The Aftereffects of Müller-Lyer and Ponzo Illusions: Differences Revealed in Sensorimotor Domain

Either effects or aftereffects of visual illusions are well studied at the visual domain while there are few studies of aftereffects at the motor tasks such as grasping or pointing at the illusory. The aftereffects of Müller-Lyer and Ponzo illusions in the sensorimotor domain were studied. We used four illusions: two versions of Müller-Lyer illusions (upper/bottom shafts appear longer) and two versions of Ponzo illusions (classical and inverted, upper/bottom shafts appear longer). They were presented to four experimental groups, each type to one of the groups. A fifth group was shown neutral stimuli (two horizontal lines, one under another). At first, one of the above described stimuli was presented ten times. Then, for testing the aftereffect, the neutral stimuli were presented thirty times. After the disappearance of each stimulus, the participant moved his/her right hand across the touch screen along its upper and lower shafts. The participants of all experimental groups experienced significant illusions, but only the classical Ponzo illusion caused significant long-time assimilative aftereffect. These results reveal the existence of an illusory aftereffect in the sensorimotor domain. Moreover, it depends on the type of visual illusion, thereby supporting the hypothesis of origin of the different visual illusions at different levels of the visual system.