A Trick Question
The headline promises more than any honest piece of writing can deliver. Neither mystics, shamans, nor neuroscientists hold the definitive answer to where visions “come from.” What we can describe, in reasonable detail, is the biological substrate underlying the psychological effects of compounds like LSD, psilocybin, mescaline, and DMT. It is not enlightenment—it is physiology. And yet, it remains deeply fascinating.
Debating what to call them—hallucinogens, psychedelics, entheogens, visionaries, or any of the more than one hundred proposed labels—is hardly worth the fight. The term hallucinogen has a bad reputation because it implies a “false reality,” yet the core of the experience is not seeing things that do not exist, but a radical transformation in how reality is understood. In a world already shaped by subjective perception, the boundary between the “true” and the “altered” is far blurrier than it seems.
The Old and Elegant Model of the Reducing Valve
The dominant narrative for decades stems from Aldous Huxley and The Doors of Perception. Drawing on an intuition from Henri Bergson, Huxley envisioned the brain as a kind of reducing valve: a mechanism that filters and narrows the onslaught of internal and external stimuli, admitting only what is strictly necessary for biological survival. If that valve were relaxed—he wrote, quoting William Blake—“the world would appear to man as it is, infinite.” Mescaline, in this view, merely loosened the filter, letting in more reality.
The metaphor was so compelling that it leapt from literature into science. When researchers discovered that the thalamus acts as a major sensory gating station, it seemed like the ideal candidate to embody Huxley’s valve. Some scientists updated the theory accordingly, framing the psychedelic experience as a modulation of thalamic gating. The trouble is that an idea does not become true simply because it is elegant. And empirical data soon began pointing in a different direction.
5-HT2A, Glutamate, and the Prefrontal Cortex
Subsequent research—including extensive animal studies—has sketched a different mechanism. When a classic psychedelic enters the brain, it binds to a specific receptor: the 5-HT2A subtype. As far as we know, psychedelics are unique in activating it with such profound behavioral consequences. This activation triggers the release of glutamate, the cortex’s primary excitatory neurotransmitter, which plays a pivotal role in learning, neuroplasticity, and alertness.
The anatomical nuance matters: this release is concentrated in layer V of the prefrontal cortex, the very seat of higher-order cognitive processing and emotional regulation. Neurons in this layer project from the prefrontal cortex to distant cortical areas, establishing a reverberating circuit that translates into rapid, heightened information processing.
The conceptual takeaway is profound: the psychedelic experience appears to be more cognitive than sensory. Rather than a flood of sensory stimuli surging through an open gate, it involves an intense, runaway mode of processing reality. This is no mere academic nuance. If psychedelics are used as models to investigate mental disorders, pinpointing the actual mechanism fundamentally shifts where we look for therapies.
What Ayahuasca Neuroimaging Shows
This is where experimental work with ayahuasca enters the picture. At the Sant Pau Institute of Biomedical Research in Barcelona, a research team led by Dr. Jordi Riba administered freeze-dried ayahuasca—which allows for reliable placebo-controlled conditions—to healthy volunteers and monitored cerebral blood flow using SPECT (single-photon emission computed tomography).
The findings contradicted the thalamic valve hypothesis: following administration, neither the thalamus nor other subcortical structures showed increased activation; instead, activity surged in cortical areas. Specifically, prefrontal regions, the insula, and the cingulate cortex—structures involved in interoceptive awareness, emotional processing, and somatic sensations—lit up, alongside parahippocampal areas tied to memory. The insula, notably, shows greater volume and activation in experienced meditators, while the hippocampus and surrounding regions encode experiences for long-term memory.
When you combine this cortical activation map with the surge in synaptic signaling driven by glutamate, the biological foundation of the torrential mental stream experienced under psychedelics—for better or worse—starts to make sense.
A Critical Perspective
It is important to approach these findings with caution. First, mapping a cerebral mechanism does not “explain” subjective experience: the hard problem of how physical matter gives rise to subjective experience (qualia) remains completely unresolved. Knowing which neurons fire does not tell us why something “feels” a particular way.
Second, these findings come from small sample sizes and techniques with inherent limitations; neuroimaging reveals correlations, not ultimate causes, and scientific models evolve. The trajectory of the “reducing valve” itself—from literary metaphor to physiological dogma—serves as a cautionary tale of how easily an appealing idea can be mistaken for an established fact.
Third, a health consideration: these compounds are neither harmless nor suitable for everyone. Intense visions can be destabilizing, particularly for individuals with psychiatric vulnerabilities, and clinical research is conducted under controlled conditions with medical screening that look nothing like casual use. This article is for educational purposes, not a user guide.
