According to Meynert they result from automatic activity of subcortical cerebral centers, which are no longer inhibited by the cerebral cortex as they are in the normal state.
The primary cause of hallucinations would thus be a suppression of the inhibitory power of the cortex, which is one of the manifestations of cortical paralysis. The hallucination is then the consequence of a supremacy of lower cerebral functions over higher ones.
Finally, according to Tambourini, hallucinations are produced by the automatic activity of a psychosensory projection-center.
Under what conditions does the automatism of the projection-center come into play? Is it under the influence of direct irritation resulting, for instance, from a tumor or from a circumscribed patch of meningitis localized exactly at this center? Such cases have occurred. Serieux2 has observed verbal motor hallucinations in a general paralytic in whose case the autopsy showed a predominance of the lesions of meningo-encephalitis at the level of the lower portion of the left third frontal convolution. The lesion must not, however, be a too destructive one. "Indeed, for a center to be able to produce hallucinations, it is necessary that conditions of integrity be preserved sufficient to permit its activity" (Joffroy).3
1 Cololian. Les hallucinations psycho-motrices verbales dans Valco-olisme. Arch, de Neurol., Nov., 1899.
2 Sur un cas d'hallucination motrice verbale chez une paralytique generate. Bull, de la soc. de med. ment. de Belgique, 1894.
3 Les hallucinations unilaterales
Siebert has also reported a case in which very pronounced hallucinations of the sense of smell persisted for a long time and subsequently disappeared by degrees. At the autopsy the hippocampus was found to be destroyed by a tumor. The author supposes that the hallucinations were caused by irritation of the center in question by the growth, and that they did not cease until this center was destroyed. (Monatschr. fur Psych, u. Neurol, Vol. VI.)
Most frequently, however, the center of projection is not the seat of any demonstrable lesion. It seems, then, that in most cases the hallucinations are the consequence, not of a direct irritation of the psychosensory center itself, but rather of an indirect irritation coming from another portion of the cortex. This explains why hallucinations are always a secondary phenomenon, and why they are but an expression, a reflection of the pathological preoccupations of the patient.
Wernicke has conceived a very ingenious theory of hallucinations, founded upon his general hypothesis of sejunction. By this term he designates a temporary or permanent interruption of the paths followed normally by a nervous impulse. This impulse cannot pass on freely, and accumulates above the point of the lesion like the water in a river above a dam. When this accumulation occurs in a psychosensory projection-center it sets up there a state of abnormal irritation of which the clinical expression is a hallucination.