Donald Hebb proposed that consciousness is synonymous with learning, and he believed that the neocortex is centrally involved in this process (Hebb 1949, 1961, 1968). In humans, electrical stimulation of the posterior cingulate cortex adjacent to the parietal cortex and M1 evokes sensations related to the vestibular sense: vertigo, dizziness, and a sense of falling (Caruana et al. 2018). This region of the cingulum innervates many regions of the neocortex that have been implicated in balancing the head with respect to the gravitational axis: the supplementary motor area, the retrosplenial cortex, the inferior parietal cortex, MT/MST, and the insular cortex (Fig. 1; Caruana et al. 2018; Chen et al. 2011; Fukushima et al. 2011; Guldin and Grüsser 1998; Rancz et al. 2015). Under normal circumstances when fully adapted, the vestibular sense via the otolith hair cells operates unconsciously through the cerebellum (Eccles et al.1967). However, once there is a change in the gain of the vestibular input, consciousness is summoned to signal the start of the adaptation process. After returning from space astronauts typically experience dizziness and disorientation which can lead to vomiting (Demontis et al. 2017). Dizziness and disorientation can last up to five days (Carriot et al. 2021). Returning astronauts have difficulty standing, stabilizing gaze through the vestibular reflex, and walking and turning their bodies at every corner (Lawson et al. 2016). In fact, they voluntarily suppress their head movements to minimize the use of the vestibular system before adapting to earth’s 1G gravitational field. The adaptation process can be eased by spending time in a swimming pool. Also, when in a space station for many days astronauts can feel that they are resting on their side, i.e., 90 degrees with respect to the vertical, visual axis, due to the lack of gravity (Carriot et al. 2021). This is especially true when the eyes are closed. Indeed, it is known that vestibular illusions can negatively affect the way astronauts pilot their space crafts (Carriot et al. 2021).

A trip to Mars and back will require four separate adaptations to changes in gravitational force, all of which will be experienced consciously: (1) going from 1 G to 0 G when leaving the earth; (2) going from 0 G to 0.38 G when landing on Mars; (3) going from 0.38 G to 0 G when leaving Mars; and (4) going from 0 G to 1 G when landing on earth (Lawson et al. 2016). Both neocortex and cerebellum are involved in the adaptive process. The neocortex consciously monitors the process, and the cerebellum finalizes the adaptation at the level of the Purkinje neurons by resetting the vestibular gain (Lisberger et al. 1984).

The next time you decide to drink too much alcohol remember that you are putting your vestibular system under conscious/neocortical examination, since the alcohol diminishes the responsivity of the vestibular hair cells by altering the fluid in the vestibular canals (Shibano 2013), thereby causing a decrease in the gain of the vestibulo-ocular reflex (Tianwu et al. 1995).

To conclude, anytime the brain needs to adapt to a new sensory experience consciousness is summoned. Awareness of all sensory changes imposed on the body is central to survival. For example, being conscious of an upset stomach that lasts for weeks can compel many to go to a doctor to avert death through cancer.

Figure 1: Regions of human neocortex that mediate the vestibular sense as verified with fMRI: the parietal cortex, the temporal cortex (including MT/MST), the insular cortex, and the premotor cortex (including the supplementary motor area). From figure 3B of Lenggenhager and Lopez (2015).

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