Visual information is a key aspect of human perception that facilitates successful interaction with the environment. When moving, we experience the perception of self-motion adapting our velocity and body posture to space. In part, this is possible because of the close link between two brain systems: the visual cortex and the vestibular cortex. The main assumption is that both systems contribute to distinguishing between self-motion and motion of the environment. However, sometimes these two systems are in conflict like when we experience self-motion when actually is the scene that moves and not us as happens in a simulator or virtual reality settings. This phenomenon of illusory self-motion is called “vection” and has been well investigated during the last decades.
An illusion for you but a conflict in your brain
Have you ever been seating on a stationary train before departing when the train on the neighboring track begins to move? You will probably feel like the train you are sitting in is starting to move instead of the train on the next track. This is a real-life situation that perfectly illustrates the experience of illusory self-motion perception. The good news is that there is a scientific explanation.
Actually what happens is that your visual and vestibular systems are in conflict. On the one hand, the visual system is confused because the illusion is almost an exact replication of the situation when the train is really moving forward. On the other hand, the vestibular system can’t resolve the conflict because the information of the head motion in space is missing and therefore, suppressed since it is not reliable. Let´s say that neurons of both systems for a brief moment, are not “talking” fluently as they normally do.
Self-motion without moving: applications in virtual reality and training simulators
Self-motion information can be processed within simulated environments without necessarily causing vection perception. For example, playing a computer game on a small device like a smartphone or gaming consoles will rarely evoke any self-motion illusion. However, there are some immersive situations like virtual simulator systems wherein high compelling levels of realism are present. These displays are often used in diverse fields such as scientific research, high-performance sports, military training (e.g. aviation) and rehabilitation. An important question to consider is whether the illusory experience of self-motion or vection in virtual reality interfaces is a desirable phenomenon or not.
Something that we know from research is that vection elicits a vivid sensation of presence mirroring our behavior almost like in the real-word. In the case of simulations involving self-motion, vection perception can be important to improve task performance allowing an optimal “transfer of training”. To optimize navigation through different virtual reality environments, the brain must accurately estimate our own motion relative to space and objects around. For this reason, self-motion perception is a highly demanding function requiring the neural integration of visual information, vestibular signals and somatosensory cues. However, within the context of simulation technologies, sometimes negative side-effects like visually induced motion sickness have been linked to vection or the sensation of illusory self-motion. Therefore, the challenge in today‘s research is to understand the brain mechanisms of visual-vestibular interaction to maximize the experience of illusory self-motion, while avoiding or minimizing the occurrence of unpleasant side-effects.
Alpha waves as a brain signature of self-motion illusions
Self-motion illusion is a phenomenon that has been investigated for all motion directions (forward-backward) and along all motion axes (linear and circular). The most frequently investigated type of vection is circular vection around the earth-vertical axis (see fig. 1). In this type of vection, the observer perceives illusory self-rotation around the earth-vertical axis. In a recent study, researchers from the University of Grenoble have investigated the neural correlates of visually induced illusions of self-motion using electroencephalography (EEG) technique. Researchers analyzed the alpha activity in sensoriomotor networks during the visually-induced illusion of self-motion in the line of the sight (roll motion). A strong presence of alpha activity can be indicative of inhibited or disengaged cognitive states. Interestingly, the authors observed an increase of alpha oscillations in posterior regions (parieto-occipital) during the ongoing perception of self-movement but only when participants were in the upright body position as compared to the supine position. This indicates that when the illusion arises (perception of self-motion), the sensory conflict between visual and vestibular systems occurs relative to the direction of gravity rather than relative to head moving (optic-flow stimuli). The main conclusion is that an increase of alpha activity in sensoriomotor networks is required to maximize the illusion.
Fig 1. Illusions of self-motion (or “vection”) are experienced when one perceives the body motion in the absence of real movement. One can experience illusory movements of the whole body or of individual body parts.
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