Arms crossed, Noémie, 39, advances boldly. The young woman places her heel once morest the tip of the opposite foot and walks step by step along a line drawn on the ground. The researchers note the number of steps she takes without wavering. Then Noémie repeats the exercise with her eyes closed, a pure challenge. She frequently staggers, the two experimenters who supervise her are on the alert.
This test is a classic for evaluating balance disorders. Noémie submitted to it in the premises of the University of Caen, at the beginning of October, where around thirty voluntary patients were gathered, from all over France. For four days, a battery of examinations enabled the researchers to assess their postural balance, their ability to walk in the presence of obstacles, their perception of angles, distances and time, and their ability to solve various cognitive tasks. Their bone density was measured, their brain screened by magnetic resonance imaging (MRI). All are suffering, like Noémie, from a deficit of the vestibular system.
The vestibular system? An unknown sixth sense. Yet it plays a crucial role in maintaining our balance, the stability of our gaze, our orientation in space. Feeling the acceleration or deceleration of an elevator, experiencing a tight bend in a car are all sensations that we owe to it. It still intervenes – but, here, its mode of action is more obscure – in our perception of time, the rhythm of our hormonal secretions, the quality of our sleep, our bone density…
However, “we only become aware of its existence in certain ‘pathological’ situations, such as seasickness, alcohol intoxication, dizziness”, pointed out, in 2006, two researchers from the Collège de France, Werner Graf and François Klam. Is it because this little gem of precision, a perfect illustration of nature’s engineering abilities, is lodged in the labyrinth of our two ears, hidden from view?
A variety of sensors
Guardian of our balance, the vestibular system does not act alone. It has two valuable allies: the visual system and proprioception. A seventh sense, for its part, which mobilizes a panoply of sensors sensitive to stretching and pressure, disseminated over our muscles, our tendons and our ligaments. These buried sentinels transmit the position data of the different parts of our body to the brain. Ultimately, our brain will integrate messages from these three systems – vestibule, visual system and proprioception. And will convert them, following a clever calculation, into coherent information. For instance, “ the body is upright, the head has turned to the right”.
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