Three new studies on the same surprising idea — that the brain's "inner map" is really a planner.

For decades, the hippocampus has been cast as the brain's inner GPS. In the 1970s, researchers found "place cells" there — neurons that fire when an animal sits in one particular spot in the world. The discovery earned a Nobel Prize and a tidy story: this is the structure that tells you where you are.

But humans do so much more that wander around. The very same circuitry surely doesn't just record your location, it rehearses your future. Many times a second, populations of these cells fire in quick sequences that trace out paths the animal hasn't taken yet, as if the brain were flickering a beam ahead of itself. Three studies—rats and humans, physical mazes and pure abstraction—converge on a bigger idea: these fast neural sequences are the machinery of planning, and they aim squarely at your goals.

Research Roundup

A maze built to fool a rat's inner compass

A gentle 8-hertz alternating current into the brains of volunteers lifted their working memory—they could keep track of ~20% more items at once. This was the finding of a little experiment I helped a few years ago. The stimulation tapped the brain at the tempo of one of its own signature rhythms, hippocampal theta, and thinking seemed sharper. But why?

A new, very clever experiment studied theta rhythms as rats solved mazes…but not just any old maze. When a rat solves a classic maze everything points the same way at once: its head, its body, and its goal are often hopelessly tangled. The clever fix is a "honeycomb maze": a floor of hexagonal platforms that rise and drop to steer a rat toward a remembered reward from awkward angles—sometimes sideways to its goal, sometimes with its back to it. With head, movement, and goal directions pulled apart, the rapid theta sweeps tracked the goal. 8 times per second, the theta frequency, sequences  of “place cells” formed little vectors pointing toward the remembered destination, no matter which way the rat was facing or moving. The effect was strongest just before correct choices.

Theta isn't merely along for the ride while your brain thinks; it's the beat to which the brain sketches where you’re going next. And as my old result hints, nudge the beat, and the thinking changes. Theta is the same rhythm that surfaces when we simply hold information in mind, and so the tempo of working memory and the tempo of previewing the future may be one and the same. Put those together and the rhythm starts to look less like weather and more like a dial: something we can, in principle, reach in and turn.

Perhaps foresight and memory might be not only watchable but adjustable, which matters most for aging brains, where this previewing machinery is among the first things to fray. A temporary 20% bump on a memory task is a long way from handing someone back the ability to plan their day. But that gap is now an experiment rather than a mystery…and a 20 years ago I'd have called it noise.

Watching Our Brain Snap Ideas Together Like LEGO

The hippocampus is a foundation part of the human brain, but it is so often studied via rats solving mazes. What if you could watch the brain of a human playing tetris or playing with LEGO [1]? We can…

A new study recorded directly from inside the brains of 28 epilepsy patients, who had electrodes implanted for clinical reasons. These volunteers solved LEGO-like puzzles that required mentally combining familiar pieces into new arrangements.

Activity in the player’s hippocampus revealed the process of mental "replay": compressed bursts in which stored building blocks are rapidly reshuffled into candidate solutions. These bursts cluster around sharp-wave ripples (brief high-frequency crackles of hippocampal activity) and each ripple nudged the prefrontal cortex a step closer to the correct inferred answer.

The better-timed a person's replay, the more efficiently they reasoned. It's a direct, real time look at a mind building new thoughts out of old parts.

[1] LEGOs? If LEGO Batman and LEGO Joker are battling atop a LEGO tower, am I watching LEGO fight, not LEGOs fight? If I have a LEGO Legolas, is he now LEGOLA? This is all very confusing. That missing ‘s’ definitely makes me think of LEGO as a weirdly macroscopic grey goo that will consume us all.

Polyrhythmic Planning

How could you build an AI to pack a whole sequence of steps into a single moment of thought? Let’s look for an answer inside ourselves, as healthy volunteers navigating an abstract space using magnetoencephalography (MEG).

A new analysis of this data reveals fast gamma bursts (>30 Hz) nested inside a slower theta wave (~8 Hz) allows each successive step of a plan to ride a different phase of the rhythm, one per burst. In fact, as people closed in on their goal this theta–gamma coupling grew stronger, but only when they were navigating correctly.

The researchers even found a division of labor that mirrors the maze-solving rats I wrote about earlier this week: one gamma band lit up for novel routes, another for familiar ones. It's evidence that a coding scheme long seen in both rat navigation and human working memory, also stitches together our plans.

3 Windows, One Picture

What makes these papers worth reading side by side is how differently they arrive at the same destination. A rat on a shape-shifting maze; epilepsy patients solving puzzles with electrodes deep in their brains; healthy people under a scanner—different species, different tools, different tasks. Yet each catches the hippocampus doing something more interesting than just remembering where things are. It drafts the future and does so with intent, biasing its rehearsals toward the goal at hand. The rhythms differ in the details—theta sweeps, sharp-wave ripples, theta–gamma coupling—but taken together, they hint that memory and imagination may be two faces of the same machinery. The humble business of finding your way to the coffee shop runs on the same circuits you use to reason your way to a new idea.

Media Mentions

SciFi, Fantasy, & Me

My honest answer is that, beyond what I’ve recently recommended, I have no new discoveries. I wish I did but nothing has pop on the genre front of late. I’m genuinely looking forward to Spider Man and Doomsday (superhero movies aren’t bad; bad movies are bad). I’ve heard good things about The Odyssey but haven’t seen it yet (and I’ve only ever been a passing fan of mythology, in any case). Oh well…next week.

Stage & Screen

• September 10, Amsterdam: How might AI change the world of investing?
• September 15, SF: Innovation Day with INSEAD!
• September 16, DC: AI and education–beyond dreams and dread.
• September 19, Phoenix: I'm giving the keynote for the Association of Science & Technology Centers annual conference.
• September 21, Stanford: We're still working on the details, but hopefully I'll be talking about my research on machine learning and neurodiversity for Stanford's Neurodiversity Project.
• September 24, UC Berkeley: It's my annual Berkeley Change-makers Lecture!
• September 24, NYC: Culture Shifting Deal Making Summit
• September 29, Cincinnati: Still baking...
• September 30, Irvine: Hybrid Intelligence for innovation!
• October 6, SF: UCSD Alumni Association
• October 6, SF: Giving a talk at the Draper Richards Kaplan Foundation
• October 6, Park City: It's Robot-Proof in the Rockies with setups.
• October 21-23, Warsaw: So much good stuff is in the works for my first visit to Poland
• October 27, Cologne: Maybe, maybe a visit to Germany!
• October, Toronto: The Future of Work...in the Future
• November 19, NYC: Secrets in the dark!

Vivienne L'Ecuyer Ming