Surgery Godfather

Chapter 2186: Chapter 1433: Monkey Pain Sensation Test (Part 2)

"Then let’s use a control group," Yang Ping didn’t hesitate. "Weber, can the Laboratory find some other primates? They don’t need to be spinal cord injury models, just healthy individuals of the same age, and we’ll run a set of baseline tests as reference."

Weber thought for a moment. "Yes, we can do behavioral tests without injuring them."

"No injury needed, we only need to measure normal responses and establish a reference range."

"We can!"

The testing protocol was launched that very afternoon. Lina and Fritz bought a pile of things at the supermarket: red chili peppers, bananas, oranges, mustard, peppermint essential oil. Tang Shun watched them pile the lab bench up like a vegetable market and couldn’t help shaking his head.

"This is the first time I’ve seen anyone do Neuroscience research with stuff off a supermarket shelf."

"This is called low-tech, high-concept," Lina said with a smile. "Name coined by Professor Yang."

The results of the first round of testing were beyond everyone’s expectations.

M7 showed almost no reaction to banana or orange scent, only a slight avoidance to peppermint, but its reaction to capsaicin—stimulating it with a single red chili—was that it let out a sharp cry, then shrank into a corner of the cage, body curled up, maintaining that posture for about twenty seconds before slowly relaxing.

More importantly, this reaction wasn’t a one-off. Two hours later, when Fritz again touched M7 with the chili, it again avoided it, but this time the cry was shorter, and the curled-up posture lasted for a shorter time, only about ten seconds.

"The response intensity has dropped," Lina stared at the video playback. "With a two‑hour interval, the habituation effect is too strong. This doesn’t look like pure pain, it looks more like learned expectation."

"You’re right," Eva nodded. "The cognitive capacity of primates is too strong; it will learn. The first time it’s afraid; the second time it knows the chili won’t actually hurt it, so it’s less afraid. Using chili to measure pain is unreliable."

Fritz’s face darkened a little. He had thought his discovery could bring a breakthrough to the team, but reality was more complicated than he’d imagined; it seemed that new ideas matured more slowly than he’d expected.

"However," Eva changed tack, "the very fact that M7 reacts to chili in the third postoperative week already tells us something. Think about it: in the first postoperative week, did we run similar tests on it?"

Fritz froze for a second, then his eyes lit up. "We did! I brought mustard close to it, and it barely reacted."

"So the change is real," Eva said. "From no reaction at all, to a reaction, and then to decreasing reaction intensity—this time course exactly matches the pattern of habituation after pain sensation returns. We don’t need to prove that M7 is in a lot of pain now, only that it can feel pain now whereas three weeks ago it couldn’t."

Yang Ping had been standing by listening without speaking. He picked up the red chili, examined it in his palm for a while, then said something no one had expected: "If I were the reviewer, I wouldn’t accept this evidence. The chili test is too crude, qualitative but not quantitative. We need a genuine mechanical pain threshold test."

"But how do you do Von Frey filaments on a macaque?" Manstein asked. "Those filaments are calibrated for rodents. Primates have completely different skin thickness and Nerves distribution."

"Then we’ll make our own set," Yang Ping said. "Use nylon filaments of different diameters, calibrate the bending force from 0.1 grams to 100 grams and create a gradient. Lina, can you design it?"

Lina thought for a moment, then nodded. "Yes. Give me three days."

Three days later, a hand‑made set of Von Frey filaments appeared on M7’s testing platform. Lina had spent a weekend in the Laboratory calibrating each nylon filament’s bending force one by one, repeatedly measuring with an analytical balance and vernier calipers to ensure that every intensity level was accurate.

M7’s mechanical pain threshold test was conducted on the Monday of the fourth postoperative week.

The method was very simple: Fritz took M7 out of the cage and placed it on a soft cushion, using one hand to gently stabilize its body and the other to hold the filament. Tang Shun filmed from the front; Eva took notes at the side.

The smallest filament, equivalent to 0.1 gram-force, was pressed onto the skin of M7’s right sole. The filament bent slightly, then Tang Shun released it. M7 showed no response.

0.5 grams, no response.

gram, no response.

grams, M7’s right leg twitched slightly.

grams, M7 turned its head, looked at its right leg, then turned back.

grams, M7 let out a short cry and withdrew its right leg.

"Ten grams is the threshold," Eva dictated. "What about baseline data? We need to know what the normal threshold is in healthy macaques."

Weber thought for a moment. "We just measured those three aged crab‑eating macaques. Their leg withdrawal thresholds were between 15 and 20 grams. M7’s ten grams is slightly below the normal range, but already very close."

"What does ’below the normal range’ mean?" Yang Ping asked.

"Possibly hyperalgesia," Weber said. "Post‑injury central sensitization, where originally harmless stimuli become harmful. This is common in spinal cord injury Patients, but it usually appears in the chronic phase. For M7 to show hyperalgesia at four weeks post‑op means its sensory pathways are recovering very fast."

Yang Ping was silent for a few seconds, his gaze resting on M7. The monkey was sitting in the cage, its right leg slightly flexed, posture much more relaxed than a week ago. Its eyes were directed toward the testing platform, its pupils reflecting the fine glitter of those nylon filaments.

"Hyperalgesia is a double‑edged sword," Yang Ping said slowly. "It proves the pathway is open, but it also means the central nervous system is undergoing abnormal changes in excitability. If left unchecked, it may evolve into chronic neuropathic pain, which is more tormenting than paralysis."

Weber nodded. He had seen too many spinal cord injury Patients—some would rather be paralyzed again than endure that persistent burning, electric‑shock, needle‑like pain.

"So we need another index," Weber said, "one that can prove the sensory pathway is recovering without causing M7 pain."

"Touch," Eva picked up immediately. "The tactile pathway and the pain pathway are separate in the dorsal horn of the spinal cord, but their recovery windows are similar. If we can show that M7’s touch sensation is also recovering, and that the recovery time points match, then we won’t need to depend on pain data."

"How do you measure touch?" Lina asked.

"With cotton," Fritz spoke up again.

This time no one laughed, because everyone noticed that when Fritz said this, his expression was serious, even a bit grave; he didn’t look like he was joking.

"Cotton?" Yang Ping turned and looked at him.

"Yes, cotton," Fritz walked over to M7’s cage and took a cotton ball out of the pocket of his White Gown. "Lightly touch its skin: if it can feel it, there will be a very subtle response—the skin will quiver a little, or the Muscle will make a tiny contraction. The response is too weak for the naked eye to judge, but a high‑speed camera can capture it."

"How do you know this?" Manstein asked curiously.

Fritz lowered his head. "I’ve studied..."

"Then we’ll use cotton," Weber said.

The tactile test began that very afternoon.

Fritz carried M7 to the soft cushion and covered its eyes with a soft towel so it couldn’t see the source of the stimulus. Then he took a slender cotton swab tipped with a fluffy cotton ball and started from the dorsum of M7’s foot, moving upward bit by bit: foot dorsum, ankle, lower leg, thigh, waist, Abdomen, chest.

M7’s responses were recorded simultaneously by two high‑speed cameras, one from the side and one from above. Weber set the frame rate to one thousand frames per second so that no tiny Muscle tremor would be missed.

The experiment was becoming more and more complex and broader in scope. Yang Ping needed more data, and he had to find what was useful within it.

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