From Dreams to Hallucinations: Jean Lhermitte's Contribution to the Study of Peduncular Hallucinosis and the Dissociation of States

Jean Lhermitte (1877-1959) was one of the pioneers of behavioral neurology, including the field of hallucinations. This article focuses on his work concerning the relationship between hallucinations, sleep, and dreams. From 1910, Lhermitte became interested in sleep and its disorders, particularly narcolepsy and its accompanying symptoms. He also reported on sleep disorders and hallucinations occurring in people with lesions of the diencephalic region ("infundibular syndrome"), and later encephalitis lethargica. In 1922, he described a syndrome of complex, predominantly visual hallucinations in patients with vascular damage to the midbrain, known as peduncular hallucinosis. Twelve historical cases of peduncular hallucinosis, including 10 from Lhermitte, are reviewed. He gave a precise phenomenological description of peduncular hallucinosis, and put forward the hypothesis that the lesion disrupted the anatomy and connections of a center regulating wakefulness and sleep, thus enabling a dissociation of the mechanisms of dream and waking states. Although the pathophysiology of peduncular hallucinosis remains to this day partly obscure, the model of a limited subcortical lesion acting through complex mechanisms and ultimately involving the cortex remains valid. Lhermitte was also a pioneer in characterizing what contemporary sleep specialists call dissociation of states.