Summary: Researchers propose “apical hypercontextualisation,” a cellular-level neurobiological mechanism that explains how classical serotonergic psychedelics—such as psilocybin, LSD, DMT and mescaline—reshape perception, cognition and conscious experience.
Moving beyond whole-brain neuroimaging, this framework focuses on the biophysics of layer V cortical pyramidal neurons, which concentrate serotonin 2A (5-HT2A) receptors along their apical dendrites and play a central role in integrating and broadcasting cortical signals.
Key Facts
- Primary receptor locus: The core psychedelic effects depend on 5-HT2A receptors, which are especially dense on the apical dendrites of layer V pyramidal neurons in the cerebral cortex.
- Two distinct dendritic inputs: Basal dendrites mainly receive local, feedforward sensory details—“what is directly present”—while apical dendrites collect top‑down feedback, contextual information, expectations and associative inputs from distant cortical areas and the thalamus.
- Bias toward contextual signaling: Agonism at 5-HT2A receptors shifts layer V neuron output toward apical-driven activity, causing cells to respond more to relationships, associations and context than to raw feedforward sensory signals.
- Mechanism for global connectivity: Because layer V pyramidal cells are principal cortical output neurons, promoting apical signal dominance causes contextual information to be broadcast widely, increasing communication across normally segregated brain networks.
- Phenomenology explained: Apical hypercontextualisation links core psychedelic phenomena—synesthesia, blurred mental boundaries, heightened sensitivity to set and setting, enhanced metaphorical or dream-like thinking and sudden, remote associations—to amplified contextual processing at the cellular level.
Source: Estonia Research Council
Psychedelics such as psilocybin (from magic mushrooms), LSD, DMT and mescaline profoundly transform perception, emotion and cognition. After years of limited attention, these substances are now the subject of renewed scientific research because of their therapeutic potential for conditions like depression, anxiety and addiction.
Despite growing clinical interest, a fundamental question remains understudied: what do these drugs do to perception and cognition in general? Most research has emphasized whether psychedelics can treat disorders, while fewer efforts have clarified their core neurobiological action and how that produces the characteristic experiences people report.
There is also a methodological gap. Many accounts rely on macro-scale neuroimaging to describe how large-scale brain networks reorganize during a psychedelic state. Neuroimaging is invaluable for revealing whole-brain patterns, but it measures aggregate activity—the summed output of millions of neurons—and does not directly show the cellular and dendritic processes responsible for those changes.
The new framework, published in Neuroscience & Biobehavioral Reviews, builds the explanation from the bottom up. It starts with cellular neurobiology and electrophysiology—examining individual neurons, their dendritic compartments and the receptors where psychedelics act—to bridge pharmacology and subjective experience.
This cellular perspective helps explain how receptor-level pharmacology links to the global patterns seen in imaging and cognition. The unifying concept offered by the authors is apical hypercontextualisation: psychedelics amplify apical dendritic signaling relative to basal input, increasing the relational and contextual influence on cortical output.
Nearly all classical psychedelic effects trace back to one receptor: the serotonin 2A receptor (5-HT2A). There is strong consensus that 5-HT2A agonism is necessary for the characteristic perceptual and cognitive effects—blocking the receptor blocks the subjective effects, and the intensity of a trip correlates with receptor occupancy.
Importantly, 5-HT2A receptors are concentrated in cortex, especially on layer V pyramidal neurons—large, highly branched cells that integrate information and form the main cortical output to subcortical structures. Their anatomy and connectivity position them as critical nodes for conscious processing.
Layer V pyramidal neurons effectively have two processing streams. Basal dendrites, located close to the soma, primarily receive local, feedforward sensory signals—detail-rich representations of immediate stimuli. Apical dendrites extend upward into the superficial cortical layers and collect top‑down contextual input: expectations, memories and associative signals that give a stimulus meaning within a broader mental context.
Where basal input supplies the direct sensory content, apical input supplies the relations that define what a perception is. The apical compartment contributes the context that binds sensory fragments into coherent mental objects and helps delineate boundaries between objects in consciousness.
Psychedelics target this very machinery. By enhancing 5-HT2A signaling on apical dendrites, they tip the balance of layer V neuron firing toward contextual and associative inputs. The result is a cortex that emphasizes relations and associations over isolated sensory features, so signals that normally remain local become amplified and broadcast across the brain.
This cellular shift provides a mechanistic account for neuroimaging observations showing increased global connectivity under psychedelics—normally segregated networks interact more when apical-driven contextual signals are propagated by layer V outputs.
Conceptually, psychedelics do not simply increase raw sensory detail; they increase how strongly representations connect to one another. Perceptual elements mingle, contextual influences dominate, and the boundaries between mental objects loosen. That explains why visual scenes can feel warped, why isolated stimuli persist while their relations shift, and why moving or complex scenes are more disrupted than simple static ones.
Cognitively, this mechanism accounts for surges of remote association, metaphorical thinking and dream-like cognition—mental links that are normally weak become prominent. It also clarifies why a person’s mindset and environment—set and setting—so powerfully shape psychedelic experiences: internal expectations and external context gain amplified influence and steer perception and insight.
Grounding psychedelic effects in a specific cellular mechanism links receptor pharmacology, dendritic physiology and subjective phenomenology. Apical hypercontextualisation explains how 5-HT2A agonism can reorganize global brain dynamics and suggests routes by which psychedelics may help reset rigid cognitive patterns in mental illness: not by changing what we represent, but by changing how representations relate to one another.
Key Questions Answered:
A: Macro-level neuroimaging describes where brain-wide networks reorganize by measuring aggregated activity. Apical hypercontextualisation provides a bottom-up cellular explanation for how 5-HT2A activation on apical dendrites of layer V neurons alters individual cell firing patterns in ways that produce those global network changes.
A: Basal dendrites handle local, feedforward sensory information—the immediate, feature-based properties of a stimulus. Apical dendrites receive top‑down feedback and associative inputs—memories, expectations and contextual relations that give sensory signals broader meaning.
A: Because psychedelics amplify apical contextual processing, internal mindset (“set”) and external environment (“setting”) become amplified contributors to perception and cognition; they can therefore direct and shape therapeutic experiences and support the reorganization of entrenched cognitive patterns.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full by the editorial team.
- Additional context was added by staff to clarify neuroscientific concepts.
About this psychedelics and neuroscience research news
Author: Merilin Reede
Source: Estonian Research Council
Contact: Merilin Reede – Estonian Research Council
Image: Image credited to Neuroscience News
Original Research: Open access. “Cellular mechanisms of serotonergic psychedelics – apical hypercontextualisation” by Karl Kristjan Kaup, Javier Hidalgo Jiménez and Jaan Aru, published in Neuroscience & Biobehavioral Reviews. DOI: 10.1016/j.neubiorev.2026.106876
Abstract
Cellular mechanisms of serotonergic psychedelics – apical hypercontextualisation
Classical serotonergic psychedelics exert their principal effects through agonism at the serotonin 2A (5-HT2A) receptor, which is highly expressed across many cortical regions and is particularly concentrated on layer V pyramidal neurons in associative and visual areas. At the cellular level, these receptors localize postsynaptically on the soma and along apical dendrites. This review synthesizes findings across subfields of psychedelic cognitive neuroscience to show how 5-HT2A agonism at apical dendrites disrupts the usual boundaries of conscious mental representations by amplifying contextual interactions between representations.
The authors propose the unifying hypothesis of “apical hypercontextualisation,” whereby enhanced apical signaling increases relational processing relative to direct stimulus representation. They map how this mechanism gives rise to core psychedelic phenomenology—perceptual distortions, associative cognition and alterations of self—and discuss implications for therapeutic mechanisms in mental health disorders.