Understanding the role of starch sheath layer in graviception of Alternanthera philoxeroides: a biophysical and microscopical study.

Plants' ability to sense and respond to gravity is a unique and fundamental process. When a plant organ is tilted, it adjusts its growth orientation relative to gravity direction, which is achieved by a curvature of the organ. In higher, multicellular plants, it is thought that the relative directional change of gravity is detected by starch-filled organelles that occur inside specialized cells called statocytes, and this is followed by signal conversion from physical information to physiological information within the statocytes. The classic starch statolith hypothesis, i.e., the starch accumulating amyloplasts movement along the gravity vector within gravity-sensing cells (statocytes) is the probable trigger of subsequent intracellular signaling, is widely accepted. Acharya Jagadish Chandra Bose through his pioneering research had investigated whether the fundamental reaction of geocurvature is contractile or expansive and whether the geo-sensing cells are diffusedly distributed in the organ or are present in the form of a definite layer. In this backdrop, a microscopy based experimental study was undertaken to understand the distribution pattern of the gravisensing layer, along the length (node-node) of the model plant Alternanthera philoxeroides and to study the microrheological property of the mobile starch-filled statocytes following inclination-induced graviception in the stem of the model plant. The study indicated a prominent difference in the pattern of distribution of the gravisensing layer along the length of the model plant. The study also indicated that upon changing the orientation of the plant from vertical position to horizontal position there was a characteristic change in orientation of the mobile starch granules within the statocytes. In the present study for the analysis of the microscopic images of the stem tissue cross sections, a specialized and modified microscopic illumination setup was developed in the laboratory in order to enhance the resolution and contrast of the starch granules.

Motor imagery helps updating internal models during microgravity exposure.

Skilled movements result from a mixture of feedforward and feedback mechanisms conceptualized by internal models. These mechanisms subserve both motor execution and motor imagery. Current research suggests that imagery allows updating feedforward mechanisms, leading to better performance in familiar contexts. Does this still hold in radically new contexts? Here, we test this ability by asking participants to imagine swinging arm movements around shoulder in normal gravity condition and in microgravity in which studies showed that movements slow down. We timed several cycles of actual and imagined arm pendular movements in three groups of subjects during parabolic flight campaign. The first, control, group remained on the ground. The second group was exposed to microgravity but did not imagine movements inflight. The third group was exposed to microgravity and imagined movements inflight. All groups performed and imagined the movements before and after the flight. We predicted that a mere exposure to microgravity would induce changes in imagined movement duration. We found this held true for the group who imagined the movements, suggesting an update of internal representations of gravity. However, we did not find a similar effect in the group exposed to microgravity despite the fact that the participants lived the same gravitational variations as the first group. Overall, these results suggest that motor imagery contributes to update internal representations of the considered movement in unfamiliar environments, while a mere exposure proved to be insufficient.NEW & NOTEWORTHY Gravity strongly affects the way movements are performed. How internal models process this information to adapt behavior to novel contexts is still unknown. The microgravity environment itself does not provide enough information to optimally adjust the period of natural arm swinging movements to microgravity. However, motor imagery of the task while immersed in microgravity was sufficient to update internal models. These results show that actually executing a task is not necessary to update graviception.

The people behind the papers - Qiang Zhu, Marçal Gallemí and Eva Benková.

The apical hook is a transient structure that functions to protect the vulnerable apical meristem from damage when the seedling penetrates the soil. Although some of the molecular players regulating its development have been identified, many aspects have remained opaque, including how an early auxin asymmetry in the hypocotyl is established. A paper in Development now provides a link between hormone signalling and the gravitropic response of the seedling's growing root in apical hook development. We caught up with co-first authors Qiang Zhu and Marçal Gallemí and their supervisor Eva Benková, Professor at the Institute of Science and Technology Austria in Klosterneuberg, to find out more about the project.

Root gravity response module guides differential growth determining both root bending and apical hook formation in Arabidopsis.

The apical hook is a transiently formed structure that plays a protective role when the germinating seedling penetrates through the soil towards the surface. Crucial for proper bending is the local auxin maxima, which defines the concave (inner) side of the hook curvature. As no sign of asymmetric auxin distribution has been reported in embryonic hypocotyls prior to hook formation, the question of how auxin asymmetry is established in the early phases of seedling germination remains largely unanswered. Here, we analyzed the auxin distribution and expression of PIN auxin efflux carriers from early phases of germination, and show that bending of the root in response to gravity is the crucial initial cue that governs the hypocotyl bending required for apical hook formation. Importantly, polar auxin transport machinery is established gradually after germination starts as a result of tight root-hypocotyl interaction and a proper balance between abscisic acid and gibberellins.This article has an associated 'The people behind the papers' interview.

Actual and Imagined Movements Reveal a Dual Role of the Insular Cortex for Motor Control.

Movements rely on a mixture of feedforward and feedback mechanisms. With experience, the brain builds internal representations of actions in different contexts. Many factors are taken into account in this process among which is the immutable presence of gravity. Any displacement of a massive body in the gravitational field generates forces and torques that must be predicted and compensated by appropriate motor commands. The insular cortex is a key brain area for graviception. However, no attempt has been made to address whether the same internal representation of gravity is shared between feedforward and feedback mechanisms. Here, participants either mentally simulated (only feedforward) or performed (feedforward and feedback) vertical movements of the hand. We found that the posterior part of the insular cortex was engaged when feedback was processed. The anterior insula, however, was activated only in mental simulation of the action. A psychophysical experiment demonstrates participants' ability to integrate the effects of gravity. Our results point toward a dual internal representation of gravity within the insula. We discuss the conceptual link between these two dualities.

Gravisensors in plant cells behave like an active granular liquid.

Plants are able to sense and respond to minute tilt from the vertical direction of the gravity, which is key to maintain their upright posture during development. However, gravisensing in plants relies on a peculiar sensor made of microsize starch-filled grains (statoliths) that sediment and form tiny granular piles at the bottom of the cell. How such a sensor can detect inclination is unclear, as granular materials like sand are known to display flow threshold and finite avalanche angle due to friction and interparticle jamming. Here, we address this issue by combining direct visualization of statolith avalanches in plant cells and experiments in biomimetic cells made of microfluidic cavities filled with a suspension of heavy Brownian particles. We show that, despite their granular nature, statoliths move and respond to the weakest angle, as a liquid clinometer would do. Comparison between the biological and biomimetic systems reveals that this liquid-like behavior comes from the cell activity, which agitates statoliths with an apparent temperature one order of magnitude larger than actual temperature. Our results shed light on the key role of active fluctuations of statoliths for explaining the remarkable sensitivity of plants to inclination. Our study also provides support to a recent scenario of gravity perception in plants, by bridging the active granular rheology of statoliths at the microscopic level to the macroscopic gravitropic response of the plant.

The gravitational imprint on sensorimotor planning and control.

Humans excel at learning complex tasks, and elite performers such as musicians or athletes develop motor skills that defy biomechanical constraints. All actions require the movement of massive bodies. Of particular interest in the process of sensorimotor learning and control is the impact of gravitational forces on the body. Indeed, efficient control and accurate internal representations of the body configuration in space depend on our ability to feel and anticipate the action of gravity. Here we review studies on perception and sensorimotor control in both normal and altered gravity. Behavioral and modeling studies together suggested that the nervous system develops efficient strategies to take advantage of gravitational forces across a wide variety of tasks. However, when the body was exposed to altered gravity, the rate and amount of adaptation exhibited substantial variation from one experiment to another and sometimes led to partial adjustment only. Overall, these results support the hypothesis that the brain uses a multimodal and flexible representation of the effect of gravity on our body and movements. Future work is necessary to better characterize the nature of this internal representation and the extent to which it can adapt to novel contexts.

Flying dreams stimulated by an immersive virtual reality task.

Despite a high prevalence and broad interest in flying dreams, these exceptional experiences remain infrequent. Our study aimed to (1) induce flying dreams using a custom-built virtual reality (VR) flying task, (2) examine their phenomenological correlates and (3) investigate their relations to participant state and trait factors. 137 participants underwent VR-flying followed by a morning nap. They also completed home dream journals for 5 days before and 10 days after the VR exposure. VR-flying successfully increased the reporting of flying dreams during the laboratory nap and on the following morning compared to both baseline frequencies and a control cohort. Flying dreams were also changed qualitatively, exhibiting higher levels of Lucid-control and emotional intensity, after VR exposure. Factors such as prior dream-flying experiences and level of VR sensory immersion modulated flying dream induction. Findings are consistent with a new vection-based explanation of dream-flying and may facilitate development of dream flight-induction technologies.

Development and regeneration of vestibular hair cells in mammals.

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.

Angular vestibuloocular reflex responses in Otop1 mice. I. Otolith sensor input is essential for gravity context-specific adaptation.

The role of the otoliths in mammals in the angular vestibuloocular reflex (VOR) has been difficult to determine because there is no surgical technique that can reliably ablate them without damaging the semicircular canals. The Otopetrin1 (Otop1) mouse lacks functioning otoliths because of failure to develop otoconia but seems to have otherwise normal peripheral anatomy and neural circuitry. By using these animals we sought to determine the role of the otoliths in angular VOR baseline function and adaptation. In six Otop1 mice and six control littermates we measured baseline ocular countertilt about the three primary axes in head coordinates; baseline horizontal (rotation about an Earth-vertical axis parallel to the dorsal-ventral axis) and vertical (rotation about an Earth-vertical axis parallel to the interaural axis) sinusoidal (0.2-10 Hz, 20-100°/s) VOR gain (= eye/head velocity); and the horizontal and vertical VOR after gain-increase (1.5×) and gain-decrease (0.5×) adaptation training. Countertilt responses were significantly reduced in Otop1 mice. Baseline horizontal and vertical VOR gains were similar between mouse types, and so was horizontal VOR adaptation. For control mice, vertical VOR adaptation was evident when the testing context, left ear down (LED) or right ear down (RED), was the same as the training context (LED or RED). For Otop1 mice, VOR adaptation was evident regardless of context. Our results suggest that the otolith translational signal does not contribute to the baseline angular VOR, probably because the mouse VOR is highly compensatory, and does not alter the magnitude of adaptation. However, we show that the otoliths are important for gravity context-specific angular VOR adaptation. NEW & NOTEWORTHY This is the first study examining the role of the otoliths (defined here as the utricle and saccule) in adaptation of the angular vestibuloocular reflex (VOR) in an animal model in which the otoliths are reliably inactivated and the semicircular canals preserved. We show that they do not contribute to adaptation of the normal angular VOR. However, the otoliths provide the main cue for gravity context-specific VOR adaptation.

Time course of the subjective visual vertical during sustained optokinetic and galvanic vestibular stimulation.

The brain is thought to use rotation cues from both the vestibular and optokinetic system to disambiguate the gravito-inertial force, as measured by the otoliths, into components of linear acceleration and gravity direction relative to the head. Hence, when the head is stationary and upright, an erroneous percept of tilt arises during optokinetic roll stimulation (OKS) or when an artificial canal-like signal is delivered by means of galvanic vestibular stimulation (GVS). It is still unknown how this percept is affected by the combined presence of both cues or how it develops over time. Here, we measured the time course of the subjective visual vertical (SVV), as a proxy of perceived head tilt, in human participants (n = 16) exposed to constant-current GVS (1 and 2 mA, cathodal and anodal) and constant-velocity OKS (30°/s clockwise and counterclockwise) or their combination. In each trial, participants continuously adjusted the orientation of a visual line, which drifted randomly, to Earth vertical. We found that both GVS and OKS evoke an exponential time course of the SVV. These time courses have different amplitudes and different time constants, 4 and 7 s respectively, and combine linearly when the two stimulations are presented together. We discuss these results in the framework of observer theory and Bayesian state estimation.NEW & NOTEWORTHY While it is known that both roll optokinetic stimuli and galvanic vestibular stimulation affect the percept of vertical, how their effects combine and develop over time is still unclear. Here we show that both effects combined linearly but are characterized by different time constants, which we discuss from a probabilistic perspective.

Bipolar mood cycles and lunar tidal cycles.

In 17 patients with rapid cycling bipolar disorder, time-series analyses detected synchronies between mood cycles and three lunar cycles that modulate the amplitude of the moon's semi-diurnal gravimetric tides: the 14.8-day spring-neap cycle, the 13.7-day declination cycle and the 206-day cycle of perigee-syzygies ('supermoons'). The analyses also revealed shifts among 1:2, 1:3, 2:3 and other modes of coupling of mood cycles to the two bi-weekly lunar cycles. These shifts appear to be responses to the conflicting demands of the mood cycles' being entrained simultaneously to two different bi-weekly lunar cycles with slightly different periods. Measurements of circadian rhythms in body temperature suggest a biological mechanism through which transits of one of the moon's semi-diurnal gravimetric tides might have driven the patients' bipolar cycles, by periodically entraining the circadian pacemaker to its 24.84-h rhythm and altering the pacemaker's phase-relationship to sleep in a manner that is known to cause switches from depression to mania.

Haltere removal alters responses to gravity in standing flies.

Animals detect the force of gravity with multiple sensory organs, from subcutaneous receptors at body joints to specialized sensors like the vertebrate inner ear. The halteres of flies, specialized mechanoreceptive organs derived from hindwings, are known to detect body rotations during flight, and some groups of flies also oscillate their halteres while walking. The dynamics of halteres are such that they could act as gravity detectors for flies standing on substrates, but their utility during non-flight behaviors is not known. We observed the behaviors of intact and haltere-ablated flies during walking and during perturbations in which the acceleration due to gravity suddenly changed. We found that intact halteres are necessary for flies to maintain normal walking speeds on vertical surfaces and to respond to sudden changes in gravity. Our results suggest that halteres can serve multiple sensory purposes during different behaviors, expanding their role beyond their canonical use in flight.

Development of a Computerized Adaptive Test of Children's Gross Motor Skills.

OBJECTIVES: To (1) develop a computerized adaptive test for gross motor skills (GM-CAT) as a diagnostic test and an outcome measure, using the gross motor skills subscale of the Comprehensive Developmental Inventory for Infants and Toddlers (CDIIT-GM) as the candidate item bank; and (2) examine the psychometric properties and the efficiency of the GM-CAT. DESIGN: Retrospective study. SETTING: A developmental center of a medical center. PARTICIPANTS: Children with and without developmental delay (N=1738). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The CDIIT-GM contains 56 universal items on gross motor skills assessing children's antigravity control, locomotion, and body movement coordination. RESULTS: The item bank of the GM-CAT had 44 items that met the dichotomous Rasch model's assumptions. High Rasch person reliabilities were found for each estimated gross motor skill for the GM-CAT (Rasch person reliabilities =.940-.995, SE=.68-2.43). For children aged 6 to 71 months, the GM-CAT had good concurrent validity (r values =.97-.98), adequate to excellent diagnostic accuracy (area under receiver operating characteristics curve =.80-.98), and moderate to large responsiveness (effect size =.65-5.82). The averages of items administered for the GM-CAT were 7 to 11, depending on the age group. CONCLUSIONS: The results of this study support the use of the GM-CAT as a diagnostic and outcome measure to estimate children's gross motor skills in both research and clinical settings.

Does gravity influence the visual line bisection task?

The visual line bisection task (LBT) is sensitive to perceptual biases of visuospatial attention, showing slight leftward (for horizontal lines) and upward (for vertical lines) errors in healthy subjects. It may be solved in an egocentric or allocentric reference frame, and there is no obvious need for graviceptive input. However, for other visual line adjustments, such as the subjective visual vertical, otolith input is integrated. We hypothesized that graviceptive input is incorporated when performing the LBT and predicted reduced accuracy and precision when roll-tilted. Twenty healthy right-handed subjects repetitively bisected Earth-horizontal and body-horizontal lines in darkness. Recordings were obtained before, during, and after roll-tilt (±45°, ±90°) for 5 min each. Additionally, bisections of Earth-vertical and oblique lines were obtained in 17 subjects. When roll-tilted ±90° ear-down, bisections of Earth-horizontal (i.e., body-vertical) lines were shifted toward the direction of the head (P < 0.001). However, after correction for vertical line-bisection errors when upright, shifts disappeared. Bisecting body-horizontal lines while roll-tilted did not cause any shifts. The precision of Earth-horizontal line bisections decreased (P ≤ 0.006) when roll-tilted, while no such changes were observed for body-horizontal lines. Regardless of the trial condition and paradigm, the scanning direction of the bisecting cursor (leftward vs. rightward) significantly (P ≤ 0.021) affected line bisections. Our findings reject our hypothesis and suggest that gravity does not modulate the LBT. Roll-tilt-dependent shifts are instead explained by the headward bias when bisecting lines oriented along a body-vertical axis. Increased variability when roll-tilted likely reflects larger variability when bisecting body-vertical than body-horizontal lines.

Human perceptual overestimation of whole body roll tilt in hypergravity.

Hypergravity provides a unique environment to study human perception of orientation. We utilized a long-radius centrifuge to study perception of both static and dynamic whole body roll tilt in hypergravity, across a range of angles, frequencies, and net gravito-inertial levels (referred to as G levels). While studies of static tilt perception in hypergravity have been published, this is the first to measure dynamic tilt perception (i.e., with time-varying canal stimulation) in hypergravity using a continuous matching task. In complete darkness, subjects reported their orientation perception using a haptic task, whereby they attempted to align a hand-held bar with their perceived horizontal. Static roll tilt was overestimated in hypergravity, with more overestimation at larger angles and higher G levels, across the conditions tested (overestimated by ∼35% per additional G level, P < 0.001). As our primary contribution, we show that dynamic roll tilt was also consistently overestimated in hypergravity (P < 0.001) at all angles and frequencies tested, again with more overestimation at higher G levels. The overestimation was similar to that for static tilts at low angular velocities but decreased at higher angular velocities (P = 0.006), consistent with semicircular canal sensory integration. To match our findings, we propose a modification to a previous Observer-type canal-otolith interaction model. Specifically, our data were better modeled by including the hypothesis that the central nervous system treats otolith stimulation in the utricular plane differently than stimulation out of the utricular plane. This modified model was able to simulate quantitatively both the static and the dynamic roll tilt overestimation in hypergravity measured experimentally.

Body height and arterial pressure in seated and supine young males during +2 G centrifugation.

It is known that arterial pressure correlates positively with body height in males, and it has been suggested that this is due to the increasing vertical hydrostatic gradient from the heart to the carotid baroreceptors. Therefore, we tested the hypothesis that a higher gravito-inertial stress induced by the use of a human centrifuge would increase mean arterial pressure (MAP) more in tall than in short males in the seated position. In short (162-171 cm; n = 8) and tall (194-203 cm; n = 10) healthy males (18-41 yr), brachial arterial pressure, heart rate (HR), and cardiac output were measured during +2G centrifugation, while they were seated upright with the legs kept horizontal (+2Gz). In a separate experiment, the same measurements were done with the subjects supine (+2Gx). During +2Gz MAP increased in the short (22 ± 2 mmHg, P < 0.0001) and tall (23 ± 2 mmHg, P < 0.0001) males, with no significant difference between the groups. HR increased more (P < 0.05) in the tall than in the short group (14 ± 2 vs. 7 ± 2 bpm). Stroke volume (SV) decreased in the short group (26 ± 4 ml, P = 0.001) and more so in the tall group (39 ± 5 ml, P < 0.0001; short vs. tall, P = 0.047). During +2Gx, systolic arterial pressure increased (P < 0.001) and SV (P = 0.012) decreased in the tall group only. In conclusion, during +2Gz, MAP increased in both short and tall males, with no difference between the groups. However, in the tall group, HR increased more during +2Gz, which could be caused by a larger hydrostatic pressure gradient from heart to head, leading to greater inhibition of the carotid baroreceptors.

Neural response in vestibular organ of Helix aspersa to centrifugation and re-adaptation to normal gravity.

Gravity plays a key role in shaping the vestibular sensitivity (VS) of terrestrial organisms. We studied VS changes in the statocyst of the gastropod Helix aspersa immediately after 4-, 16-, and 32-day exposures to a 1.4G hypergravic field or following a 7-day recovery period. In the same animals we measured latencies of behavioral "negative gravitaxis" responses to a head-down pitch before and after centrifugation and found significant delays after 16- and 32-day runs. In an isolated neural preparation we recorded the electrophysiological responses of the statocyst nerve to static tilt (±19°) and sinusoids (±12°; 0.1 Hz). Spike sorting software was used to separate individual sensory cells' patterns out of a common trace. In correspondence with behavior we observed a VS decrease in animals after 16- (p < 0.05) and 32-day (p < 0.01) centrifugations. These findings reveal the capability of statoreceptors to adjust their sensitivity in response to a prolonged change in the force of gravity. Interestingly, background discharge rate increased after 16 and 32 days in hypergravity and continued to rise through the recovery period. This result indicates that adaptive mechanisms to novel gravity levels were long lasting, and re-adaptation from hypergravity is a more complex process than just "return to normal".

The neural basis of Drosophila gravity-sensing and hearing.

The neural substrates that the fruitfly Drosophila uses to sense smell, taste and light share marked structural and functional similarities with ours, providing attractive models to dissect sensory stimulus processing. Here we focus on two of the remaining and less understood prime sensory modalities: graviception and hearing. We show that the fly has implemented both sensory modalities into a single system, Johnston's organ, which houses specialized clusters of mechanosensory neurons, each of which monitors specific movements of the antenna. Gravity- and sound-sensitive neurons differ in their response characteristics, and only the latter express the candidate mechanotransducer channel NompC. The two neural subsets also differ in their central projections, feeding into neural pathways that are reminiscent of the vestibular and auditory pathways in our brain. By establishing the Drosophila counterparts of these sensory systems, our findings provide the basis for a systematic functional and molecular dissection of how different mechanosensory stimuli are detected and processed.