SherlockMS and the Case of the Rubber Snake

Naturally, I was thinking about cases. Not the ordinary sort where someone steals a ring and then acts surprised when it’s found. No, my thoughts drifted toward the refined cases. The ones where the culprit lives inside the mind, and the evidence appears only as signals.
Then came the knock. Three times. Hesitant. Like someone who knows they’re stealing my time and hopes I won’t notice. A courier handed me an envelope: “Urgent. Research center. Scanner suite. Unexplained fear responses.” I smiled. “Unexplained” is what people say when they can’t be bothered to look closely.

The Crime Scene: A Scanner Room, an Animal and a Strange Calm

I entered the institute the way a judge enters a courtroom: unhurried, but with the unspoken threat that someone might cry in a moment.
They led me to a room with two main characters:

• an MRI/PET setup, groomed like an opera singer
• and a snake in a terrarium, looking like it had seen better days or better prey
  

“We expose participants gradually,” an assistant explained. “Like a miniature exposure therapy. Closer, away, closer again…” “Yes,” I said. “Humans call it therapy. The brain calls it a siege.”
Then came the sentence that turned this into a case: “Some react massively: pupils dilate, heart racing. Others… don’t. And the imaging shows things we interpret differently. Some even claim someone is… opioid-like manipulating the response.”
I raised an eyebrow. “You mean: someone covered fear with a pharmacological blanket?”  “Exactly!” I sighed. “How creative. And how wrong. Show me your traces.”

First Trace: Pupils Rarely Lie

I asked to see the pupil data. When the snake was near: clear dilation. When it was far: less. With a harmless control object: almost nothing.
“Look,” I said, tapping the curves, “this is not theater. This is the autonomic nervous system in its pure form. No debate. Pupils are like bad actors: they cannot lie convincingly.” The researchers nodded enthusiastically because nodding is easier than thinking.

Second Trace: The Brain Prepares, Not Just Panics

Then came the fMRI images. And here it became delightful.
As the snake approached, you didn’t just see “fear.” You saw preparation:

• brainstem defense circuits: ancient, fast, uncompromising
• thalamus: the bouncer deciding what gets in
  
• attention networks: danger has VIP status
• and deliciously underestimated motor and premotor regions: the brain rehearses the sprint before the person even knows they want to run
  

I said aloud: “This isn’t ‘I feel afraid.’ This is ‘I’m about to leave.’” A young PhD student whispered, “But… shouldn’t the amygdala” “Ah,” I interrupted, “the famous amygdala, mascot of every PowerPoint slide. Important, yes. But I strongly advise against treating emotions like a badly sorted toolbox: hammer = fear, screwdriver = joy. The brain is subtler than your textbook graphics.”

Third Trace: The Opioid Alibi Trick

Now for the real scandal. They had run a PET scan using a ligand that binds to μ-opioid receptors; I like to call them the brain’s “calm-and-bond switches.” Not because they always calm (they don’t), but because humans enjoy believing that.
And here was the oddity: during threat, the system showed more ligand binding to these receptors.
One part of the team said, “Aha! More opioid activity! The body is flooding itself with endorphins!” I laughed so quietly it was even more humiliating. “No. That’s the classic error. A beginner’s error. The kind that ruins careers in London.”
I explained it in terms even a stressed researcher could digest:

• If lots of endogenous opioids are released, receptors are already occupied → the PET ligand finds fewer free sites.
  
• If fewer endogenous opioids are around, more receptors are “available” → the PET ligand binds more strongly.

“In other words,” I said, “during acute threat the opioid calming system withdraws rather than blankets you. Not because the brain is foolish, but because it reallocates resources: alertness, muscle tone, focus. There will be time for ‘cuddle mode’ later.” The PhD student looked as if I had just stolen her childhood.

The Culprit: Not the Snake. Not the Participant. But Time

But why did some react strongly and others weakly?
Here it becomes elegant: people differ in baseline opioid system “settings”, call it baseline tone. Some have more available μ-receptors, some fewer.
And here comes the punchline, which I personally enjoyed:
•    Those with higher receptor availability show stronger acute brain responses to threat (yes; stronger, not weaker).
•    And that baseline also predicts how responses change with repeated exposure: the brain adapts, habituates, but not equally in everyone.
“It’s like two people in the same city hearing the same alarm,” I said. “One reacts immediately and then quickly learns to tune the sirens out. The other responds flatly and yet stays tense inside. You mustn’t only read the peak; you must read the dynamics.”

The Twist: The Snake Was… Not Real

I walked to the terrarium, leaned in, studied the snake, and smiled. “You know,” I said, “I appreciate your safety measures. Very… practical.” “What do you mean?”
I gestured toward the setup, eye contact sharp as a blade. “You didn’t use a real snake in the scanner, did you?”
Silence. A researcher cleared his throat. “Well… for practical reasons”
“Never mind,” I cut in. “It’s brilliant. And it’s the finest part of this case: The fear was real, even though the snake wasn’t.” I let the words hang in the room like the ozone after a thunderstorm.
“The brain,” I said, “doesn’t operate on ‘real’ versus ‘not real.’ It operates on meaning, proximity, loss of control. A rubber tube is enough, if it appears at the right moment, at the right distance.”

Closing: I Solve the Case. And the Brain Remains Astonishing

In the end, the solution was crystal clear:

• Acute threat activates not only “fear,” but defense, attention, and motor preparation.
  
• The μ-opioid system behaves less like a warm blanket and more like a regulator that steps back in the heat of the moment; so the organism can run at maximum sharpness.
  
• Human differences live not only in “how strong,” but in “how it changes with repetition.”

I left the institute as if they had handed me a case too pretty to be true. London smelled of rain and possibility. Later, I was back in my room of course. I watched the flames and said, only half to myself: “Most people believe courage is the opposite of fear. How tedious. Courage is often just fear with functional motor circuitry… and an opioid system that knows when to stay quiet.”
Your SherlockMS