Aristotle’s Illusion in Parkinson’s Disease: Evidence for Normal Interdigit Tactile Perception

Abstract

Sensory alterations, a common feature of such movement disorders as Parkinson’s disease (PD) and dystonia, could emerge as epiphenomena of basal ganglia dysfunction. Recently, we found a selective reduction of tactile perception (Aristotle’s illusion, the illusory doubling sensation of one object when touched with crossed fingers) in the affected hand of patients with focal hand dystonia. This suggests that reduced tactile illusion might be a specific feature of this type of dystonia and could be due to abnormal somatosensory cortical activation. The aim of the current study was to investigate whether Aristotle’s illusion is reduced in the affected hand of patients with PD. We tested 15 PD patients, in whom motor symptoms were mainly localised to one side of the body, and 15 healthy controls. Three pairs of fingers were tested in crossed (evoking the illusion) or parallel position (not evoking the illusion). A sphere was placed in the contact point between the two fingers and the blindfolded participants had to say whether they felt one or two stimuli. Stimuli were applied on the affected and less or unaffected side of the PD patients. We found no difference in illusory perception between the PD patients and the controls, nor between the more affected and less/unaffected side, suggesting that Aristotle’s illusion is preserved in PD. The retained tactile illusion in PD and its reduction in focal hand dystonia suggest that the basal ganglia, which are dysfunctional in both PD and dystonia, may not be causally involved in this function. Instead, the level of activation between digits in the somatosensory cortex may be more directly involved. Finally, the similar percentage of illusion in the more affected and less or unaffected body sides indicates that the illusory perception is not influenced by the presence or amount of motor symptoms.

Actual and Illusory Perception in Parkinson's Disease and Dystonia: A Narrative Review

“…When the Aristotle illusion paradigm was applied to PD patients, patients and healthy controls were found to experience the same illusory doubling perception ( 64 ). Hence, tactile perception involving an inter-digit functional relationship appears to be preserved in PD ( 64 ) (Table 1 ).…”

“…Since the Aristotle illusion is associated above all with primary somatosensory cortex activity ( 105 ), the fact that this illusion is selectively altered in focal hand dystonia ( 65 ) highlights the importance of the role played by somatosensory cortical alterations in this form of dystonia though not in other types of dystonia, such as blepharospasm and cervical dystonia ( 65 ), or in PD ( 64 ). Following the same line of reasoning, the reduction in Aristotle's illusion in focal hand dystonia may be interpreted as the behavioral consequence of alterations in the extent of cortical activation induced by tactile stimulation ( 65 ).…”

“…This hypothesis is supported by the fact that finger representation in the primary somatosensory cortex is abnormal in focal hand dystonia ( 111 – 113 ). By contrast, it has been suggested that the functional activation of finger representation in the somatosensory cortex is retained in patients with PD, which may explain why the functional relationship between fingers during tactile perception is preserved in this disease ( 64 ). Moreover, the results of a neurophysiological study ( 105 ) suggest that the mechanisms underlying Aristotle's illusion are related above all to activity in the primary somatosensory cortex rather than in the basal ganglia.…”