Moving in a Moving World: A Review on Vestibular Motion Sickness

Abstract

Motion sickness is a common disturbance occurring in healthy people as a physiological response to exposure to motion stimuli that are unexpected on the basis of previous experience. The motion can be either real, and therefore perceived by the vestibular system, or illusory, as in the case of visual illusion. A multitude of studies has been performed in the last decades, substantiating different nauseogenic stimuli, studying their specific characteristics, proposing unifying theories, and testing possible countermeasures. Several reviews focused on one of these aspects; however, the link between specific nauseogenic stimuli and the unifying theories and models is often not clearly detailed. Readers unfamiliar with the topic, but studying a condition that may involve motion sickness, can therefore have difficulties to understand why a specific stimulus will induce motion sickness. So far, this general audience struggles to take advantage of the solid basis provided by existing theories and models. This review focuses on vestibular-only motion sickness, listing the relevant motion stimuli, clarifying the sensory signals involved, and framing them in the context of the current theories.

Keywords: cross-coupling stimulus; linear oscillations; motion sickness; sensory conflict; vestibular.

Figure 1

Scheme of the different sensory systems contributing to the perception of self-motion. For the vestibular system, the otolith organs and the semicircular canals are represented separated to allow evidencing the inherent limitations of the signal from each sensor before central processing. Specifically, the semicircular canals signal in response to continuous rotation decline over time (solid black line decaying in the plot in the top/left corner) leading to a similar decrease in the sensation of self-rotation (as depicted by the change in the curve arrow above the two heads). The otolith organs, instead, perceive only the overall GIA vector (sum of gravity and inertial acceleration) and cannot, therefore, distinguish between a head tilt and linear translation.

Figure 2

Scheme of the Coriolis/cross-coupling stimulus and the induced sensory conflict. (A) Constant-velocity rotation. The three heads correspond to three subsequent time points during a constant-velocity rotation. The three arrows above each head correspond to the actual rotational velocity (solid black arrow), to the perceived velocity (dashed arrow), and to the amount of velocity decayed over time (gray arrow) due to the properties of the vestibular system. The sum of the solid black and gray arrows must always correspond to the dashed arrow. In the beginning (leftmost head), the subject has just been accelerated to the constant velocity. The perceived velocity (dashed arrow) is only slightly lower then the actual one (solid black arrow). As time passes (central and rightmost heads), the decayed velocity signal increases and the perceived velocity decreases. (B) Head tilt during constant-velocity rotation. Heads and arrows as in (A) after the tilt (rightmost head), no actual rotational velocity exists along the head vertical axis (no solid black arrow). However, as the decayed velocity is different from 0, the absence of an actual velocity leads to a perceived velocity different from 0 (recall that sum of the solid black and gray arrows must always correspond to the dashed arrow, so in the absence of a black arrow, the dashed arrow is equal to the gray arrow). The actual velocity is now along the head interaural axis and is perceived by the semicircular canals. The perceived velocity has, therefore, now two orthogonal components. (C) Brain expectations and conflict. Leftmost head: the perceived gravity signal (double-line, black arrow below the head) is stable. Accordingly, any expected perceived rotational velocity (dashed arrow above the head) must be aligned with the gravity vector. Rightmost head: the perceived rotational velocity is tilted by 45° (thick dashed arrow above the head) as it corresponds to the sum of the two perceived rotational velocity vectors in (B) (thin dashed arrows above the head). Accordingly, the gravity vector (double-line, black arrow below the head) should rotate in the head reference frame as the head changes orientation relative to gravity. Expected and perceived sensory signals do not match, leading to a sensory conflict.