Perception normally balances external sensory signals with internal predictions based on prior knowledge. In a double-blind, placebo-controlled, cross-over experiment with healthy participants, the N-methyl-D-aspartate receptor (NMDAR) antagonist S-ketamine shifted perception toward the external mode, favoring sensory input over prior knowledge. A case-control study found that people with paranoid schizophrenia, a condition linked to NMDAR hypofunction, also spend more time in the external mode. This NMDAR-dependent shift suggests that schizophrenia symptoms may arise from recurring disconnections between perception and prior knowledge about the world.
In communities where hearing voices is common and expected, people are taught cultural models to identify and explain voice-like events. These communities share common features: they teach discernment to accurately identify the legitimate voice of a spirit or being, and they use two methods to enable experiencing spirits—trained attention to inner experience and repeated speech to the invisible other. The ability to hear spirits becomes more skilled with practice, and what is heard becomes clearer over time. Despite shared cultural ideas, individual variation in experience exists, some possibly reflecting psychotic process. Voice-like events may be shaped by cognitive expectation, trained practice, and an experiential pathway.