Summary: Researchers introduce “apical hypercontextualisation”—a bottom-up neurobiological mechanism explaining how psychedelics (psilocybin, LSD, DMT, mescaline) transform perception and cognition.
Moving beyond macro-scale whole-brain neuroimaging, the framework focuses on the cellular biophysics of layer V cortical pyramidal neurons, which express high concentrations of serotonin 2A receptors along their apical dendrites.
Key Facts
- Primary Receptor Site: Psychedelic action hinges on $5\text{-HT}_{2\text{A}}$ receptors, which are densely localized on the apical dendrites of layer V pyramidal neurons in the cerebral cortex.
- Dual Dendritic Processing: Basal dendrites process direct, local, feature-specific “what is right in front of me” inputs, while apical dendrites gather remote, contextual signals, expectations, and memories from distant cortical regions and the thalamus.
- Shift to Associative Signals: 5-HT2A receptor agonism skews layer V output toward the apical compartment, causing neurons to fire primarily based on relationships and context rather than raw sensory inputs.
- Explaining Whole-Brain Hyper-Connectivity: Because layer V pyramidal cells are the principal output channels of the cortex, broadcasting apical-driven signals creates widespread communication across normally segregated brain networks.
- Phenomenological Integration: Apical hypercontextualisation explains hallmark psychedelic experiences, including synesthesia, loosening of mental boundaries, heightened sensitivity to “set and setting,” and metaphorical or dream-like thought processes.
Source: Estonia Research Council
Psychedelics like psilocybin (from magic mushrooms), LSD, DMT and mescaline profoundly alter how we perceive, feel and think. After decades of neglect, they are again the subject of serious scientific inquiry, largely because of their promise for treating depression, anxiety, addiction and other conditions.
But this therapeutic promise has run ahead of a more basic understanding: while a great deal of attention has gone to whether psychedelics help treat mental disorders, far less has gone to what they actually do in a general sense — the fundamental way they change perception, thought and consciousness. Only by understanding these basic mechanisms can we ever make sense of the therapeutic effects.
There is also a gap in how that basic question has been approached. Most attempts to explain the mechanisms of psychedelics have been pitched at the level of whole-brain neuroimaging — describing how activity across large networks reorganises during a trip. But neuroimaging only ever captures aggregate activity: the summed behaviour of millions of cells at once, not what the individual neurons, receptors and dendrites underneath are actually doing.
The aim of the new framework published in Neuroscience & Biobehavioral Reviews is to build the explanation from the bottom up, starting with cellular neurobiology and electrophysiology. Thus, the framework starts with the individual neurons, receptors and dendrites that psychedelics actually act on.
This matters because a picture grounded in cells can connect the pharmacology of the drug to the lived experience in a way that network maps alone cannot. From this cellular foundation, the authors argue that psychedelics have a single unifying effect, which is called apical hypercontextualisation.
Almost everything about the effects of psychedelics starts at one receptor: the serotonin 2A receptor, or 5-HT2A. There is near consensus in the field that the bulk of psychedelic effects depend on this receptor. Block it, and the psychedelic effects are blocked; the intensity of the trip even tracks how many of these receptors the drug occupies.
Researchers agree on that, but the new framework tries to understand how exactly the effect on receptors is linked to global effects seen at the level of brain imaging or cognition.
Crucially, the 5-HT2A receptor is not scattered randomly in the brain. It is most abundant in the cortex, and especially on one type of cell: the layer V pyramidal neuron. These are large, elaborately branched cells that act as the brain’s key integrators and as the main output stream from the cortex to deeper structures. Because of these properties, they have long been considered central to consciousness itself.
Here is the part worth slowing down on. A layer V pyramidal neuron has, in effect, two very different sets of branches, and they do different jobs. The basal dendrites, near the cell body, mostly receive local input — the direct, feature-by-feature “what is right in front of me” signal.
The apical dendrites are different. They form a long trunk that reaches up and branches into a tuft in the topmost layer of the cortex, and they gather input from far away: distant cortical regions and the thalamus. That input is not object-based or feature-based — it is contextual and associative.
A useful way to picture the difference is this. The basal branches carry the more direct, local signal of the stimulus as it arrives. But that signal, on its own, means very little. The apical branches supply everything the stimulus is bound up with: the wider setting, memories, expectations, and its links to other things active in the mind.
And this is the key idea behind contextualisation — a mental object is not first defined by itself and then related to other things. What it is comes from how it relates. The apical compartment is where those relations live, and it has even been suggested to set the boundaries between one mental object and the next.
And this is exactly where the 5-HT2A receptor sits most densely. The main target of psychedelics is the very part of the neuron responsible for context and relation.
Now we can much better understand what psychedelics do. They tip these neurons toward their apical, context-gathering side. Normally, a layer V neuron fires based on a balance between its direct, local input and its wider contextual input. Psychedelics, acting through the 5-HT2A receptor, weaken the local side and strengthen the contextual side so the cell is driven more by relations and associations than by the raw stimulus in front of it.
Because these neurons are among the brain’s main output cells, feeding into the thalamus and back out across the cortex, tilting them this way has a large downstream consequence: the contextual signal gets broadcast widely, spreading across many regions instead of staying local.
This is exactly what whole-brain neuroimaging has been showing all along — psychedelics make brain activity more global, with normally separate networks talking to each other far more. The contribution of the present framework is to show where that global shift comes from at the level of the cell.
The bigger picture is that psychedelics do not alter the direct neural representation of a stimulus so much as they amplify the relations between representations. They do not enhance the raw sensory information itself; they enhance the impression that information makes and how strongly it connects to everything else. A signal that would normally stay in its lane instead spreads, mingles and gets contextualised by whatever else is active. The boundaries between mental objects loosen.
This one idea ties together the classic features of the psychedelic experience. Visually, it explains why psychedelics can leave a lone stimulus intact but disrupt how stimuli relate — strengthening context-dependent illusions, warping the edges between objects, and impairing moving or complex scenes more than simple static ones.
In cognition, it explains the flood of remote associations, metaphorical and dream-like thinking, and unexpected insight — the mind making connections it usually cannot reach. It even accounts for why “set and setting” matter so much: if subtle internal representations are amplified and broadcast, then a person’s mindset and surroundings can steer the whole experience.
Grounding psychedelic action in this cellular machinery is an attempt to explain psychedelics in properly neurobiological terms: from the physical, biological material the drugs act on. It links receptor pharmacology, dendritic physiology and lived experience in one framework, and it explains, rather than merely restates, what the neuroimaging has been showing — with direct implications for how these drugs might reset rigid patterns of thought in mental illness. Ultimately, psychedelics do not change what we represent, but how our representations relate to one another.
Key Questions Answered:
A: Macro-level neuroimaging maps aggregate brain-wide network reorganization, describing where changes occur across millions of cells. Apical hypercontextualisation provides a bottom-up cellular explanation, showing how 5-HT 2A receptor activation on specific dendritic compartments of layer V neurons alters individual cell firing to drive those global network shifts.
A: Basal dendrites gather local, feedforward sensory information, the raw, feature-based details of an immediate stimulus. Apical dendrites extend into upper cortical layers to gather top-down feedback, including memories, expectations, and relationships to other mental states, supplying the wider context necessary to interpret sensory input.
A: Because psychedelics amplify the apical, context-gathering side of cortical neurons, internal expectations (“set”) and external surroundings (“setting”) are not merely background variables. They become actively amplified and integrated into primary perception, guiding the direction of the therapeutic experience and enabling the resetting of rigid cognitive patterns.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this psychedelics and neuroscience research news
Author: Merilin Reede
Source: Estonian Research Council
Contact: Merilin Reede – Estonian Research Council
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Cellular mechanisms of serotonergic psychedelics – apical hypercontextualisation” by Karl Kristjan Kaup, Javier Hidalgo Jiménez, Jaan Aru. Neuroscience & Biobehavioral Reviews
DOI:10.1016/j.neubiorev.2026.106876
Abstract
Cellular mechanisms of serotonergic psychedelics – apical hypercontextualisation
Classical serotonergic psychedelics primarily exert their profound effects through agonism at the serotonin 2A (5-HT2A) receptor, which is abundantly expressed in many cortical regions, particularly in layer V pyramidal neurons of associative and visual areas.
At the cellular level, these receptors are predominantly localized postsynaptically on the soma and apical dendrites. This review synthesizes evidence from distinct subfields of psychedelic cognitive neuroscience to illuminate how 5-HT2A agonism at apical dendrites of layer V pyramidal neurons disrupts the usual boundaries of conscious mental representations, boosting their interaction with surrounding contextual influences.
We propose “apical hypercontextualisation” as a unifying hypothesis, whereby enhanced apical signaling amplifies relational processing over direct stimulus representation.
Finally, we overview key psychedelic phenomenological features – such as perceptual distortions, associative cognition, and self-alterations – demonstrating how they emerge from contextual amplification, with implications for therapeutic mechanisms in mental health disorders.
