Can Brain Stimulation Help Jump-Start Awareness?

Post by Annika Matthiesen

The takeaway

Stimulating specific brain regions may help “jump-start” brain activity in patients who have limited awareness and disrupted brain communication. While the effects were modest, the study suggests this noninvasive approach could help support recovery of awareness and responsiveness.

What’s the science?

People with severe disorders of consciousness have limited awareness and responsiveness because communication between different parts of the brain can be disrupted. Currently, there are very few effective treatments to improve wakefulness or recovery in these patients. This week in NeuroImage, Sangare and colleagues aim to improve brain activity and awareness in patients with limited wakefulness through brain stimulation.

How did they do it?

The authors studied 18 patients with disorders of consciousness. The authors used brain stimulation on the frontal cortex, an area at the front of the brain involved in higher-level thinking and control. Before and during treatment, the researchers measured how patients responded to sounds, sights, movement, and attempts to communicate. They also used EEG to record the brain’s electrical activity through sensors placed on the scalp, to track changes in brain function. Patients received stimulation or no stimulation (control), and the authors compared their responses before and after treatment. Six months later, the researchers followed up again to see whether any changes lasted over time.

What did they find?

The researchers found some signs that the brain stimulation may have helped improve patients’ responsiveness and awareness, as the treatment group showed slightly greater behavioral improvement than the control group after stimulation. However, they did not observe changes in the overall level of consciousness. EEG results showed more active and complex brain activity in certain regions after treatment, which may suggest stronger communication between different parts of the brain. Overall, stimulation increased electrical activity across seven areas throughout the brain, although these changes did not consistently lead to major improvements in consciousness or behavior. Together, the findings suggest the stimulation may influence brain function, but larger studies are needed to determine whether it can produce meaningful recovery for patients with severe disorders of consciousness.

What’s the impact?

This study found that brain stimulation may help improve brain activity and responsiveness in patients with limited wakefulness by strengthening communication across different parts of the brain. If future studies confirm these findings, noninvasive brain stimulation could offer a safe way to improve awareness and brain communication in people with a wide array of serious neurological conditions.

Access the original scientific publication here.

Changing the Direction of Traveling Brain Waves Influences Attention and Memory

Post by Shalana Atwell

The takeaway

Neural information spreads across the cortex through brain oscillation patterns called “traveling waves.” Altering the direction of these traveling waves using traveling-wave transcranial alternating current stimulation (twtACS) influences cognitive performance.

What's the science?

Cortical “traveling waves” are rhythmic patterns of brain activity that move across the surface of the cortex, like ripples spreading across a pond, and they have been observed across species and during many different cognitive tasks. Cognitive dysfunction is a hallmark of many neurodegenerative diseases, psychiatric disorders, and aging populations, and previous research has shown that standard tACS can enhance certain aspects of human cognition. While tACS can synchronize brain rhythms, its effect is confined to one location and cannot generate dynamic patterns of neural activity. This week in PNAS, Lee and colleagues created and validated a stimulation method to impose a directionally controlled traveling electric field across the cortex and demonstrated how the wave's directionality influences specific cognitive tasks.

How did they do it?

The authors used computer models of human head and brain anatomy to calculate how currents from multiple scalp electrodes would flow through the brain and then optimized the phases of the injected sinusoidal currents so that the location of the maximal electric field (the “peaks”) would sweep across the cortex over time (backward: from anterior to posterior), forming a traveling phase gradient. The team applied twtACS to two human patients with electrocorticography (ECoG) electrodes implanted for clinical reasons and showed that the measured phase at cortical electrodes closely matched the target pattern from the simulations and exhibited clear traveling phase gradients in the backward direction.

A nonhuman primate was implanted with a dense grid of intracortical microelectrodes spanning frontal to motor cortex, allowing measurement of multiunit activity (MUA; pooled local spiking) across space. The animal underwent four consecutive conditions at 10 Hz: baseline (no stimulation), standard tACS (same phase everywhere), forward twtACS (from posterior to anterior), and backward twtACS. For each condition, the authors computed the preferred phase of spiking relative to the stimulation cycle at each electrode. They confirmed that neural population activity aligned with the propagation of twtACS-induced electric fields.

Healthy participants received 10 Hz twtACS targeted to right frontal and parietal regions, with conditions optimized to produce either forward or backward traveling fields. During 20 minutes of stimulation, participants performed two tasks in fixed order: (1) A visual attention task where they covertly attended to flickering disks on the left or right, reporting whether a brief target appeared in the attended disk. (2) An episodic memory task where they encoded 60 images during stimulation, then, after a 10-minute rest, performed an old/new recognition test.

What did they find?

In ECoG patients, the phase of the recorded electric fields across electrodes showed robust gradients, confirming that twtACS can impose direction-specific electric fields across the human cortex that mimic observed cortical traveling waves.

Compared to tACS and baseline, twtACS produced systemic shifts in the preferred phase of spiking along the anterior-posterior axis. The spatial phase gradients in the MUA closely matched those of the electric field, indicating that population spike timing aligned with the propagating twtACS-induced field.

In the visual attention task, performance differed between forward and backward stimulation and depended on whether participants attended to the left or right visual field. Specifically, performance accuracy was higher with backward twtACS when participants attended to the right visual field. In the memory task, accuracy was higher under forward twtACS conditions. Together, these tasks indicate that the directionality of the imposed wave was critical to performance, and they are broadly consistent with previous work linking forward waves to feedforward sensory processing and memory encoding and backward waves to top-down control and spatial attention.

What's the impact?

This study provides evidence that manipulating the direction of traveling cortical waves can actively shape cognition. These findings support the idea that the direction of traveling waves is closely related to task demands, with different directions favoring feedforward vs feedback processing. This approach could eventually provide new therapeutic strategies for cognitive dysfunction in aging and psychiatric or neurological disorders that are thought to involve disrupted large-scale brain coordination.

Access the original scientific publication here.

A Mega-Analysis of the Effects of Psychedelics on Brain Networks

Post by Anastasia Sares

The takeaway

In a re-analysis of data from 11 different MRI studies, researchers found that psychedelics increase connectivity between many different brain networks, but only slightly decrease connectivity within networks. This analysis synthesizes and harmonizes findings in a field where research to date has been inconsistent and contradictory.

What’s the science?

Psychedelics are a class of compounds that primarily target serotonin receptors in the brain, changing neuronal activity and connectivity (see a previous BrainPost for more information on how these drugs work on a molecular level). There are currently many clinical trials looking at their potential therapeutic benefits for neuropsychiatric disorders like depression. Alongside this, researchers are trying to understand how exactly psychedelics act in the brain.

While we are beginning to understand the effect of psychedelics on brain networks, the picture is not consistent from study to study. Each psychedelic compound is a little bit different, and sample sizes in this kind of research tend to be small, so each brain network study has somewhat different findings—in fact, some papers report opposite results. Recently, in Nature Medicine, Girn and colleagues performed a “mega-analysis” from 11 different magnetic resonance imaging (MRI) datasets from studies on psychedelics to try to settle some of these debates and uncover the general effects of psychedelics on brain function.

How did they do it?

One common way of studying brain function that has been applied to many research topics, including psychedelics, is called resting-state functional MRI (or rs-fMRI). In regular functional MRI (fMRI), researchers measure the blood flow in the brain while a person lies in the scanner doing some kind of task: seeing images, hearing sounds, or thinking about certain things. As neurons use glucose and oxygen, the blood vessels in the brain open and increase the flow to supply them with more, and this can be tracked with the fMRI signal. However, in rs-fMRI, this signal is measured while people lie in the scanner doing nothing in particular—in other words, they are “at rest.” The brain is still active at rest, and a lot can be learned from studying this kind of data.

The authors collected and re-analyzed data from multiple laboratories that had scanned people under the effects of psychedelics using rs-fMRI. Most of these studies were randomized controlled trials with some kind of psychedelic versus a placebo. They didn’t just compile the results: instead, they started from scratch with the raw data and re-analyzed it with their own pipeline. MRI analysis is complex, and there is room for a lot of variation in processing methods, so by re-analyzing the data themselves, they hoped to iron out some of those inconsistencies. The type of analysis they conducted looked at functional connectivity, which tracks the fMRI signal to see which parts are correlated in their activity over time (and are therefore probably working together). They were primarily interested in testing one finding from previous studies: that psychedelics decrease connectivity within regions that usually work together, and they increase connectivity between regions that are usually distinct.

What did they find?

The main claim the researchers were testing was partially confirmed: compared to participants with a placebo, people who were under the influence of psychedelics showed increased connectivity between networks, especially general networks having to do with attention, self-reflection, sensory processing, and executive function. On the other hand, while they did see somewhat decreased connectivity within brain networks, these effects were much less robust. This amounts to more “cross-talk” between regions and only slightly less “internal chatter.” However, the one study using ayahuasca showed a completely different pattern: there was an overall decrease in connectivity between most brain areas. Since there was only one study on ayahuasca that had only 9 participants and lacked a placebo condition, more research will be needed to understand its effects. Overall, these results align better with some theories about how psychedelics work (like the subcortical connectivity account) while providing less support for others (like the ‘network disintegration’ account).

What’s the impact?

As clinical trials progress and potential therapies are identified, this research helps us understand the mechanisms behind those therapies and the mental health conditions they could treat. Knowing about the brain mechanisms of psychedelics might help us to better predict who could benefit from their use (personalized medicine), identify potential side effects, and develop sensible regulations.

Access the original scientific publication here.