Have you ever felt one of the following?
· Queasiness after a car ride or roller coaster
· Trepidation while hiking in the dark
· Dizziness when you stand after prolonged sitting
· The surprise of a misstep
All these experiences have something in common: sensory information changed compared to your 'normal'. The result of each of these experiences might be described as DISORIENTING.
Disorientation is defined as the condition of having lost one's sense of direction. We orient ourselves through our relationship with gravity; with our ability to sense it, through graviception.
Graviception is the sensation of gravitational “force.”
It involves sensory integration of multiple inputs from different systems: visual, vestibular, and somatosensory. This results in perception of body position, an awareness of what is upright, and provides a sense of equilibrium.
Awareness through sensing
Graviception is different than balance and postural control, but it contributes to both. It is commonly assessed in the literature by the Subjective Visual Vertical test. (1)
While this may seem straightforward, there are a few misconceptions when it comes to sensing gravity.
To better understand all the components of graviception we need to dig deeper – deeper into the body.
The vestibular system may not always be the
main driver of graviception
The vestibular system seems to get most of the credit for sensing gravity. However, the literature on graviception reveals it’s more complex and differentiates between vestibular graviception and extra-vestibular (ie, outside of the vestibular system) graviception.
Where outside? In the ventral cavity.
A few studies have shown that graviceptors in the viscera, referred to as somatic graviceptors, play a role in sensing gravity.
Intriguing right? Mittelstadt thought so. He studied paraplegic patients with their kidneys removed and found graviception can occur independent of mechanoreception in the skin, legs, and spine. He identified sensory inputs from two areas: the region of the kidneys and the cardiovascular system. (2,3)
In a different study Trousselard et al found that the stomach, specifically the mechanoreceptors in the fundus, contribute to graviception. (4)
Additionally, research by Vaitl et al revealed that blood distribution influenced by gravity in different positions (standing, sitting or recumbent) serves as a cue for graviception. (5)
synchronized sensing perhaps
Gravity therefore impacts interoception via shifts in body fluids.
Interoception is the sense of the internal state of our body. It evolves from the integration of multiple visceral inputs to signal our body’s physiological state and "sense of self". (6)
Perceiving gravity depends on more than
inputs from graviceptors.
Graviceptors refer to the sensory receptors and systems that contribute to sensing the pull of gravity and our perception of upright.
Perceiving gravity depends on more than inputs from graviceptors in your otoliths and viscera. It also involves integration of these inputs with other senses in the thalamus and cerebellum AND includes predictions we make based on models in multiple levels of the cortex (7, 8)
While Jeff Hawkins doesn’t specifically address graviception in his book A Thousand Brains: A New Theory on Intelligence,his ideas on how we learn and build models through movement mesh well with this concept.
Hawkins proposes that our neocortex houses not a single model of the world but thousands. These models are created through sensory input and reference frames and learned through sensing and moving.
The ability to maintain a stable impression of the visual world during sustained movements of our eyes, head and body is called “orientation constancy”. This is driven in large part by our internal estimate of the direction of gravity. (9)
For example, we have an orientation model that has a prior setting in which the head is normally upright in space during the day. When lying down at night the model recognizes that the head and body are tilted 90° and sometimes rolled (sidelying).
We use this model to compensate for quick changes in the head and body positions. Things go awry when information from one of our graviceptors conflicts with one another - or our internal map of gravity.
While the cortical representation of gravity is required to determine our body's orientation in space and influence the way we move, it also does more.
Graviception results in more than
optimal movement
“Your brain is not for thinking.” says Lisa Feldman Barrett in Seven And A Half Lessons About The Brain
Rather your brain’s most important job “is to control your body- to manage allostasis- by predicting energy needs BEFORE they arise so you can efficiently make worthwhile movements and survive”.
Barrett refers to this as body budgeting
Just like financial budgeting we make estimates on what our physiological needs are. In both scenarios when our needs aren’t met, we feel stressed.
Graviception may act as a stress regulator. If we consider this to be true it provides a potential explanation for the calming effect for interventions like visceral manipulation and grounding, and behaviors like rocking.
"It is gravity that is the tool; it is gravity that is the therapist." ~ Dr. Ida Rolf.
Why should we care about the
ability to sense gravity?
Graviception applies to all of us. Your perception of it is relative and plastic.
Understanding graviception has widespread applications in both rehab and fitness realms.
Patients with vestibular dysfunction, pusher syndrome after a stroke, and individuals with scoliosis may be challenged with sensing gravity. (10, 11) Disorientation can present as motion sickness, dizziness or even persistent pain/discomfort related to increased muscular tone.
Disorientation may be a factor with individuals who don’t verbalize this complaint because what they feel is stressed.
The importance of graviception extends beyond the rehab world
Studies in sports performance demonstrate that experts in soccer and gymnastics are better at perceiving body orientation which in turn positively influences postural control and motor skills. (12, 13)
High level soccer players are better able to manage disturbances to proprioceptive and exteroceptive inputs compared to regional level players. This superior postural control may be the result of several factors, including a higher graviception sensitivity that has evolved through experience and exposure.
CONCLUSION
Gravity is described as a force of attraction that exists between all objects with mass.
We tend to take gravity for granted.
Perceptions derive from sensations but not all sensations result in perception. We adapt to inputs when they remain constant over prolonged periods of time. If our graviceptors are working well, we have less of a conscious felt experience.
It’s when inputs give conflicting information with our internal models, we feel “off”.
There are still unanswered questions about gravity. What we do know is how we move in our world depends on it. It is the foundation for how we learn through our senses and develop references frames to create models of our world and our own body schema.
Our evolved graviception is a big part of what makes us human. In the future this sense will continue to be challenged in new ways as we encounter more experiences like virtual reality and space travel where gravity conditions and available inputs change. (14)
We will continue to be challenged to #levelup
REFERENCES
1. Chetana N, Jayesh R. Subjective Visual Vertical in Various Vestibular Disorders by Using a Simple Bucket Test. Indian J Otolaryngol Head Neck Surg. 2015;67(2):180-184. doi:10.1007/s12070-014-0760-0
2. Mittelstaedt H. Evidence of somatic graviception from new and classical investigations. Acta Otolaryngol Suppl. 1995;520 Pt 1:186-7. doi: 10.3109/00016489509125224. PMID: 8749115.
3. Mittelstaedt, Horst. “Interaction of eye-, head-, and trunk-bound information in spatial perception and control.” Journal of vestibular research : equilibrium & orientation 7 4 (1997): 283-302 .
4. Trousselard M, Barraud PA, Nougier V, Raphel C, Cian C. Contribution of tactile and interoceptive cues to the perception of the direction of gravity. Brain Res Cogn Brain Res. 2004 Aug;20(3):355-62. doi: 10.1016/j.cogbrainres.2004.03.008. PMID: 15268913.
5. Dieter Vaitl, Horst Mittelstaedt, Ralf Saborowski, Rudolf Stark, Friedhelm Baisch,
Shifts in blood volume alter the perception of posture: further evidence for somatic graviception, International Journal of Psychophysiology, Volume 44, Issue 1, 2002,
Pages 1-11.
6. Quigley KS, Kanoski S, Grill WM, Barrett LF, Tsakiris M. Functions of Interoception: From Energy Regulation to Experience of the Self. Trends Neurosci. 2021;44(1):29-38.
7. Paul R. MacNeilage, Stefan Glasauer, Gravity Perception: The Role of the Cerebellum, Current Biology, Volume 28, Issue 22, 2018, Pages R1296-R1298,
8. Delle Monache S, Indovina I, Zago M, Daprati E, Lacquaniti F, Bosco G. Watching the Effects of Gravity. Vestibular Cortex and the Neural Representation of "Visual" Gravity. Front Integr Neurosci. 2021 Dec 1;15:793634.
9. Julien Barra, Adélaïde Marquer, Roxane Joassin, Céline Reymond, Liliane Metge, Valérie Chauvineau, Dominic Pérennou, Humans use internal models to construct and update a sense of verticality, Brain, Volume 133, Issue 12, December 2010, Pages 3552–3563
10. Morgane Le Berre, Charles Pradeau, Anthony Brouillard, Monique Coget, Caroline Massot, Jean-François Catanzariti, Do Adolescents With Idiopathic Scoliosis Have an Erroneous Perception of the Gravitational Vertical?, Spine Deformity, Volume 7, Issue 1, 2019
11. H.-O. Karnath, S. Ferber, J. Dichgans ,The origin of contraversive pushing: Evidence for a second graviceptive system in humans, Neurology Nov 2000, 55 (9) 1298-1304; DOI:10.1212/WNL.55.9.1298
12. Paillard T, Bizid R, Dupui P. Do sensorial manipulations affect subjects differently depending on their postural abilities?. Br J Sports Med. 2007;41(7):435-438. doi:10.1136/bjsm.2006.032904
13. Bringoux L, Marin L, Nougier V, Barraud PA, Raphel C. Effects of gymnastics expertise on the perception of body orientation in the pitch dimension. J Vestib Res. 2000;10(6):251-8.
14. Roy-O’Reilly, M., Mulavara, A. & Williams, T. A review of alterations to the brain during spaceflight and the potential relevance to crew in long-duration space exploration. npj Microgravity 7, 5 (2021)