Perception alternates between external and internal modes.
Perception involves a tradeoff between exploration — where the brain samples new information — and exploitation — where the brain relies on prior knowledge and expectations. Exploration makes perception sensitive to change but makes it vulnerable to ambiguity and noise. Knowledge about the world can help deal with ambiguity but increases the risk of circular inferences. Adaptive perception thus requires a balance between exploration and exploitation.
Our research shows that the brain solves this problem by alternating between two functional modes: an external mode, where perception is determined by sensory inputs, and an internal mode, where perception is shaped by knowledge and memory. These fluctuations reflect a fundamental cognitive strategy: switching between information-seeking (exploration) and belief-based inference (exploitation). We show that this dynamic balance can break down in psychotic states, offering a mechanistic explanation for hallucinations and delusions.
Analyzing behavioral data from large-scale experiments, we found that perception fluctuates between an external, exploratory mode (high accuracy, driven by current input) and an internal, exploitative mode (strong reliance on perceptual history). These dynamics help balance sensitivity to new information with the ability to have stable and robust perceptual experiences, even when sensory signals are ambiguous and noisy.
Pharmacological manipulation with S-ketamine, which blocks NMDA receptors, shifts the brain into a prolonged external (explorative) mode. This exposes perception to the destabilizing effects of noise and ambiguity and may cause people to build models of the world that don’t align with reality. We replicated the effect of NMDAR antagonism on mode in people living with schizophrenia, a diagnosis often associated with NMDAR hypofunction. These results point to a causal link between NMDAR neurotransmission, mode switching, and psychotic symptoms.
During the internal mode, the brain exploits predictive templates for perceptual decision-making. Predictive templates that don’t align with reality can lead to misperceptions or false alarms — a hallmark of hallucinations. GLM-HMMs 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 like schizophrenia. Stabilizing these dynamics could form the basis for targeted therapies aimed at restoring cognitive flexibility and adaptive perception in people living with delusions and hallucinations.