Perception alternates between external and internal modes.
Perception depends on a balance between exploration and exploitation. At some moments, the brain needs to stay open to new sensory evidence. At others, it needs to rely on prior knowledge to stabilize perception. Too much exploration makes perception noisy and unstable; too much exploitation makes it rigid and prone to circular inference.
Our research suggests that the brain manages this tradeoff by alternating between two functional modes: an external mode, in which perception is dominated by current sensory input, and an internal mode, in which perception is shaped more strongly by knowledge, memory, and perceptual history. We show that this balance is a basic feature of perception and that it can break down in psychotic states.
Across large-scale behavioral datasets, perception fluctuated between an external, exploratory mode that tracked current input closely and an internal, exploitative mode that relied more strongly on perceptual history. These fluctuations appear to balance sensitivity to change with the need for stable perception under noisy and ambiguous conditions.
Pharmacological manipulation with S-ketamine, which blocks NMDA receptors, shifted the system into a prolonged external mode. This leaves perception more exposed to noise and ambiguity and may contribute to models of the world that drift away from reality. We observed a related mode imbalance in people living with schizophrenia, pointing to a causal link between NMDAR neurotransmission, mode switching, and psychotic symptoms.
During the internal mode, the brain exploits predictive templates to guide perceptual decisions. When those templates are misaligned with reality, they can produce misperceptions or false alarms. GLM-HMM analyses indicate that these templates are amplified in individuals with psychosis-proneness.
We used an integrated approach across:
Our findings suggest that the explore-exploit balance is a core computational principle of perception and a potential biomarker for psychiatric disorders such as schizophrenia. Stabilizing these dynamics could become a path toward therapies that restore cognitive flexibility and more adaptive inference in people living with delusions and hallucinations.