Trance, an absorptive state with narrowed external awareness used by shamans for insight, was studied with fMRI in 15 experienced shamanic practitioners listening to rhythmic drumming. During trance, three brain regions—posterior cingulate cortex, dorsal anterior cingulate cortex, and left insula/operculum—showed stronger hubs (higher eigenvector centrality). The posterior cingulate cortex, a default network hub for internal thought, coactivated with control-network regions, suggesting amplified internal neural streams. Auditory pathway seeds were less connected, indicating perceptual decoupling from repetitive drumming. This network reconfiguration may support extended internal thought and insight.
Rhythmic flicker light stimulation at 10 Hz reliably induces transient visual hallucinations in healthy people, while arrhythmic flicker does so less. Using fMRI, rhythmic flicker produced stronger activation in higher order visual cortices and selectively increased connectivity between ventroanterior thalamic nuclei and those cortices, compared to arrhythmic control. The strength of this connectivity correlated positively with the subjective intensity of hallucinations. Because the ventroanterior thalamus and higher order visual areas do not receive primary visual inputs, the findings suggest the thalamus coordinates cortical activity to generate hallucinatory experiences, offering insight into pathological hallucinations.
Flicker light stimulation (FLS) induces hyperconnectivity between the lateral geniculate nucleus (LGN) and early visual areas, as well as proximal upstream areas of the ventral visual stream (e.g., hV4, VO1). An exploratory analysis indicates that higher-order thalamic nuclei, such as the anterior and mediodorsal nuclei, are strongly affected by FLS, with connectivity changes to upstream cortical visual areas directly reflecting a frequency-dependent increase in experienced visual phenomena. These findings help identify specific thalamocortical interactions involved in the emergence of visual hallucinations.
An altered state of consciousness (ASC) induced by non-pharmacological Ganzfeld stimulation—where participants are exposed to intense, homogeneous visual and auditory input—progressively decouples the thalamus from the cortex, as measured by fMRI. This decoupling contrasts with psychedelic-induced ASCs, which typically show increased thalamo-cortical coupling. The Ganzfeld ASC also increased eigenvector centrality in core regions of the default mode network (DMN), opposite to the reduced DMN activity seen with psychedelics. These findings suggest an imbalance between sensory bottom-up and internally generated top-down signaling.