We have all experienced it. You’re enjoying some scroll time in the car, the driver decides to play ‘red light, green light’, and suddenly, you’re nauseous. Why does this happen?
Short answer: you suspect you have been poisoned??!!
The history
Motion sickness has been recognized for millennia. It mostly occurred out at sea (source behind the etymology of nausea), but also occurred during cart (and camel!) travel. There was some recognition that this may have to do with an imbalance of the senses, but the exact mechanism was unclear. It was a significant concern, as it could turn the tide of battle if troops were too weary to engage in combat once they landed ashore. Several remedies ranging from ingesting bizarre fluids and distracting oneself with delightful scents were used with variable success. To learn more about these bizarre fluids, and for other interesting facts on the history of motion sickness, please take a look at the review by Huppert et al. [1].
The science
It wasn’t until this past century that we developed theories on why this occurs. Today, the widely accepted explanation behind motion sickness is the sensory conflict hypothesis, originally reported by Reason and Brand in 1975 [2]. Essentially, confusion ensues due to disagreement between the signals reported by your eyes (visual), the balance system in your ears (vestibular), and what the rest of your body senses (somatic). There is an element of comparison to an expected pattern of sensory inputs (often stemming from prior experience), which explains the protective effects of exposure therapy. In other words, disagreement in reported signals is acceptable, as long as it is expected [3]. If you are curious about more details on how each of the three balance systems work, the lecture on sensory feedback of human motion by Dr. Park does an excellent job of providing a succinct description of the mechanisms [4].
There has also been considerable effort put into learning about the parameters surrounding motion sickness. Understanding the limitations provides critical engineering design guidance for naval and air forces, as well as habituation techniques that can optimize the training regimen. Even though we have done a great job of characterizing the conditions that result in motion sickness, it remains unclear why we find ourselves nauseous during that experience. Is there some evolutionary advantage to getting dizzy, feeling fatigued, or vomiting? Or is it all an unintended consequence of our natural wiring.
The why
There are a few competing hypotheses that aim to attach reason to this madness. Guedry et al. propose that this unpleasant reaction is a feedback mechanism to discourage inefficient movement learning during development [5]. There are other claims that it is a consequence of hypoperfusion to the brain due to inefficient response of compensatory mechanisms during the disorientation [6]. However, a leading hypothesis is one that was proposed by Treisman in 1977 [7]. Treisman proposed that the brain interprets any conflicting signals in the balance system as an early warning sign of the presence of a neurotoxin, and promotes the expulsion of the stomach’s contents as a safety mechanism. This act may also have served as a warning sign to others in our group that this may not be the meal they are looking for. The nausea one experiences afterwards may have been part of a conditioning plan to program aversion from this menu item in the future. Lackner mentions work done by Money and Cheung, which I was not able to access, that provides experimental evidence to support this claim whereby surgical manipulation of the vestibular system in canines resulted in decreased emetic response to certain poisons [8].
The end
In conclusion, the reason that we get nauseous during motion sickness may just be an evolutionary accident resulting from safety mechanisms meant to keep us from ingesting poisons. I think the jury is still out on this one, but that is an interesting end to our investigation.
[1] D. Huppert, J. Benson, and T. Brandt, “A Historical View of Motion Sickness—A Plague at Sea and on Land, Also with Military Impact,” Front. Neurol., vol. 8, Apr. 2017, doi: 10.3389/fneur.2017.00114.
[2] J. T. Reason and J. J. Brand, Motion sickness. London: Acad. Pr, 1975.
[3] J. T. Reason, “Motion Sickness Adaptation: A Neural Mismatch Model,” J. R. Soc. Med., vol. 71, no. 11, pp. 819–829, Nov. 1978, doi: 10.1177/014107687807101109.
[4] ME585 3.2.2 Sensory feedback of human motion. [Online Video]. Available: https://www.youtube.com/watch?app=desktop&v=zJ_G1o5VY5A
[5] F. E. Guedry, A. R. Rupert, and M. F. Reschke, “Motion sickness and development of synergy within the spatial orientation system. a hypothetical unifying concept,” Brain Res. Bull., vol. 47, no. 5, pp. 475–480, Nov. 1998, doi: 10.1016/S0361-9230(98)00087-2.
[6] J. M. Serrador, T. T. Schlegel, F. O. Black, S. J. Wood, and S. J. Wood, “Cerebral Hypoperfusion Precedes Nausea During Centrifugation,” 2005.
[7] M. Treisman, “Motion Sickness: An Evolutionary Hypothesis,” Science, vol. 197, no. 4302, pp. 493–495, Jul. 1977, doi: 10.1126/science.301659.
[8] J. R. Lackner, “Motion sickness: more than nausea and vomiting,” Exp. Brain Res., vol. 232, no. 8, pp. 2493–2510, Aug. 2014, doi: 10.1007/s00221-014-4008-8.