Effects of Five-Day “Dry” Immersion on the Strength of the Ponzo and the Müller-Lyer Illusions

The parameters of the verbal and sensorimotor responses to the Müller-Lyer and Ponzo illusions were determined in two groups spending five days in support unloading produced by “dry” immersion. Subjects in the “immersion” (IM) group were not exposed to any treatment other than immersion. Subjects of the “immersion + weight loading” (IM + L) group underwent weight loading using a Penguin axial loading costume for 4 h each day. Differences in the verbal and sensorimotor responses were seen in the two groups, along with differences in assessments of the two illusions. Verbal reports indicated that the strength of the Ponzo and Müller-Lyer illusions decreased linearly as the experiment progressed; sensorimotor responses indicated that being in the Penguin suit led to increases in illusion strength; the strength of the Müller-Lyer illusion increased after immersion fi nished. It is suggested that the main factor affecting illusion strength is gravitational unloading, which decreases the level of activation of the left hemisphere, leading to use of a metric representation system mainly associated with activity in the right hemisphere.