In a landmark study, scientists have found that the precise timing of electrical activity in our brains determines how well we process the world around us. This new knowledge could have massive implications for how we understand and treat focus, attention and memory in Alzheimer's disease and attention-deficit/hyperactivity disorder (ADHD).
University of Bremen researchers have demonstrated how a unique pathway – where sensory cues travel as signals on waves of gamma rhythms to our processing center – is responsible for our ability to focus on one thing and block out all the other noise along the way.
"In an environment full of voices, music, and background noise, the brain manages to concentrate on a single voice," said brain researcher Dr. Eric Drebitz from the University of Bremen. "The other noises are not objectively quieter, but are perceived less strongly at that moment."
Neuroscientists have long puzzled over how the brain cuts through the noise of a busy world to focus on what really matters. This new research unravels the mystery, showing how the secret lies not just in how strongly brain cells fire, but in when they fire. And it's the precise timing of electrical activity, locked to the brain’s natural gamma rhythms, which determines whether information flows smoothly from one brain area to the next or gets garbled en route. The discovery could help explain why attention falters in conditions such as ADHD, schizophrenia, and Alzheimer’s disease – which all show disrupted gamma activity.
"Until now, it was unclear how this survival-critical mechanism of selecting relevant information is controlled," said Drebitz. "When you cross a street and a car suddenly appears from the side, the brain immediately focuses its processing on this one piece of visual information – the movement of the vehicle. Other impressions, such as signs, passersby, or billboards, fade into the background as they distract our attention and slow down our reaction. It is only through this targeted prioritization that we are able to react quickly and take evasive action."
To put it as simply as possible, groups of neurons often synchronize their activity in rhythmic patterns. Gamma rhythms, which oscillate about 30 to 90 times per second, have been thought to be particularly important for linking up different parts of the brain for perception, attention and memory. Earlier research had shown that when gamma waves align between brain regions, communication improves, but until now it hasn't been well understood if this synchrony drives "targeted prioritization" or if it's simply a byproduct of other changes.
To better isolate this function, the researchers recorded brain activity in macaque monkeys as they performed a visual attention task. Visual information flows through a series of brain checkpoints known as areas V1, V2, V3, and V4, with each stage handling more complex features. Here, the team was focused on area V2, which handles basic features like edges and textures, and area V4, which processes more complex aspects of shapes and objects.
Using microstimulation to inject artificial bursts of activity into V2, the researchers timed these bursts relative to the gamma rhythms in V4. Essentially, they wanted to see if a signal that arrived in sync with the rhythm was more efficient at rapid-fire processing in the brain than signals that were out of sync. When stimulation landed during the receptive phase of the gamma cycle in V4, it altered neural activity and behavior; when it arrived too early or too late, the effect vanished, showing that gamma phase was not just associated with information flow but controlling it.
"The artificially triggered signals only influenced the activity of the nerve cells in V4 when they arrived during a short phase of increased receptivity," said Drebitz. "If the same signal arrived too early or too late, it had no effect. If it arrived within the sensitive time window, it not only changed the activity of the nerve cells, but also the behavior of the animals: They reacted more slowly and made more mistakes – from which it can be concluded that the test signal, which contained no information for the task, became part of the processing and thus interfered with the performance of the actual task.
“Whether a signal is processed further in the brain depends crucially on whether it arrives at the right moment – during a short phase of increased receptivity of the nerve cells,” he added. “Nerve cells do not work continuously, but in rapid cycles. They are particularly active and receptive for a few milliseconds, followed by a window of lower activity and excitability. This cycle repeats itself approximately every 10 to 20 milliseconds. Only when a signal arrived shortly before the peak of this active phase did it change the behavior of the neurons.”
Essentially, instead of boosting all signals equally, the brain uses its gamma rhythm as a timing gate. Inputs that arrive in sync with the rhythm are amplified, while those that come at the wrong moment are dampened or ignored. That timing mechanism is what allows the brain to pick out a voice in a noisy room or focus on one object. When gamma rhythms are disrupted, as has been observed in ADHD, schizophrenia and Alzheimer’s disease, signals may arrive out of step and fail to get through cleanly, resulting in poorer attention and memory functioning.
In ADHD brains, poor gamma synchronization might be driving attention deficits, making distractions harder to shut out. In 2016, a breakthrough study found that exposing Alzheimer’s disease-model mice to 40-Hz – aka gamma wavelength – flashing lights (later combined with sounds) reduced amyloid plaques and activated immune cells that help clear toxic proteins. Scientists have continued to develop light-flickering stimulation to induce gamma-wave oscillations in Alzheimer's disease patients. And we've covered some of these developments over the years.
Overall, these new findings demonstrate that well-functioning cognition has a lot to do with timing. The brain’s rhythms set the beat and information flows best when signals align with peak activity in the nerve cells. This discovery opens the door to developing novel interventions to improve attention and memory, as well as designing brain-computer interface technology that enhances selective processing and the storing of information.
“The results provide a basis for developing more precise models of the brain," said Drebitz. "They show how information is selected and prioritized before it leads to perception, learning, and behavior."
The study was published in the journal Nature Communications.
Source: University of Bremen
