Struggling with a difficult mental task? Zapping the brain region linked to our motivation to make a mental effort might help – for a while. People who had this part of their brain stimulated by electrical currents excelled at a difficult task, but also became more mentally fatigued afterwards.

“Their performance is much better; however, this good performance comes with lower motivation in the end,” says at the University of Munich, Germany.

Xia and her colleagues put electrodes on the heads of 31 volunteers with no known health conditions. During a half-hour session in an MRI scanner, the volunteers spent 20 minutes doing the 3-back memory test, where you have to remember and identify the third item back as an ongoing sequence is presented to you – a very demanding mental task. They were then offered €1 for doing an easy test or €2 for a harder one.

During the 3-back memory test, the electrodes delivered transcranial direct current stimulation (tDCS) to a region of the brain called the dorsolateral prefrontal cortex to randomly chosen volunteers, who were unaware they were receiving it.

Other studies have shown the dorsolateral prefrontal cortex is involved in we make. “Our dorsolateral prefrontal cortex helps us to work hard,” says Xia.

The volunteers who had tDCS reacted faster and more accurately than those who didn’t receive this stimulation. For instance, their average reaction time was 0.98 seconds, compared with 1.12 seconds with sham stimulation.

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Xia thinks stimulating the dorsolateral prefrontal cortex will boost brain performance generally. “I can imagine any cognitive task will be better, but afterwards you will feel more fatigued,” she says.

After the tests, the volunteers who got tDCS had higher levels of the neurotransmitter glutamate in their dorsolateral prefrontal cortex, according to a non-invasive technique called magnetic resonance spectroscopy. The higher their glutamate levels, the more likely they were to choose the €1 task over the €2 one.

Some researchers think mental fatigue is a result of the buildup of glutamate, which can harm cells at high levels. Xia thinks her team’s findings are the strongest evidence of this yet. The findings suggest it is important to take a break when you feel mentally tired, she says.

Xia thinks future studies should also look at the dorsolateral prefrontal cortex in people with conditions such as depression, which can affect motivation.

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“I think the claim that the stimulation made people ‘work harder’ is well substantiated,” says neuroscientist Emile Radyte at Samphire Neuroscience, a UK-based company exploring the use of tDCS to treat conditions such as premenstrual syndrome. But the study doesn’t prove that higher glutamate levels were the cause of the greater mental fatigue, she says.

Radyte says her trials have shown that if productivity is hindered by conditions such as premenstrual syndrome, applying tDCS over the dorsolateral prefrontal cortex can restore it. But she isn’t sure about using it to boost mental performance. “A 20-minute laboratory task does not establish a net productivity benefit over a working day,” says Radyte.

It is a good study, says at University College London, but the greater mental fatigue could be a result of using more neural energy, rather than the buildup of glutamate. It’s also hard to precisely target the dorsolateral prefrontal cortex with tDCS without an MRI scanner to guide you, says Lavie. “Drink coffee instead,” she says.

Human brain organoids have been implanted into mice that are missing half their brain. Tiny versions of the human cerebral cortex – a region involved in memory, movement and thinking – were put into mice that lacked this major brain region. Not only did the organoids integrate into the mice’s brains, but they also restored most of their cognition.

“The mouse is [relatively] sick without a cortex, then you transplant these organoids in, and it behaves more like the normal mouse,” says at the University of Cambridge, who wasn’t involved in the research.

at Stanford University in California and his colleagues genetically engineered mice to lack more than 90 per cent of their cortex, or about half their brain. These mice experience only some memory and movement deficits, since their brain adapts to compensate for their missing cortex, says Pașca.

In about half of these mice, the team surgically implanted four human brain organoids, generated by bathing human stem cells in chemicals for about 40 days. These filled the cavity in each mouse’s skull, producing what the researchers called XCX mice. The remaining mice, apallial mice, didn’t receive the transplants.

A few months later, the apallial mice performed worse in a memory test, where they had to navigate a maze, than a third group of normal mice with intact brains. But this deficit was almost completely reversed in the XCX mice. The XCX mice also had a gait between that of the apallial and normal mice.

Scans revealed that the organoids had grown to form cortex-like grafts. “Half the volume of the brain is [initially] gone, and now largely 90 per cent of that missing volume is covered by human cells,” said Pașca.

This builds on prior studies that have implanted human brain organoids into rat or mouse brains, but without such a large part of the rodent brain being missing initially, says Pașca. These studies showed only that such organoids can alter cognition, rather than improving it, he says. For instance, in one study, light stimulated human brain organoids in rats to make them anticipate water.

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The grafts in the latest study were also the first to contain a kind of nerve cell, called von Economo neurons, that influences social skills, as well as neurons that extended from the cortex to the spinal cord. The latter could explain why these grafts reduced impairments in motor skills, says Balmuș.

The team’s organoids were larger and more akin to the human cerebral cortex than those used in previous experiments, says Pașca. But according to at the Institute of Molecular Biotechnology in Vienna, Austria, they still lack the proper arrangement and structure of the cortex. “It’s a bit of a mishmash of neurons” and other cells, he says. “Most of the cell types are there, but they’re not separated into regions like in a normal cortex.”

In another experiment, the team showed that human organoid transplants made mice respond to hypoxia, or low oxygen levels, in a way similar to people. Mice have evolved to be highly resilient to low oxygen levels, due to living underground, so it is hard to replicate this in a normal mouse, says Balmuș. This suggests that these experimental animals could provide a better way to study cerebral palsy, a condition that causes movement problems and can be caused by hypoxia around birth, says Balmuș.

But such experiments come with ethical concerns, says Knoblich. These mice “deserve the same kind of protection that we ascribe to any animal experiment, meaning that we need to make absolutely sure that there’s no sensation of pain, that there is no unnecessary suffering, and that the risk of the experiment is matched to the medical gain,” he says. The latest study achieved this, he says.

A common concern is whether mice with human brain organoids will gain a human-like level of consciousness. “The data so far doesn’t say that putting [a human organoid] in makes the mouse smarter or more conscious than a normal mouse,” says Knoblich. “It’s similar to taking a wheel from a car and putting it on a tree and saying that’s a car.”

A brain implant has enabled people with paralysis to speak while gesturing, such as nodding or waving. These sorts of exchanges, done via an avatar, could help them communicate with more nuance.

“In our everyday life, gestures play an important role in communication,” says at the University of California, San Francisco. “Saying ‘maybe’ and nodding your head has a very different connotation than if you shake your head.”

Experimental brain implants have boosted the ability to either speak or move among people with paralysis, but simultaneously haven’t been very successful. To achieve this, implants need to cover a large part of the sensorimotor cortex, a brain region involved in speech and movement, says Brosler.

Now, Brosler and her colleagues have tested an iPhone-sized implant in this part of the brain in two people with paralysis. The first participant, who the researchers call Bravo-1r, became paralysed after a stroke, while the second, Bravo-6, had amyotrophic lateral sclerosis, the most common form of motor neuron disease.

Both participants had lost almost all movement in their limbs. They also couldn’t speak and could only make unintelligible sounds.

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First, the team recorded the participants’ brain activity while they repeatedly attempted to say 10 phrases – such as “hello”, “nice to meet you” and “how’s it going?” – or perform 10 gestures, like waving, clapping and shaking their head. In some cases, the participants attempted to do these separately, while in others they tried both simultaneously.

The team then used each of the participants’ brain recordings to train customised machine learning models to predict their intended phrases and gestures.

To put the models to the test, the participants attempted to repeat various pairs of phrases and gestures that were displayed on a screen. The models interpreted their brain activity and used that to control the movement of an avatar resembling the participants, while the speech predictions flashed up on the screen. 

The team chose to use avatars, rather than attempting for the individuals themselves to speak and gesture, because this offered a relatively simple way to prove that both actions could be simultaneously decoded from brain activity, says Brosler.

Paralysed man can feel objects through another person’s hand

Keith Thomas, a man in his 40s with no sensation or movement in his hands, is able to feel and move objects by controlling another person's hand via a brain implant. The technique might one day even allow us to experience another person's body over long distances.

Bravo-1r’s intended gestures and speech were predicted with 88 and 84 per cent accuracy, respectively, while Bravo-6’s were predicted with 66 and 70 per cent accuracy. “If those models were operating on complete chance, the accuracy would be like 9 per cent,” says Brosler. “It was really exciting.”

“It’s a very nice piece of work, with robust results,” says at the University of Lausanne in Switzerland. But it’s based on a very limited set of phrases and gestures, and these still need improving, he says. “For daily use without frustration, there needs to be higher accuracy.”

Brosler says the team is working to address this by further training the models and tweaking the algorithms. She also says that future models could move a person’s own body – if their joints and muscles are in a good enough condition – and produce a synthetic voice, rather than having to do this via an avatar.