In 2008, a cockatoo named Snowball took part in an experiment at a bird shelter in Indiana. He was set on the back of an armchair, and a song he knew came on: "Everybody", by the Backstreet Boys. A computer had sped it up or slowed it down without changing its pitch. The question was whether he would stay with the beat. His two owners stayed in the room but didn't dance, so that he couldn't copy them. One of them was Irena Schulz, who ran the shelter. In the first two sessions, she bobbed her head to the beat anyway. It was slight, and she didn't mean to.
Her head raises this lecture's question. Why does a beat pull at a body that's meant to keep still?
Snowball was the one the scientists were watching. Moving to a beat was thought to be something only humans do. Dogs have lived with people and their music for thousands of years, and you don't see them do it. But the person in that room is a puzzle too. She was meant to keep still beside a song with a beat, and her head kept time without her. You've probably caught a foot of your own doing the same.
The last two lectures were about predicting which note comes next. A beat raises a different prediction. It's about when.
I'll break the question into four smaller ones. Is the beat in the sound, or in you? Babies bounced to a drum rhythm answer that one. Then which part of the brain keeps it? Adults lying still in a scanner show us. How well do the famous findings about the beat hold up? Two of them shrank when bigger studies checked. And who else can keep a beat? Snowball comes back for that, with a sea lion named Ronan.
A beat seems to live in the music, in the drums and the bass. Researchers define it as a steady pulse that you perceive in music. It's what you'd tap your foot to. You also group the beats. Feel them in twos, one-two, one-two, and you have a march. Feel them in threes, one-two-three, and you have a waltz. That grouping is called meter. Usually the music marks it for you, with louder or longer notes on the strong beats.
But the beat doesn't have to be in the sound at all. Play a run of identical clicks at an even pace. People can hear every second one as stronger, or every third. Nothing in the clicks is stronger. A beat can even fall on silence. In syncopation, a note lands just before the beat, and nothing sounds on the beat itself. You still feel the beat there.
Two Canadian psychologists asked whether the body helps decide which grouping you hear. They played seven-month-old babies a drum rhythm for two minutes. It had no accents at all, so it could be felt as a march or as a waltz. Half the babies were bounced on every second beat as it played. The other half were bounced on every third. Each baby heard two versions of the rhythm, one with louder beats in twos and one with louder beats in threes. The babies controlled how long each version played. They listened longer to the one that matched their bouncing.
Was it something they saw? A second group was blindfolded while they were bounced, and they showed the same preference. A third group sat still and watched an adult bounce, in twos or in threes. What would watching do? Nothing. Those babies preferred neither version, and neither did babies who were never bounced. Their own bodies had to move. Adults showed it too. Those who bounced along in twos later picked out the march version as the one they'd heard. Those who bounced in threes picked the waltz.
The two psychologists put it in a line: "We hear the melody in music, but we feel the beat." The babies hadn't moved themselves. They had been moved, so the psychologists propose that the sense of balance and motion did the work. The beat is partly something you add, then, and the way your body moves helps decide which one you hear. Something in your brain must be doing the adding.
The adding happens somewhere you might not expect: in the parts of the brain that plan and time movement. Call them the movement system. Jessica Grahn and Matthew Brett put 27 people in a scanner in Cambridge, England, the blood-flow kind from the second lecture. Fourteen of the 27 were musicians, and the rest had no musical training.
They heard short rhythms made of plain tones. Each rhythm played three times, and they said whether the third was different. They were told to keep their whole body still, apart from pressing the answer button. The rhythms came in three kinds. In one, the notes were spaced so that a steady beat came through. In another, the very same note lengths were shuffled, so no steady beat emerged. The third used lengths too awkward for any beat to fit. Rhythms with a beat are easier to remember, so the team tuned the task until people scored about the same on all three. A harder task couldn't then explain a difference in the brain.
Every kind of rhythm woke up the movement system. But the rhythms with a beat drove two parts of it harder. One was the basal ganglia, the brain's deep movement-timing machinery. The other was the supplementary motor area, a planning area in the brain's outer layer. The beat made the same difference in musicians and in people with no training. Meanwhile, the area that becomes active when the body moves stayed quiet. The listeners had kept still. The planners were busy, and nothing was moving.
The deep timing machinery does seem to matter for the beat. Parkinson's disease damages the basal ganglia. In a later study, Grahn and Brett found that people with Parkinson's were worse than healthy volunteers at telling apart rhythms with a beat. With the other rhythms, they weren't reliably worse.
A scanner is too slow to follow each beat. Recordings of the brain's magnetic signals can. A team led by Takako Fujioka in Toronto recorded twelve people as they watched a movie of their choice, with steady tones playing. They weren't asked to listen for anything. The recordings followed a fast brain wave called the beta wave. It has long been tied to movement: it dips when you plan or make a movement, and recovers afterwards.
In these still, distracted viewers, the beta wave dipped just after every tone. Then it climbed back, and peaked just before the next tone arrived. Faster tones brought the climb sooner. With random tones, the climb lost its timing. The wave was getting ready for a beat it could predict. It rose and fell in movement areas too, including the planning area.
These listeners weren't moving, or even paying attention. So a later review of this work calls the movement system's part in hearing a beat "obligatory." It suggests that may be why music makes people want to move. As I see it, this finishes something the third lecture started. There, the brain predicted which note would come next. Here it predicts when the next beat will come, and it does that with the movement system.
So far, two pieces fit. You add the beat, and the way your body moves helps decide which beat you hear. The movement system keeps that beat, and runs a little ahead of it, while the body stays still. What we haven't asked is how well the famous evidence holds up, and what makes one rhythm pull at you harder than another.
The first famous finding seemed to show a march or a waltz inside the brain. It came in 2011, from Sylvie Nozaradan and her colleagues in Brussels. They recorded the brainwaves of eight volunteers, through electrodes on the scalp. Each heard a pulsing tone, a little more than twice a second. They imagined the pulses in twos, as a march, or in threes, as a waltz. They learned to do it by tapping and counting aloud. Then they had to keep it going in their heads, sitting still.
The sound never changed, and the brainwaves rose and fell with its pulse. Imagining a march added something the sound didn't have: a second, slower wave, at half the pulse's rate. Imagining a waltz added one at a third of the rate. On the team's reading, the meter itself could be seen in the brainwaves.
Eight people is a small study for a claim that size. A psychology journal later invited labs to repeat it, under a plan written and reviewed before any data came in. Every lab watched its volunteers for movement. One volunteer was dropped for moving rhythmically. In the end, results from 152 people in thirteen labs counted.
The plan set its tests in advance. By those tests, the extra waves came out at most a third of their 2011 size. That was too small to tell from no effect at all, so the study failed to replicate. A second analysis, not in the plan, put all 152 people together. It found a small effect, in the expected direction. Seeing it took nearly twenty times as many people as the first study. And the extra wave didn't predict who could tell where their imagined beats fell. The team's verdict is careful: it may be "premature to conclude that there is no such effect."
That takes away the most vivid picture of an imagined beat: the waltz in your head, showing up in your brainwaves. It doesn't touch the bounced babies, who were judged by how long they listened, though that study was small too. And the scanner result on the movement system has turned up in several studies, even when people were listening for changes in loudness or pitch instead.
The second finding was about the pull itself. Researchers call the pleasurable urge to move to music groove, and music with groove is often full of syncopation, notes that land just before the beat. The music researcher Maria Witek and her colleagues ran an online survey. Sixty-six people rated fifty funk drum breaks for how much each made them want to move.
The ratings rose with syncopation, peaked in the middle, and fell again. It was the upside-down U from the last lecture, where enjoyment rose and fell with complexity, with groove at the sweet spot. Witek's team argued that in the middle, the beat is clear enough to predict, and the off-beat notes can still surprise. They called syncopation "an invitation to the body" to move with the beat.
A decade later, a team in Switzerland tested the sweet spot again. Witek was one of its authors, testing her own finding. This time every drum pattern was copied from a real record by a famous drummer, and no hump appeared.
So the team went back to Witek's ratings. Fourteen of her fifty breaks had been composed by the experimenters, to reach the far ends of syncopation. The made-up breaks got low ratings, and they sat at the two extremes of syncopation, the least and the most. They were mainly what bent the line into a hump. Without them, the hump was still there, but it did almost nothing to predict which breaks people rated highly. The Swiss team argues that listeners can probably hear when drumming doesn't sound real, and rate it down for that.
That doesn't end the argument. Six earlier studies had found the full hump, and three found part of it. The Swiss team says that most of them, like Witek's, mixed real patterns in the middle with made-up ones at the ends. But one study made up every pattern, at every level, and still found the hump. That suggests made-up drumming isn't the whole story. And the Swiss patterns all came from great drummers, with none at the extremes, where a drop-off would show. I'd score the sweet spot for groove as unproven, not disproven. The Swiss study did find two routes to the urge to move. Listeners heard simpler patterns as more regular, and regularity made them want to move. They heard complex patterns as more interesting, and that made them want to move too.
Both checks teach the same caution. A first study of eight people, or of sixty-six raters online, can find a pattern that a much bigger study can't. The beta-wave studies are small too, so I'd hold them more loosely than the scans.
Which brains can keep a beat at all? Two abilities are involved: hearing a beat, and moving in step with it. Moving in step means matching the beat's speed and landing on it. Researchers call that entrainment. People land on the beat, or a hair ahead of it. That means predicting it, since the movement has to start before the sound arrives. Fujioka's team suggests that the beta wave's climb helps plan exactly that kind of movement.
Hearing a beat comes first. A team in Budapest played a rock drum pattern to sleeping babies two or three days old, with their mothers beside them. Now and then, the first beat of the pattern was left out, and the babies' brains responded as if something had gone wrong. A review pointed out a snag: dropping that beat also dropped drums, so the babies might just have noticed fewer sounds.
So the same team tried a cleaner test. Now a drum hit was sometimes made much quieter, either on the beat or between beats, so the change was the same sound either way. Each sequence came in two forms, one with steady timing and one with the gaps jumbled. The babies' brains reacted more to a quiet hit on the beat than between beats, but only when the timing was steady. The order of the sounds was the same in both forms. So the babies weren't just learning which sound follows which, the statistical learning of the third lecture. Two macaque monkeys given the same test sensed the steady timing, but showed no sign of the beat. On these two studies, both from one lab, babies track a beat within days of birth. Which rhythms sound normal is learned later, as the third lecture showed with babies of twelve months.
Moving in step comes much later. Another team filmed 120 babies, from five months to two years old, on a parent's lap. The babies moved more to music than to speech, and faster to faster music. But the team found no sign of them landing on the beat. In tests like these, that seems to wait until about preschool age, and it goes on improving for years.
Snowball's test asked whether a bird was capable of entrainment: matching the speed and landing on the beat. Aniruddh Patel ran it. In the second lecture, he argued that music and language share some of their processing resources. Someone had e-mailed him a video of Snowball. "This is much more than just a cute pet trick," he recalls saying. His team made eleven versions of the song, at different speeds.
In more than half his dancing trials, at most of the speeds, Snowball had stretches of head bobs locked to the beat. A check against chance made luck very unlikely. In the study, nobody trained him or rewarded him with food. His early history is unknown, and a previous owner had danced with him. And the owner's bobbing head? The trials with her bobbing and the trials with her still came out about the same. A little over half had locked stretches either way. He wasn't copying her.
Patel's original explanation was that keeping a beat rides on vocal learning, the ability to learn and imitate new sounds. Copying a sound takes tight links between hearing and movement. He argued that keeping a beat does too. People and parrots learn sounds. Dogs, cats and monkeys don't. In early studies, macaques that learned to tap along landed about a quarter of a second after each beat. That's faster than a plain reaction, so they predict a little, but not the way people do. With other training they got closer, and later work suggests monkeys can learn to anticipate a steady metronome.
Then there's Ronan, a California sea lion at a university marine lab in Santa Cruz. Sea lions show no sign of learning sounds in the wild. On Patel's rule, she shouldn't keep a beat. At three, Ronan was trained to bob her head in time to a metronome, with fish as rewards. She soon kept time to speeds she had never heard, and later to music.
In 2021, Patel answered that sea lions' vocal skills haven't been tested with modern methods. He pointed out that some seals, which are related to sea lions, are skilled sound-imitators. He also noted that Ronan's timing slid around the beat as the tempo changed, unlike a person's. And he revised his rule rather than dropping it. Vocal learning comes in degrees, he now argues, and more of it should mean more humanlike beat keeping.
At fifteen, Ronan was tested again, beside ten students. They chopped an arm down in time to the same snare drum she bobbed to. Her timing varied less than any student's. Some students beat her on one measure or another, but none on all of them. And her timing no longer slid with the tempo. At the end, the students were thanked. Ronan got a toy filled with fish and ice.
Ronan is one animal, tested by her own trainers on a plain, steady beat, beside students trying an arm chop for the first time. Her trainers call the vocal-learning idea "a stretch". They think any complex brain may have the basic equipment, and that things like motivation, control of the body and practice decide who uses it. The argument is open.
Why did Irena Schulz's head move? On the best evidence, her brain was keeping the beat with its movement system. The basal ganglia and the supplementary motor area work harder for a rhythm with a beat, even in a body that's still. A wave tied to movement climbs toward each beat before it arrives. That's the explanation these researchers favor, though it isn't proven. Even the experiments had to fight the pull. A head bobbed in the bird shelter, and a volunteer in the thirteen labs was dropped for moving rhythmically. My own gloss goes one step further. Sitting still to music means holding back a movement your brain is already preparing. Others read the same activity as the brain simulating movement to keep time, with nothing held back at all.
The beat, in the end, is a joint product. The sound offers the timing, and you supply the grouping, with your body's movement able to tip it toward a march or a waltz. Landing a hair ahead of a beat is the mark of a brain predicting it. The most striking evidence shrank when thirteen labs checked it: a waltz imagined into the brainwaves. The sweet spot for groove is now in dispute too. And keeping a beat isn't only human, though which brains can do it is not settled.
Ronan also points ahead. At fifteen she keeps time better than she did at three. The next lecture asks what years of practice do to a human brain, and whether it matters when you start.