This theory-building pre-print (2021) provides a mechanistic explanation of how psychedelic and psychotic hallucinations are enacted through ascending and descending circular message-passing networks. Psychedelic phenomena are proposedly related to the amplification descending top-down predictions that over-integrate sensory information into multimodal hallucinations, whereas psychotic states entail the amplification of bottom-up sensory information that becomes overinterpreted as delusions and unimodal hallucinations.
Abstract
“Psychedelics are known to distort perception and induce visual and multimodal hallucinations as well as synaesthesia. This is in contradiction with the high prevalence of distressing voices in schizophrenia. Here we introduce a unifying account of unimodal and multimodal erroneous percepts based on circular inference. We show that amplification of top-down predictions (descending loops) leads to an excessive reliance on priors and aberrant levels of integration of the sensory representations, resulting in crossmodal percepts and stronger illusions. By contrast, amplification of bottom-up information (ascending loops) results in overinterpretation of unreliable sensory inputs and high levels of segregation between sensory modalities, bringing about unimodal hallucinations and reduced vulnerability to illusions. We delineate a canonical microcircuit in which layer-specific inhibition controls the propagation of information across hierarchical levels: inhibitory interneurons in the deep layers exert control over priors, removing descending loops. Conversely, inhibition in the supragranular layers counterbalances the effects of the ascending loops. Overall, we put forward a multiscale and transnosographic account of psychosis with important theoretical, conceptual and clinical implications.“
Authors: Pantelis Leptourgos, Vincent Bouttier, Sophie Denève & Renaud Jardri
Summary of From hallucinations to synaesthesia
Hallucinations are percepts occurring while the person is awake and without corresponding external stimulation of the relevant sensory organ. They can also be induced by psychotomimetic drugs. Classic psychedelics include naturally occurring chemicals such as mescaline, psilocybin and DMT, as well as synthetic compounds such as lysergic acid diethylamide (LSD). Various cultures have used these drugs for more than a century to improve physical performance or gain spiritual guidance. All classic psychedelics are serotonergic agonists with a high affinity for 5HT2A receptors. These drugs increase activity in a variety of cortical regions and cause profound changes in the functional connectivity of the default-mode network and within/between resting-state networks and task-positive networks.
Psychedelics induce profound changes in people who consume them, including perceptual abnormalities, emotional alterations and cognitive alterations, as well as mystical experiences and a diminished sense of self.