The corpus callosum, a brain structure connecting the two hemispheres, was thicker in the isthmus region among 22 ayahuasca users compared to 22 matched controls. A positive correlation was observed between callosal thickness in the rostral body and the number of past ayahuasca sessions, though neither finding survived correction for multiple comparisons. No brain region was thicker in controls than in ayahuasca users, and no region was negatively linked to ayahuasca use. This provides preliminary evidence of an association between ayahuasca use and callosal structure, but replication with larger samples and longitudinal designs is needed.
In healthy men, a thinner posterior cingulate cortex (PCC) is linked to a stronger feeling of disembodiment after a low dose of ketamine, a drug that can rapidly relieve depression. The study measured cortical thickness in two brain regions—the PCC and the pregenual anterior cingulate cortex (pgACC)—and found that only PCC thickness correlated with the altered sense of self (disembodiment). No such link appeared for the pgACC. These results suggest the PCC plays a key role in ketamine's effects on self-experience, a feature shared with other fast-acting antidepressants that also produce psychedelic-like effects.