Characterization of Senses for Head-Body Orientation: Exploring Effects of Vision, Vestibular and Muscular Proprioception

Spatial awareness of our orientational reference is derived from a network of sensory feedback modalities, collaborating to create a singular egocentric spatial representation of the body. With specific regards to the head-body orientation, the sensory inputs used to map this representation are vestibular, visual, and cervical proprioceptive feedback. However, visual and vestibular feedbacks have limited capability to properly distinguish the difference between rotations of the head and rotations of the body. In contrast, densely distributed muscle spindles within the neck muscle (Longus Colli and Multifidus), indicate that cervical proprioceptive feedback is perhaps dominating in the formation of the perception of head-body orientation. The significant impacts caused by defective cervical proprioception (e.g., dizziness, offset in spatial awareness, and gait and balance deficit) also suggests the critical role of the cervical proprioceptive feedback. We hypothesized that the information from visual and vestibular feedback provide redundancies in the detection of the relative orientation between the head and the body, upon the normal operation of cervical proprioception. In addition, we hypothesis that cervical proprioception impacts the relative head-body orientation perception the greatest of the three sensory modalities. To test these hypothesizes, healthy human subjects were recruited (5 females, 5 males) and asked to turn their head by 45-degrees and return back to the starting position, which is a standard task to test the perception of relative head-body orientation. The error between the starting and returned position was used to test the effectiveness of cervical proprioception. In addition, the deviation from the initial reference position was recorded across each set of tests for the 10 subjects and recorded to examine the precision of the participants across the tests. The repositioning test was done under 8 different conditions of sensory feedback combinations, to isolate and gauge the contribution of each sensory feedback individually and collectively. To test the effect of cervical proprioception, either the head was turned with the body staying still or both the head and the body turned as one. To test the effect of visual feedback, subjects performed tests blindfolded or unblindfolded with eyes open. To test the effect of vestibular feedback, subjects were tested with a slower rate of return, below the threshold for vestibular perception, towards the initial position versus a normal paced rate of return towards the initial position. Upon the completion of these tests, we hoped to gain more insight and create a profile how these sensory modalities (or loss of each sensory modality) contribute to our perception of the head-body orientation. This will help in designing sensory augmentation in a better way. For example, if neck proprioception dominates the formation of perception of head-body orientation, we may focus on proprioceptive augmentation instead of visual or vestibular augmentation, in treating the defective neck proprioception. The tests revealed that cervical proprioception provided the greatest contribution towards the head-body orientation in instances of isolated available feedback (p < 0.01 for both proprioception vs vestibular and visual) and created the most error and deviation in instances where cervical proprioception was lacking versus instances in where vestibular or visual feedback were missing (pvis = 0.05, pvest < 0.01 for error; pvis = 0.03, pvest = 0.01 for deviation). Given these results, direct augmentation of the cervical spine muscle proprioceptors should be the most desired approach for resolving cervical proprioceptive errors that effect our orientational references. A functional biphasic electrical stimulator has been developed to begin testing on the effects and efficacy of electrical augmentation of the orientation perception sensation