How LSD reshapes brain circuitry to blur the lines between perception and thought

A recent small study analyzes how the psychedelic drug LSD reshapes brain activity. The research shows that the substance boosts widespread neural synchronization while blurring the boundaries between sensory perception and abstract thought. Through computer modeling and brain scans, researchers found that LSD alters the balance of excitement and inhibition in specific brain circuits, potentially pulling the mind out of entrenched patterns. The findings were published in PLOS Computational Biology.

Psychedelics are seeing a resurgence in psychiatric research. Clinical trials suggest these substances hold potential for assisting in the treatment of conditions like depression, anxiety, and addiction. Mental health disorders often involve rigid, stubborn patterns of thinking. Psychedelic compounds seem to induce the opposite effect, introducing temporary flexibility to brain activity.

To understand how a drug can drastically alter human consciousness, scientists look at how different networks function in the brain. Even when a person is resting, regions of the brain constantly communicate. Distinct networks process everything from simple sensory inputs, like touch and sight, to abstract cognitive tasks, like self-reflection and attention.

Healthy brain function relies on a delicate seesaw effect known as the excitatory and inhibitory balance. Excitatory neurons act like a biological accelerator, sending electrical signals that encourage other neurons to fire. Inhibitory neurons act like the brakes, preventing overactivity and keeping the system organized.

Lingyu Zhang, a researcher at the Beijing University of Posts and Telecommunications, alongside colleagues across several other institutions, wanted to map how this balance changes under the influence of LSD. Measuring the exact chemical equilibrium directly in a living human brain is incredibly difficult with current noninvasive technology. To get around this limitation, the research team turned to computational modeling coupled with neuroimaging data.

The researchers utilized an existing data set from a small study of 15 healthy adults. During the original experiment, participants underwent functional magnetic resonance imaging. This type of brain scan measures changes in blood flow over time, allowing researchers to detect which areas of the brain are highly active. Each person received two scans on separate days, one occurring after an injection of a placebo, and the other occurring after an intravenous dose of LSD.

Zhang and the research team took this scanning data and looked for patterns of synchronization. They wanted to see if the rhythmic waves of activity in different brain regions peaked and dipped at the exact same moment. Phase synchronization occurs when multiple regions align their rhythms. The researchers grouped these synchronized moments together to categorize distinct brain states.

Under the placebo condition, the brain hopped smoothly between various modular states. Some of these states were dedicated purely to processing sensory information. Other states were tied strictly to the default mode network, which is a group of associative brain regions dealing with mind wandering, memories, and an individual’s sense of self.

When participants took LSD, their brain dynamics shifted in a profound manner. The researchers found that LSD enhanced global brain synchrony. Instead of operating in segregated, independent networks, the entire brain was much more likely to fire together in a unified state.

This highly synchronized global state seemed to act like a magnet, drawing the brain away from its compartmentalized routines. The probability of the brain transitioning from this unified state back into specialized cognitive control networks was markedly reduced. Due to the limited sample size, some minute differences in transition probabilities between minor states were not statistically significant. However, the overarching trend toward increased global synchrony remained visible.

To understand the hidden machinery behind this shift, the researchers built a dynamic computer simulation. They combined the brain scan data with detailed maps of structural connections in the human brain. This allowed the team to calculate the estimated ratio of excitation to inhibition in tiny neural circuits across the entire cerebral cortex.

The computer model revealed that LSD alters the brain’s internal chemical balance, doing so unevenly. The drug affects regions responsible for basic sensory perception quite differently than it affects regions responsible for abstract thought.

In areas of the brain related to sensory and motor processing, the model showed a sharp drop in the excitatory-to-inhibitory ratio. The biological brakes became much stronger in these regions. This chemical shift suppresses how tenaciously the brain anchors itself to external sensory inputs.

Conversely, the model estimated that the activation ratio increased in associative brain regions. Taking off the brakes in these abstract processing centers could make neurons uncharacteristically active. The researchers suggest this neural remodeling fosters cognitive flexibility, allowing participants to experience intense introspection.

By turning down sensory areas and dialing up abstract areas, LSD essentially levels the playing field between the two. The strict boundaries that usually separate concrete perception from abstract cognition begin to dissolve. This physiological mechanism aligns closely with the subjective experiences often reported by users of psychedelics, such as a dissolving sense of self and an altered perception of the world.

The team also discovered that the sensory and motor cortices might serve as primary drivers for these brain-wide changes. The suppression of these early sensory pathways appears to cascade upward. This disruption travels up the hierarchy of the brain, scattering the higher-order networks that typically impose order on human cognition.

Psychedelics are known to bind to a specific type of serotonin receptor in the brain, known as the 5-HT2A receptor. This receptor triggers chemical chain reactions that can alter the release of glutamate, which serves as the brain’s primary excitatory neurotransmitter. The researchers noted that their computer model’s map of altered excitement and inhibition closely overlapped with known anatomical maps of serotonin and glutamate receptors.

This theoretical overlap hints at the biological mechanism at play. The LSD binds to serotonin receptors, which in turn manipulate the excitatory neurotransmitters at localized points in the sensory cortex. The ripple effect ultimately changes the entire brain’s operational rhythm, forcing it out of rigid habits.

The authors pointed out several limitations to their analysis that warrant caution. Because this original data set came from a small study, larger clinical trials will be necessary to confirm the results. Expanding the participant pool would help ensure the findings apply reliably to the broader population.

The research focused exclusively on the cerebral cortex, which is the brain’s wrinkled outer layer. The computational models did not include deeper subcortical structures like the thalamus. The thalamus acts as a major relay station for sensory information. Previous research suggests this region plays a vital role in how hallucinogens affect the mind, meaning future studies will need to incorporate it to provide a complete picture.

The study also did not match the brain scanning data with subjective psychological questionnaires from the participants. The researchers noted that future investigations should explore how these measured changes in brain connectivity correlate with a person’s specific emotional or perceptual experiences. Learning exactly how the loss of sensory anchoring matches an individual’s reported hallucinations would bring science one step closer to practical therapeutic applications.

The study, “Lysergic acid diethylamide-derived excitatory/inhibitory ratio change enhances global synchrony in functional brain dynamics,” was authored by Lingyu Zhang, Weiyang Shi, Ziyang Zhao, Zhichao Wang, Congying Chu, Bokai Zhao, Jiaqi Zhang, Qianhui Liu, Yueheng Lan, and Tianzi Jiang.