Turning green in microgravity: facial color changes as best physiological indicator of space sickness

This study identifies increased facial skin greenness as a simple, non-invasive, and reliable objective indicator of motion sickness severity, particularly in space environments, outperforming other physiological measures like heart rate and blood pressure.

The Gist

Imagine your brain is like a highly sophisticated navigation system in a car. Usually, your eyes, your inner ear (which feels motion), and your body all agree on where you are and how you're moving. But sometimes, they start arguing. Your eyes say, "We're moving fast!" while your inner ear says, "No, we're sitting still!" This confusion is called sensory conflict, and it's the main reason people get motion sickness (feeling carsick, seasick, or space-sick).

This study asked a big question: Can we tell someone is about to get sick just by looking at their body, before they even say "I feel nauseous"?

The researchers wanted to find a "lie detector" for motion sickness. They tested two very different scenarios:

1. Virtual Reality (VR): Like sitting in a chair while watching a rollercoaster on a screen (your eyes say "moving," your body says "still").
2. Parabolic Flight: A real airplane that flies in giant arcs to create moments of weightlessness (like the "Vomit Comet"). This messes with your inner ear in a way VR can't.

The "Lie Detectors" They Tested

The team strapped 29 volunteers to various sensors to measure:

• Heart rate & Blood pressure: Like checking the engine's RPM and fuel pressure.
• Skin conductance: Measuring sweat (like a lie detector test).
• Cortisol: A stress hormone (like a "panic button" chemical in the blood).
• Skin temperature: Checking if the person is getting hot or cold.
• Skin Color: This is the star of the show. They used a special camera to measure exactly how red, blue, or green a person's face was.

The Big Discovery: "Turning Green"

The most exciting finding was about facial color.

You've probably heard the phrase, "He turned green." Well, it turns out that's not just a figure of speech! The study found that as people got sicker, their faces literally became greener.

• In the Airplane: As the plane did its weightless loops, the people who got sick (and eventually vomited) showed a distinct shift toward green on their cheeks. The greener they got, the worse their nausea was.
• In VR: The same thing happened, though less dramatically because people stopped the simulation before they got too sick.

The Analogy: Think of your skin like a mood ring. When you get motion sick, your body redirects blood flow to cool you down and deal with the stress. This changes the way light bounces off your skin, making it look slightly greenish. It's a silent, non-invasive alarm bell that says, "Warning! Motion sickness detected!"

What Didn't Work (The False Alarms)

The researchers hoped to find other easy predictors, but most of them were unreliable:

• Heart Rate: It stayed mostly the same. You can't tell if someone is carsick just by checking their pulse.
• Sweat (Skin Conductance): It went up for everyone, whether they got sick or not. It was too noisy to be a specific marker.
• Stress Hormones (Cortisol): Everyone's stress went up, so it didn't help distinguish who would actually vomit.
• Past History: Asking people, "Did you get carsick as a kid?" didn't help predict if they would get sick in space or VR. Just because you were a "sick kid" doesn't mean you'll be a "sick astronaut."

The "One-Size-Fits-All" Myth

A major takeaway is that motion sickness is not the same in every situation.

• The way your body reacts to a VR headset is different from how it reacts to a real airplane.
• For example, blood pressure dropped significantly in the airplane but stayed steady in VR.
• This means you can't just test someone on Earth and be 100% sure how they'll handle space travel. The "rules" change depending on the environment.

The Bottom Line

This study gives us a new, simple tool: Look at the face.

If you want to know if someone is getting motion sick, don't just ask them. Look at their skin. If they start turning a shade of green, they are likely feeling very nauseous. It's a simple, non-invasive, and surprisingly accurate way to measure the invisible feeling of sickness.

While we still can't perfectly predict who will get sick before it happens, we now have a much better way to measure how bad it is while it's happening. So, next time you're on a bumpy ride, if your friend's face looks a little green, maybe offer them a bag before they ask for one!

Technical Summary

1. Problem Statement

Motion sickness (MS) is a debilitating condition caused by sensory conflicts (e.g., visual-vestibular or intra-vestibular conflicts) that impairs cognitive and motor performance. While subjective questionnaires (like the MSSQ) exist, they rely on self-reporting and past experiences, which have been shown to poorly predict susceptibility to Space Motion Sickness (SMS) or cybersickness.

• The Gap: There is a critical lack of reliable, objective physiological predictors for MS, particularly for spaceflight contexts where Earth-based susceptibility tests often fail.
• The Question: Do different sensory conflict paradigms (Virtual Reality vs. Parabolic Flight) elicit distinct physiological responses? Can baseline physiological measures predict individual susceptibility to MS in these environments?

2. Methodology

Study Design & Participants

• Participants: 29 healthy volunteers (mean age 27 ± 7 years; 14 women, 15 men).
• Protocols: Participants were exposed to two distinct sensory conflict paradigms:
  1. Virtual Reality (VR): A visuo-vestibular conflict inducing cybersickness (simulated car ride and rotating tunnel).
  2. Parabolic Flight (PF): A vestibulo-vestibular (otolitho-canal) conflict inducing space motion sickness (31 parabolas on an Airbus A310 Zero G, alternating 0g, 1.8g, and 1g).
• Grouping: 21 participants completed both protocols; others completed only one. Participants were classified as "Sick" or "Non-Sick" based on objective endpoints (vomiting in PF; premature session termination due to severe nausea in VR).

Physiological Measures
The study monitored a broad set of objective markers before, during, and after exposure:

• Facial Skin Colorimetry: Measured via a colorimeter on the left cheek (RGB channels).
• Autonomic & Stress Markers: Skin conductance (GSR), Heart Rate (HR), Heart Rate Variability (HRV), Systolic/Diastolic Blood Pressure.
• Hormonal: Salivary Cortisol.
• Thermal: Skin surface temperature (mastoid process).

Subjective Measures

• MSSQ: Motion Sickness Susceptibility Questionnaire (baseline).
• SSQ: Simulator Sickness Questionnaire (pre/post VR and PF).
• MSAQ: Motion Sickness Assessment Questionnaire (post-PF only).

Statistical Analysis

• Two-way repeated-measures ANOVAs (Time × Sickness status).
• Correlation analyses (Spearman's ρ) between physiological changes and subjective scores.
• Linear regression to test the predictive power of baseline physiological values.

3. Key Results

A. Physiological Responses to Exposure

• Cortisol: Significantly increased in both VR and PF post-exposure. In PF, the increase was robust regardless of sickness status. In VR, a trend toward increase was seen only in "Sick" participants.
• Skin Conductance: Increased post-VR, particularly in sick participants. No significant change in PF.
• Temperature: Showed opposing trends. Skin temperature increased in VR (likely due to headset heat) but decreased in PF (likely due to cabin cooling and environmental factors). Neither trend strongly correlated with sickness severity.
• Blood Pressure:
  • VR: Sick participants had lower systolic pressure than non-sick, but no change over time.
  • PF: Systolic pressure significantly decreased post-flight. This drop was strongly correlated with nausea severity.
• Heart Rate & HRV: No significant changes or correlations with sickness severity in either paradigm.

B. The "Green" Finding (Skin Colorimetry)

• VR: Sick participants exhibited a significant increase in the Green channel of facial skin color immediately post-exposure.
• PF: Sick participants showed a significant increase in the Green channel from pre-flight to mid-flight (the period of highest microgravity exposure).
• Correlation: Increased skin greenness was strongly correlated with subjective nausea ratings in both paradigms.
  • In PF, the magnitude of greenness increase correlated with the parabola number of the first vomit (earlier vomiting = greener skin).
  • This is the first study to objectively quantify the colloquial "turning green" phenomenon using RGB colorimetry.

C. Predictive Value of Baseline Measures

• MSSQ (Subjective History): Did not predict physiological changes or sickness severity in either VR or PF.
• Baseline Physiology:
  • VR: Baseline cortisol correlated with oculomotor symptoms but did not predict variance.
  • PF: Baseline skin temperature and HRV showed weak predictive value for central and peripheral symptoms, respectively, but failed to predict the most debilitating symptom: gastrointestinal nausea.
• Cross-Paradigm Correlation: Physiological responses in VR did not correlate with responses in PF. An individual's reaction in one environment does not predict their reaction in the other.

4. Key Contributions

1. Objective Marker Identification: The study identifies facial skin greenness (Green channel intensity) as the most reliable, non-invasive, and objective physiological indicator of motion sickness severity across both terrestrial (VR) and space-analog (PF) environments.
2. Paradigm Specificity: It demonstrates that while some markers (cortisol, greenness) are consistent, others are paradigm-specific. Specifically, systolic blood pressure reduction is a unique marker for parabolic flight (vestibular-driven) MS, whereas it is not a reliable marker for VR-induced cybersickness.
3. Failure of Prediction: The study confirms that baseline physiological measures and past motion sickness history (MSSQ) are poor predictors of acute susceptibility to novel inertial environments (like spaceflight).
4. Methodological Innovation: The use of portable colorimetry to track real-time skin color changes provides a novel, low-cost alternative to subjective reporting or complex imaging.

5. Significance and Implications

• Spaceflight Safety: Since Earth-based susceptibility tests fail to predict space sickness, the ability to monitor facial greenness in real-time offers a potential tool for mission control to detect early onset of SMS and intervene before vomiting or cognitive decline occurs.
• Clinical & Operational Utility: The "green" metric is simple, non-invasive, and requires minimal equipment, making it suitable for monitoring in VR training, drone piloting, and actual space missions.
• Physiological Insight: The findings suggest that the "greening" of the face is likely due to microcirculatory changes (vasoconstriction/vasodilation) and shifts in hemoglobin oxygenation associated with the autonomic response to nausea, rather than just pallor.
• Future Research: The study highlights that gastrointestinal symptoms (the most severe form of MS) remain difficult to predict via baseline measures, suggesting that visceral afferent sensitivity (gut-gravity receptors) may be a critical, unmeasured factor in individual susceptibility.

In conclusion, while individual susceptibility to motion sickness remains difficult to predict a priori, the study establishes facial skin greenness as a robust, real-time a posteriori indicator of severity, bridging the gap between subjective experience and objective physiological monitoring.