Perception

Perception is the brain's process of constructing a coherent experience from raw sensory data — turning wavelengths into colors, pressure waves into sound, and proprioceptive signals into a felt sense of the body in space. It is not a passive recording of the external world; it is an active interpretation, continuously shaped by expectation, memory, and attention.

This constructive character is what makes perception so sensitive to psychoactive substances. The reality any experience feels like is a model built by the nervous system, not a direct readout of the physical world. When that model is altered — by a compound, a practice, or a shift in brain state — what changes is not the world itself but the process generating it.

How it works · its role

Perception begins with sensory receptors detecting physical signals and encoding them as electrical impulses. These travel along dedicated pathways to specialized cortical regions: the visual cortex reconstructs spatial scenes, the auditory cortex parses sequences into sound and speech, and somatosensory cortex maps touch and position onto a body schema.

At every stage the brain applies prior knowledge — predictions about what incoming signals probably mean. This predictive processing means perception is always partly top-down: the brain's expectations shape what is registered as much as the raw incoming data does. Attention acts as a gate, amplifying certain streams and suppressing others before they reach awareness.

Relevance to substances & effects

Psychoactive substances alter perception at several points in this chain. Serotonergic psychedelics — including LSD, psilocybin, and mescaline — act primarily at the 5-HT₂A receptor in cortical neurons, disrupting the balance between incoming sensory data and the brain's top-down predictions. The result is heightened sensory intensity, novel pattern formation, and the dissolution of ordinary object boundaries.

Dissociatives such as ketamine block NMDA receptors, degrading the integration of sensory signals with bodily context and producing a characteristic detachment from one's own perceptions. Cannabinoids act through the endocannabinoid system and tend to alter time perception and sensory salience more than they produce frank hallucinations. Stimulants can sharpen attention and amplify sensory vividness, though at high doses or in prolonged use the same class can produce paranoid misinterpretation of sensory input.

Because perception is modular — vision, hearing, time sense, and body sense each follow partly independent pathways — substances tend to produce characteristic patterns of distortion rather than uniform changes. This is why effect records on these pages distinguish visual distortions, auditory changes, and time distortions as separate entries, even when a single compound is responsible for all of them.