Timothy C. Hain, MDMost recent update:
June 18, 2009
Rarely people experience an illusion that the world is upside down. They see the ceiling as the floor, and the floor as the ceiling.
Inversion illusions may occur both under certain normal circumstances as well as in medical disorders. In normal persons, inversion illusions may occur when gravity reference signals are gone. In the space-lab, astronauts no longer have gravity to orient themselves and may experience inversion. Divers may experience inversion illusions when somatosensory cues indicating the direction of downward (i.e. foot pressure), is absent.
Structures in inner ear, showing utricle
|Orientation of utricle (horizontal) and saccule (vertical)|
Medical disorders that are associated with tilts or inversions seem to relate to disturbances of inner ear function (the otoliths sense gravity), central connections of the otoliths, or disturbances of central visual processing.
The location of the otolithic apparatus in the ear is shown in the illustrations above. Thinking physiologically, one might conjecture that inversion illusions should be a result of saccule disturbances. Also, unilateral disturbances of otolith function should, in theory at least, only cause tilts of the world. However, one might conjecture based on experiences of normal persons, that any disorder that increase the uncertainty of processes that estimate orientation to vertical, might also be associated at least occasionally with inversion illusions.
Otoliths pathways extend in the brainstem from the 8th nerve entry zone in the medulla, upwards to the cerebellum, the oculomotor nuclei, and end in temporal cortex close to auditory cortex. Discrete lesions of the brainstem, such as from tumor or MS placques, to the authors knowledge, are not reported as sources of inversion illusions. However in Wallenberg's syndrome, a stroke syndrome also called lateral medullary syndrome, otolith connections are disturbed and inversion illusions have been reported (Hornsteen 1974).
|Human brainstem. Inversion illusions have been reported due to strokes related to disease of the vertebral arteries, the posterior inferior cerebellar arteries (PICA), and from the basilar artery (large vessel going down middle of brainstem).|
At the highest level, perhaps an example of a disturbance of vision, a syndrome called "top of the basilar syndrome", can be accompanied by an inversion illusion. Because both brainstem pathways and visual cortex are affected by syndromes at the "top of the basilar", inversion syndromes from this cause might theoretically fall into two categories, one due to a otolith connection disturbance, and the other due to visual distortions.
Graphics are courtesy of Northwestern University, and are used with permission.
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