The postural control system has two functions: to ensure that balance is maintained by bracing the body against gravity, and to fix the orientation and position of the features that serve as a frame of reference for perception and action with respect to the external world.[10] Postural control relies on multisensory processing and motor responses that seem to be automatic and occur without awareness. Studies have shown that people afraid of heights or falling have poor postural control, especially in the absence of strong visual cues. These individuals rely heavily on vision to regulate their posture and balance.[11] When faced with high or unstable ground, the vestibular system in these individuals senses the instability and attempts to correct it by increasing postural sway to reactivate visual balance feedback (postural sway refers to the phenomenon of constant displacement and correction of the position of the center of gravity within the base of support).[12] This often fails, however, resulting in a feeling of increased instability and anxiety, which is often interpreted as fear.