The human brain is somewhat similar to a rudimentary radio with five channels. Electrical signals from neurons coordinate across the brain, generating oscillations known as brain waves. Each wave corresponds to a state of the brain. Some come fast and furious, with a high frequency usually associated with when we’re awake and thinking. Others are more relaxed, with a slower undulation that occurs during deep, restful sleep.
In a way, the brain switches channels as we go about our day to match our internal state of mind to outside requirements – though at any point, the channels can bleed over.
But there’s a mysterious outcast: a frequency called theta waves. They happen while we’re awake or asleep. For decades, these waves have taunted neuroscientists trying to decipher their functions. Theta waves seem to help mice navigate mazes, but also support memory in humans.
It’s not just academic curiosity. Our ability to navigate complex new environments and keep those memories declines with age. It’s especially tough for people with Alzheimer’s disease. By finding the source of theta waves, we could potentially enhance them – using neurostimulation or other methods – to slow cognitive decline.
A study in Neuron took a first step. Thanks to Xbox and some virtual mall shopping, a team led by Dr. Arne Ekstrom from the University of Arizona dug deep into what drives theta waves. The study recruited people with epilepsy who already had electrodes implanted into their brains to hunt down the source of seizures.
When imagining a previous route, the participants’ brains sparked with theta activity, a response far stronger than that elicited by simply navigating the route with an Xbox joystick.
The brain has a way of internally generating theta waves using memory, said the team.
An Ocean of Brain Waves
Our brains operate on multiple electrical frequencies. Using electroencephalography (EEG), we can record the speed and sequence of brain activity and capture its relative pace. Like calm or stormy, choppy waters, these brain oscillations rise and fall at different frequencies, each representing a different state of mind.
Beta waves, for example, spark when the brain is completely engaged – like when you’re intrigued by a conversation. Alpha waves are slower and usually present when you’re sitting down and ready for a rest.
Then there are theta waves. These waves are larger in amplitude and cycle even slower at 3 to 12 hertz per second. Earlier studies found that they pop up when you’re zoned out: during highway driving, on a long run, or in the shower. These waves are tentatively linked to creativity – ideas or solutions suddenly come to you – or when you’re daydreaming or meditating.
Despite decades of research, we still don’t really understand what they encode. While we’re awake theta waves are mostly found in the hippocampus, a brain region critical for both memory and navigation. So it’s no surprise that the waves appear in mice and rats as they try finding their way across complex mazes, suggesting they help integrate sensations and movement as the rodents explore a new environment.
What is surprising is that the waves also appear in completely still humans challenged with memorizing lists of words or pictures. One study found that the oscillations were critical for associating different concepts. In another, artificially enhancing theta waves with an off-the-shelf entrainment device – which uses a combination of sound and lights to stimulate certain brain wave bands – increased memory performance for recalling words in 50 volunteers.
There’s a direct link between theta activity and memory performance, the UC Davis team concluded at the time.
So what is it that theta waves do? Do they help guide us as we navigate the world? Or do they help us lay down precious memories?
The new study had the two theories go head-to-head.
A Digital Shopping Spree
The team started with a group of volunteers: 12 men and women with epilepsy who unfortunately didn’t respond to medication. Each already had up to 17 electrodes implanted in their brains to search for the source of seizures.
The task itself was one of my worst nightmares: navigating a mall. Here they did it virtually using an Xbox joystick, with the digital mall displayed on a laptop computer.
The first step was getting familiar with six different shopfronts and their locations – for example, an ice cream shop, camera store, and comic shop – like wandering around a new mall in person for the first time.
After two rounds, the volunteers were challenged with a navigation task. They were “teleported” to a random store in a first-person view, and were prompted with a message on the screen to find another store using the Xbox joystick. The trial ended after they navigated to every store.
Then came the memory task, and it was all hands off. The participant still started at a random storefront. Instead of using a joystick, they were instructed to mentally simulate the walk over to another target store – pressing “A” on the controller to indicate when they began and when they arrived. All the while, the graphics were gone, with only a tiny white cross on the screen for the volunteers to focus on. In all they performed 44 trials, with their brain activity monitored the entire time.
It’s hard to read what’s going on in a person’s head: are they actually mentally simulating the route or simply relaxing? The study added several guardrails. First, the volunteers had to successfully complete another mental navigation task, but in a familiar environment: they were instructed to imagine standing in their bedrooms at home and walking to their kitchens.
Next, the researchers cleverly inserted “catch trials.” Here, the participants used the controller to simulate their imagined routes during memory tasks, which helped ensure that they were actually remembering the route.
Finally, with a dose of statistics, the team found that the time it took for each participant to find a target store correlated between both tasks, suggesting they were imagining the same route rather than simply daydreaming.
The brain wave analyses came back with a clear answer. Both navigation and mental simulation sparked theta waves, becoming stronger as the trials progressed. However, simply remembering the route – without any movement – generated far larger waves that lasted longer. They found the same results regardless of whether analyzing each individual electrode or each volunteer.
It seems that memory is a far stronger driver of theta waves compared to simple navigation, said the team. Theta waves seem to naturally occur in the brain even without outside stimulation, supporting the idea that they’re internally generated in the brain and crucial for memory.
That’s great news. With memory closely knitted to theta waves, it’s possible to tap into the unique frequencies and improve memory during aging or in patients with dementia or other mental disorders. Researchers are already exploring different brain stimulation methods – from electrical to magnetic – with initially promising results.
But perhaps more broadly, the study adds to a recent trend that explores brain waves as a novel route for treating difficult neurological disorders, including Alzheimer’s and stroke. Dozens are now in clinical trials. They may turn the tide for tackling previously uncrackable neurological disorders. Time will tell.
