The bull came out of the gate the way fighting bulls are bred to: fast, head low, horns forward. Standing in the sand of a small ranch bullring near Córdoba, Spain, a slim, dark-haired man held a cape in his right hand and a small metal box in his left. He was not a matador. He was a neuroscientist from Yale University. When the animal was a few steps away, he pressed a button. The bull braked, skidded to a stop in a cloud of dust, and stood there, blinking. Then it turned and trotted off.
The scientist was José Manuel Rodríguez Delgado, and the box was a radio transmitter. Beforehand, he had threaded fine wires deep into the bull’s brain and connected them to a tiny radio receiver strapped to its horns. When The New York Times put the story on its front page on May 17, 1965, it called the stunt “probably the most spectacular demonstration ever performed of the deliberate modification of animal behavior through external control of the brain.” Sixty years later, as companies like Neuralink put wireless implants into human brains, Delgado’s bull is starting to look less like a curiosity and more like a preview.
A brain scientist from bullfighting country
Delgado was born on August 8, 1915, in Ronda, a hilltop town in Andalusia with a long bullfighting history. His father was an eye doctor, and Delgado planned to become one too, until he read the work of Santiago Ramón y Cajal, the Spanish Nobel laureate who showed that the brain is built from individual nerve cells. Delgado later described being captivated by “the many mysteries of the brain. How little was known then. How little is known now!”
His training was interrupted by the Spanish Civil War. Delgado served as a medical corpsman on the Republican side, and when Francisco Franco’s forces won in 1939, he spent five months in a prison camp. Afterward he had to repeat parts of his medical degree before earning a doctorate in physiology in Madrid. In 1946 he won a fellowship to Yale, and in 1950 he joined its physiology department for good.
Yale’s physiology department was led by John Fulton, whose work on chimpanzees had helped inspire the lobotomy, the surgery that destroyed parts of the frontal lobes to calm psychiatric patients. Delgado hated the idea. “I thought Fulton and Moniz’s idea of destroying the brain was absolutely horrendous,” he told the science writer John Horgan decades later. He wanted a gentler tool: electricity.
Electricity instead of a scalpel
The model for that tool came from Switzerland. Starting in the 1920s, the physiologist Walter Rudolf Hess had shown that he could make cats hiss with rage, grow hungry or fall asleep by passing tiny electrical currents through wires placed in specific parts of their brains. In 1949, Hess shared the Nobel Prize in Physiology or Medicine with António Egas Moniz, the Portuguese neurologist who had pioneered the lobotomy. The same prize celebrated two opposite ideas: cut the brain, or nudge it with electricity.
“My idea was to avoid lobotomy,” Delgado said, “with the help of electrodes implanted in the brain.” In 1952 he co-authored what he later called the first peer-reviewed paper on electrodes implanted deep in human brains. Over the next two decades he implanted electrodes in about 25 human patients, most of them people with severe epilepsy or schizophrenia at a state mental hospital in Rhode Island whose illness had resisted every other treatment. Stimulating different spots could bring on laughter, sudden friendliness, fear, relaxation, or the urge to flex a hand. One patient whose hand clenched on cue reportedly told him, “I guess, doctor, that your electricity is stronger than my will.”
The early setups were clumsy. Wires ran out through the skull and skin to bulky equipment, which kept subjects tethered and invited infections. So Delgado, whom a Yale colleague called a “technological wizard,” built something new: the stimoceiver, a combined radio stimulator and receiver small enough to ride on the head. Later versions were about the size of a large coin. It could send brain recordings out by radio and deliver pulses of current on command, letting a monkey, a cat or a person move around freely while it worked. That idea, a wireless link into the brain, is exactly what today’s implants are built around.
Into the ring with Lucero
Delgado was fascinated by aggression. Europe had just lived through a civil war, a world war and the Holocaust, and he wanted to know what in the brain makes violence possible. “I thought: Which is the animal which is characterized by his aggressive behavior?” he recalled. “The fighting bull!”
The experiments took place on La Alamirilla, a ranch near Córdoba owned by a bull breeder named Ramón Sánchez. Delgado told Horgan the work happened in 1963, the year most accounts give, but his own detailed write-up, and the 1965 Times story, which said it was done “last year,” both point to 1964. According to that write-up, an encyclopedia chapter he wrote in Spanish in 1981 that was rediscovered and translated by the neuroscientist Timothy Marzullo in 2017, the work went like this.
The bulls, about three years old, were first knocked out with a tranquilizer dart. Delgado and the ranch hands then drilled a hole about 2 centimeters wide on each side of the skull, held the head steady in a frame, and slid in bundles of stainless steel wire a tenth of a millimeter thick. Each bundle carried seven contacts spaced 5 millimeters apart, reaching from the outer motor cortex down to deeper structures, so each animal ended up with 14 to 28 electrodes. There was no atlas of the bull brain to guide them. The wires were fixed in place with dental cement, and the connectors were tucked into bandages wrapped around the horns.
A few days later, the tests began. Delgado’s handheld transmitter, about 14 by 6 by 6 centimeters, could send pulses 20 to 100 times a second at currents of up to about 1 milliamp. In the first bull, named Cayetano, stimulating the left caudate nucleus made the animal turn its head to the left. Turning the current up made it walk in a full circle, then faster circles. Stimulating the right side sent it circling the other way. The bull did not bellow or try to flee. Delgado described it as looking “surprised in its own conduct, but not irritated.”
The second bull, Lucero, is the one in the famous photographs. With a cape in one hand and the transmitter in the other, Delgado faced it himself. “The size of the bull and its horns appear to grow, one perceives its smell, one hears its snorts, and one becomes conscious of its power,” he wrote. When Lucero was within 2 or 3 meters, he switched on the current, aimed at the caudate nucleus and thalamus. The bull stopped violently, tail down and head up. As soon as the current was switched off, it charged again. Once, Delgado admitted, the transmitter failed, and the bull reached him, “fortunately without more consequence than a good scare.” Word spread through Córdoba, and a Spanish television crew and hundreds of onlookers came to watch.
Did he switch off anger, or just the legs?
Delgado’s interpretation was bold. He said the stimulation had “detained” the bull’s aggressiveness, and that after repeated sessions the animals would tolerate people in the ring for several minutes without attacking. The press took it further, and readers pictured a real-life version of Ferdinand, the gentle bull from the children’s book.
Other scientists were skeptical. The caudate nucleus sits deep in the brain as part of the basal ganglia, a set of structures that help start, stop and steer movement. Damage there is involved in movement disorders such as Huntington’s disease. The neuroscientist Elliot Valenstein argued that Delgado had not turned off the bull’s rage at all. He had simply jammed its ability to keep charging straight ahead, the way stimulating Cayetano’s caudate had forced it into circles. The fact that Lucero charged “furiously” the moment the current stopped fits that view.
Marzullo, reviewing the 1981 chapter, reached a similar conclusion. The bull experiments were never formally published, there were no counts of how many animals responded, no circuit diagrams, and no examination of where the electrodes really ended up. The stimulation clearly changed the bulls’ movement, he wrote, but calling it a reduction of aggression “is rather over-arching.” Delgado himself conceded, late in life, that the experiment “naturally could be interpreted in one way or the other,” though he stood by his view. It is the same trap that still catches brain researchers today: it is easy to read too much into a brain signal or a behavior, as the dead salmon that lit up an fMRI scanner famously showed.
Paddy, and a brain that talked to a computer
Delgado thought the newspapers had picked the wrong experiment. The one he considered more important involved a female chimpanzee named Paddy. Her stimoceiver was linked to a computer that listened for a particular burst of electrical activity, called a spindle, coming from her amygdala, a region tied to emotion. Every time the computer detected a spindle, it fired a pulse into another part of her brain that produced an unpleasant sensation.
Within two hours, Paddy’s brain was producing half as many spindles. Within six days, the count had dropped by 99 percent. She also became quieter and less attentive, which shows how blunt the tool still was. But the principle was new: a machine reading the brain’s signals and responding automatically, with no human pressing the button. Delgado suggested this kind of loop might one day stop seizures or panic attacks.
He was right, eventually. In 2013, the U.S. Food and Drug Administration approved the NeuroPace RNS System, an implant that watches for the electrical signature of an oncoming epileptic seizure and fires a pulse to head it off. It is, in effect, a far more refined version of Paddy’s experiment.
A prophet of the “psychocivilized society”
In 1969, Delgado published a book called Physical Control of the Mind: Toward a Psychocivilized Society. It summarized his experiments and then went much further. Humanity, he argued, had learned to tame nature and should now learn to shape its own brains. The old command “Know thyself” should give way to “Construct thyself.” Used wisely, he wrote, brain technology could help create “a less cruel, happier, and better man.”
Many readers found the book terrifying, not inspiring. It was illustrated with photos of monkeys, cats and two young women wearing stimoceivers on their heads. It did not help that some of his work was funded by the Office of Naval Research and an Air Force research laboratory, along with civilian agencies. Critics imagined cyborg soldiers, while Delgado insisted his military sponsors never asked for anything of the kind. Around the same time, other brain-implant researchers made far more disturbing proposals, and the Michael Crichton novel The Terminal Man in 1972 turned brain stimulation gone wrong into a thriller. Strangers began accusing Delgado of secretly implanting devices in their heads, and one woman sued him and Yale, although he had never met her.
In 1974, he moved back to Spain to help start a new medical school at the Autonomous University of Madrid. He said the job offer, not the controversy, was the reason. There he switched to stimulating the brain from outside the skull with electromagnetic fields, an early cousin of today’s transcranial magnetic stimulation. Because he published mostly in Spanish, much of his later work faded from view. He died in San Diego on September 15, 2011, at age 96.
From the bullring to Neuralink
Delgado once complained that he had done many important things but had never followed any of them in depth. Others did. Electrical stimulation deep in the brain became a standard therapy: deep brain stimulation was approved in the United States for tremor in 1997 and for Parkinson’s disease in 2002, using wires placed not far from the regions Delgado probed in his bulls. Cochlear implants now give hearing to people who were deaf by stimulating the auditory nerve.
The other half of his stimoceiver, listening to the brain, took off too. In the early 1990s, the University of Utah engineer Richard Normann patented a tiny bed of 100 silicon needles that could record from many neurons at once. The Utah array became the workhorse of brain-computer interface research. In 2004, a paralyzed man in the BrainGate trial used one to move a computer cursor with his thoughts, and implants like it are now helping people who have lost their voices to speak.
Neuralink, which implanted its first human patient in January 2024, records through many flexible threads instead of rigid needles. It reads signals far more precisely than anything Delgado could have built, and it sends them out by radio, just as his stimoceiver did. Its Blindsight project aims to add the other half, stimulation, to help blind people see. The goals are different, too. Today’s implants are mostly about restoring abilities to people who have lost them, not about controlling behavior.
The questions Delgado left behind
The worries that followed Delgado have not gone away. They have simply moved from the bullring to privacy law. In 2021, Chile became the first country to amend its constitution to protect brain activity and the information that comes from it. In 2024, Colorado added “neural data” to the sensitive information covered by its consumer privacy law, and California followed later that year. Those laws matter less for medical implants, which are already tightly regulated, than for the growing number of consumer gadgets that read brain signals from outside the head. A headband like the MUSE 2 Brain Sensing Headband, for instance, uses EEG sensors to track brain activity during meditation.
None of today’s devices can steer a person the way Delgado steered his bull, and he doubted they ever would. Near the end of his life, he told Horgan that scientists still had no idea how the brain encodes complex information, and that “the whole brain is involved in everything.” But his bull remains a vivid reminder of what is at stake when we wire into the brain: the power to help, and the power to override. As he put it, “Can you avoid knowledge? You cannot! Can you avoid technology? You cannot!”
Explore the brain at home
MUSE 2 Brain Sensing Headband — A lightweight EEG headband that reads your brain’s electrical activity through the skin, with no surgery involved, and turns it into real-time sound feedback in a phone app during meditation. A small, harmless taste of the brain signals Delgado spent his career chasing.
Learning Resources Human Anatomy Model Brain — A 31-piece take-apart brain model on a stand, with color-coded regions, so you can pull off the outer layers and see how the deep structures sit beneath the cortex. Good for students and curious adults alike.