Is Vestibular Information Involved In Mental Imagery

Whole body rotations involve a rigid rotation of one's own body with respect to an external frame of reference. Unlike movements of a body part, physical whole body rotations inevitably involve a stimulation of the vestibular end organ. When body movements are imagined, however, participants are completely still, which precludes any vestibular stimulation. Still, mechanisms that are normally associated with the processing of vestibular information during real body rotation may also be involved when the body movement is physically absent but instead vividly imagined. To the present, there is still a lack of knowledge on the role of the vestibular system (in particular its cortical projection areas) in cognitive tasks. The principal roles of the vestibular system include maintenance of balance, reflex contractions of the trunk and limbs, reflex control of eye movements, and detection and constant perception of the position and movement of the head. In consequence, the vestibular system connects with numerous brain areas. It is known to project to the thalamus, and is thought to project to the hippocampus to aid with formation of spatial memory. Some anatomical studies have suggested potential pathways for the thalamus to carry vestibular information to the hippocampus, although direct pathways may also exist (Smith, 1997).

Primate studies have identified several cortical areas as being vestibular in nature (Guldin and Grüsser, 1998). Vestibular responses have been reported physiologically in area 2v at the tip of the intraparietal sulcus (Schwarz and Fredrickson, 1971; Büttner and Buettner, 1978; but see Grüsser et al., 1990), area 3av in the central sulcus (Akbarian et al., 1994), the parietoinsular vestibular cortex next to the posterior insula (PIVC) (Grüsser et al., 1990), and area 7 in the inferior parietal lobule (Faugier-Grimaud et al., 1997; Akbarian et al., 1994). These areas have been shown to receive converging information from the vestibular, visual, and somatosensory systems and are thought to be involved in the analysis of self-motion, and/or modifying the reference frame in which visual and other sensory inputs are represented (Bremmer et al., 1997a, 1997b).

These cortical connections have been confirmed in human studies primarily through the use of fMRI and PET during caloric stimulation of the semicircular canals or galvanic stimulation of the vestibular system (Bottini et al., 1994; Lobel et al., 1998; Bense et al., 2001). Some authors contend that vestibular thalamo-cortical projections end in the parietoinsular cortex and the parieto-occipital cortex: Areas primarily involved with perception of self-motion and judgments of verticality. Dieterich and Brandt (2001) have shown that patients with parietal lesions have difficulties aligning a disk with a line on it to the earth vertical when any surrounding visual cues were absent. Interestingly, Akbarian and colleagues (1994) have also demonstrated cortical vestibular projections back to the vestibular nuclei, which is possibly important to understand the influence of top-down control of vestibular information. The influence in the reverse direction is presumably just as strong and it is rather surprising that still so little is known about how cognitive functions are influenced by vestibular information (e.g., Mast et al., 2006).

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