Dissociative Drugs & the Nature of Perception
Dissociative Drugs & the Nature of Perception
"Perceptions are constructions, but they’re not constructed ex nihilo."
Links to related written content and the podcast episodes that informed them, can be found at the bottom of this post.
You’re at the dentist getting a tooth pulled. Before the tooth is yanked, you will need a local anesthetic injected into your gums. You’re squeamish about needles, so the dentist has you breathe in a mixture of oxygen and nitrous oxide (N2O, aka “laughing gas”). Within a couple of inhalations, you’re carefree—completely detached from your anxieties.
You were awake for all of it—the jab of novocaine into your gums, molars getting pulled by force, the bright lights. It was all happening and even vaguely amusing. Somehow, it didn’t feel like it was happening to you. When the mask was removed, a few breaths was enough to rekindle your familiar sense of identity. You sort of remember what happened. Like waking from a dream, the details slipped away as you recalled your plans for the day.
What happened?
Dissociative Drugs & the NMDA Receptor
Nitrous oxide is a dissociative drug. Depending on the dose, these substances can induce euphoria, dissociation, or anesthesia (you experienced the first two at the dentist).
NMDA receptors are a key molecular target of dissociative drugs. They are a type of ion channel widespread throughout the brain. When open, these channels allow electrochemical energy to flow. Without going into excruciating detail, let’s consider a special property they have.
NMDA receptors are coincidence detectors. Unlike other ion channels, the NMDA receptor only opens up when a neuron receives multiple input signals at the same time. This helps neurons encode when something very exciting, and worth remembering, has taken place.
When simultaneous inputs cause NMDA receptors to open, molecular processes required for neuroplasticity are triggered within neurons. For example, a neuron might start making extra proteins, using them to build new connections or strengthen existing ones.
Dissociative drugs block NMDA receptors. Because these receptors are so widespread, this can have consequences for everything from perception to memory. It’s why you were able to remain awake while your tooth was being pulled, yet were “not all there.”
Ketamine is another dissociative drug. It has a long history of use as a medical anesthetic—take enough of it and you’ll be knocked out. More recently, it has become famous for inducing rapid-onset antidepressant effects when given at low doses (see M&M #29).
In clinical settings, ketamine is usually given by IV injection. When taken recreationally, insufflated through the nose as a powder or nasal spray. At medium doses, this leads to inebriation, not unlike alcohol intoxication. At higher doses, a dissociative state called the “K-hole” is experienced. Most find it baffling and hard to describe. You are awake and aware of your surroundings, yet somehow detached from your sense of yourself.
As an example, someone on ketamine might look down at their arm but fail to perceive it as their arm. There’s no visual distortion—you can see clearly—but your own body is now perceived as just another object in the world rather than a part of you.
What is this altered state, exactly? To explore this, let’s consider some observations to help us understand the nature of our normal, sober perception.
Brain Expectations
Dissociative states feel weird and inexplicable because they break associations that are almost never broken. Unless you’ve been in an accident, your arm has always been attached to your torso. Your eyes register data about what it looks like. Your skin registers data about what it feels like. Your will sets it in motion. Each information stream is congruent with the others.
If you touch your left arm with your right hand, you simultaneously perceive the sight and sensation of the touch. This multisensory experience is seamless and mundane, masking a simple fact: the signals from your eye reach your brain before those from your skin. Like a flash of lightning preceding thunder, these are distinct channels of information about the same event, traveling at different speeds. So why don’t we experience a lag—seeing the touch before feeling it—as we do with lightning and thunder?
Because the brain initiated the decision to touch the left arm with the right hand, it “wants” to construct one multisensory experience to represent what it already knows is a singular event. To accomplish this, the visual information flowing in from the eyes is briefly held in memory, “waiting” for the touch information to arrive. Your conscious experience, including the perceived simultaneity of these two sensory streams, is constructed after the brain receives everything it’s expecting.
Consider a common experience with sensory uncertainty: putting your hand under a water faucet when you don’t know whether it’s ice cold or scalding hot. You know what both feel like. Neither is pleasant. One is worse. For a split second, it feels hot and you jerk your hand back before realizing—wait, no—maybe it’s not. You probe again, confirming it’s cold. But it sure did feel hot, didn’t it?
That moment of false perception was your brain “filling in the blank,” which was there due to the time delay between the signals from your eyes and skin. While waiting for the real temperature information to arrive, the brain it had to fill in something. If it didn’t, you would have spent that moment with your hand under the faucet, getting burned. There is adaptive value in having a brain that misperceives reality in this way.
The same idea lurks behind multisensory tricks like this, where the sound you hear is dictated by which words you read:
The point is to recognize that perception is a constructive process. As neuroscientist Anil Seth put it:
The brain is constantly making a best guess about the causes of its sensory signals and calibrating those best guesses using sensory signals from the world. Perception comes from the inside-out, not from the outside-in. —Anil Seth, M&M #44
It’s important to emphasize that this perspective is not solipsistic—it doesn’t deny that there’s an external reality imposing constraints on the structure of your conscious experience. Perceptions are constructions, but they’re not constructed ex nihilo. Outside-in constraints flow in through our sense organs. The importance of these constraints can be experienced directly—it’s the difference between waking and dream consciousness. Your dreams reflect how the brain structures conscious experience in the absence of outside-in sensory constraints.
Once you understand certain things about how the brain structures waking experience, you can hack consciousness to induce striking sensory illusions…
Join the community: have you ever experienced a dissociative state? If so, share your experience in the comments.
To learn more about the topics covered in this essay, try these episodes of the Mind & Matter podcast:
Lisa Monteggia: Ketamine, SSRIs, Depression, Psychiatric Disorders & the Brain | #29
Anil Seth: Consciousness, Perception, Hallucinations, Psychedelics, Selfhood, Neuroscience & "Being You" | #44
Karl Deisseroth: Psychiatry, Autism, Anxiety, Depression, Dissociation & "Projections: A Story of Human Emotions" | #35
Read other parts of the ‘What is Psychological Identity?’ series: