toassume that the impact of the fluid distribution shouldbe greater during the runs in which the Gz gravity gra-dient is high. Our results showing the largest differencein perception of verticality between the COM and theOTO subject groups when the gravity gradient is 75%seems to support this assumption.

It is interesting to note that the cardiovascular re-sponses were not significantly different in both groupsof subjects when the first group (OTO) was exposed to1 g at the otolith and the second group (COM) to 1 g atthe heart. Also, the measured cardiovascular responsesto short-radius centrifugation causing 1–1.5 g at theheart are similar to standing, at least for a comparisonbased on centrifuge runs lasting 4 minutes. The gravitygradient for these low g levels seems not to interferewith tilt perception or cardiovascular responses. Theseresults are in agreement with previous studies and theyconfirm the efficacy of short-radius centrifugation as apossible countermeasure to the cardiovascular decon-ditioning that occurs during spaceflight [7,14].

One might expect to find that the degree of changein cardiovascular reflexes during centrifugation wouldcorrelate with individual measures of tilt perception.This was the case for the subjects within a group butnot between groups. For example, RRI and RSA werenot significantly different in the COM subjects who felt

being vertical and in the OTO subjects who felt tiltedby only 32.1◦ relative to the horizontal. The cardio-vascular responses in the OTO group, however, seemto depend on the g level, with stronger RRI and RSAresponses corresponding to a stronger stimulation ofthe graviceptors. The small number of subjects coupledwith the intrinsic variability in subjective and objectiveresponses could be the reason for the lack of correla-tion between the magnitude of tilt perception and tran-sitory cardiovascular responses across groups. It mayalso be possible that different pathways are used in theprocessing of acceleration information contributing totilt perception and control of cardiovascular orthosta-sis, just as vestibular perceptual responses are dissoci-ated from oculomotor function [3,4,17].

Our results are in agreement with those of Wood etal. [22] who elicited changes in the perception of ver-ticality in supine subjects using static visual cues anddemonstrated a visually mediated cardio-respiratoryresponse. Transient decreases in mean blood pressurewere measured in response to the illusory tilts. Theseresponses were very similar to those that these samesubjects manifested when exposed to an actual passivehead-up tilt. As in our study, a significant correlationbetween the magnitude of physiological and subjectivereports could not be demonstrated. The authors sug-gest that visual information contributing to tilt percep-tion may also be processed on different pathways thanthose subserving visual control of cardiovascular or-thostasis [22].

When one moves from the supine position to an up-right stance, there are a number of mechanisms thatserve to resist hypotension. As well, hypotension canbe elicited by natural or electrical vestibular stimu-lation. Compensation for orthostatic hypotension isalso impaired by vestibular lesions. Yates and Bron-stein [23] have proposed that the background excitabil-ity of many types of sympathetic preganglionic neu-rons may be regulated by central vestibular or periph-eral signals. Under conditions of vestibular pathologyor altered gravitational states, the role that is playedby graviceptors and perception of verticality in or-thostasis may become more critical. Centrifugationprovides the potential for future enhanced cardiovas-cular countermeasures to combat orthostatic intoler-ance in crewmembers returning from spaceflight [2,5,6]. Our data suggest that controlling for the subject’sperception of verticality during centrifugation testingis a wise course of action. The subjects’ position onthe centrifuge could be individually adjusted to allowfor the reproduction of the naturally perceived stand-

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up position in all the subjects and an optimum stimula-tion of the otolith-autonomic reflex for cardiovascularorthostasis.

Acknowledgements

The authors are grateful to Marie-Pierre Bareille andArnaud Beck (MEDES Toulouse, France) for their as-sistance with subjects’ selection and testing, and toAngie Bukley for editing the manuscript. This ex-periment was supported by grants from the EuropeanSpace Agency, the Centre National d’Etudes Spatiales,and the Belgian Federal Science Policy (PRODEX pro-gram).

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