Postural adaptation to prolonged optical reversal of vision in man

Effects of long-term vision reversal on postural control have been studied by measuring the ability of subjects to maintain balance whilst standing or walking on narrow (0.75 in.) or wide (2.25 in.) flat wooden rails, with eyes open or closed. In all situations tested, the initial period of vision reversal was associated with marked, often catastrophic (strong safety rails were necessary), disorganization of control, indicating predominance of visual over vestibular and other non-visual afferent influences. Thereafter, the walking test with eyes open showed progressive recovery of postural control, continuing in the longest instance over 3–4 weeks of maintained vision reversal. However, restoration of normal vision after 27 days adaptation was again associated with severe loss of control, suggesting that functional reversal had taken place in relevant visuo-spinal influences. Subsequent readaptation to normal conditions occupied up to 2 weeks. Marked changes in the results of standing tests with eyes shut indicates that modification of non-visual postural mechanisms may have occurred. The results as a whole revealed systematic patterns of adaptive changes in postural control associated both with vision reversal and with return to normal vision. The plastic nature of these changes is indicated by their long-term retention, yet complete reversibility. Qualitative observations included marked nausea, reversed apparent movement of the visual scene on rotating the head after adaptation but with prisms off and strange sensations of movement when lying down with eyes closed.