Consciousness has long been described as one of science’s toughest puzzles. Researchers still do not fully understand how physical brain tissue gives rise to thoughts, emotions, and subjective experience. A relatively new technology, known as transcranial focused ultrasound, may offer a powerful way to investigate this mystery more directly.
Although the technology has been around for several years, it has not yet become a standard tool in neuroscience research. Now, two researchers at MIT are preparing new experiments using the technique and have published a paper that serves as a detailed guide, or “roadmap,” for applying it to the study of consciousness.
“Transcranial focused ultrasound will let you stimulate different parts of the brain in healthy subjects, in ways you just couldn’t before,” says Daniel Freeman, an MIT researcher and co-author of the paper. “This is a tool that’s not just useful for medicine or even basic science, but could also help address the hard problem of consciousness. It can probe where in the brain are the neural circuits that generate a sense of pain, a sense of vision, or even something as complex as human thought.”
Unlike other brain stimulation methods, transcranial focused ultrasound does not require surgery. It can reach deeper areas of the brain with greater precision than techniques such as transcranial magnetic or electrical stimulation.
“There are very few reliable ways of manipulating brain activity that are safe but also work,” says Matthias Michel, an MIT philosopher who studies consciousness and co-authored the paper.
The study, titled “Transcranial focused ultrasound for identifying the neural substrate of conscious perception,” appears in Neuroscience and Biobehavioral Reviews. In addition to Freeman and Michel, the authors include Brian Odegaard, an assistant professor of psychology at the University of Florida, and Seung-Schik Yoo, an associate professor of radiology at Brigham and Women’s Hospital and Harvard Medical School.
Why Studying the Brain Is So Challenging
Understanding the human brain is particularly difficult because researchers typically cannot experiment on healthy people in invasive ways. Outside of neurosurgery, scientists have limited options for exploring deep brain structures. Imaging tools such as MRIs and various forms of ultrasound can show anatomy, while the electroencephalogram (EEG) records electrical signals across the brain. However, these methods mainly observe activity rather than directly influencing it.
Transcranial focused ultrasound works differently. It sends acoustic waves through the skull and concentrates them on a precise target, sometimes only a few millimeters wide. This allows researchers to stimulate specific brain regions and observe the effects, making it a promising tool for carefully controlled experiments.
“It truly is the first time in history that one can modulate activity deep in the brain, centimeters from the scalp, examining subcortical structures with high spatial resolution,” Freeman says. “There’s a lot of interesting emotional circuits that are deep in the brain, but until now you couldn’t manipulate them outside of the operating room.”
Testing Cause and Effect in Consciousness
One of the most important advantages of this technology is its ability to help identify cause-and-effect relationships in the brain. Many current studies of consciousness rely on observing brain activity while people process visual stumuli or perform tasks linked to awareness. While these studies reveal correlations, they do not always show whether a brain signal creates a conscious experience or simply follows it.
By actively changing brain activity, transcranial focused ultrasound may help researchers determine which neural processes are essential for consciousness and which are secondary effects.
“Transcranial focused ultrasound gives us a solution to that problem,” Michel says.
Competing Ideas About How Consciousness Works
In their paper, the researchers outline how the technology could be used to test two broad theories of consciousness. One view, known as the cognitivist approach, argues that conscious experience depends on higher-level mental processes such as reasoning, reflection, and the integration of information across the brain. This perspective often emphasizes the role of the frontal cortex.
The alternative view, sometimes called the non-cognitivist approach, suggests that consciousness does not require complex cognitive machinery. Instead, specific patterns of brain activity may directly produce particular experiences. From this perspective, consciousness might arise in more localized brain regions, including areas toward the back of the cortex or deeper subcortical structures.
The researchers propose using focused ultrasound to explore questions such as the role of the prefrontal cortex in perception, whether awareness depends on local brain activity or large-scale networks, how separate brain regions combine information into a single experience, and what part subcortical structures play in conscious awareness.
What Pain and Vision Can Reveal
Experiments using visual stimuli could help identify which brain regions are required for conscious perception. Similar approaches could also be applied to pain, another fundamental component of conscious experience. For example, people often pull their hand away from a hot surface before they consciously feel pain. This raises questions about where and how the sensation of pain is actually generated.
“It’s a basic science question, how is pain generated in the brain,” Freeman says. “And it’s surprising there is such uncertainty … Pain could stem from cortical areas, or it could be deeper brain structures. I’m interested in therapies, but I’m also curious if subcortical structures may play a bigger role than appreciated. It could be the physical manifestation of pain is subcortical. That’s a hypothesis. But now we have a tool to examine it.”
Experiments and Growing Interest at MIT
Freeman and Michel are not only outlining ideas for future research. They are actively planning experiments that will begin with stimulation of the visual cortex and later move to higher-level regions in the frontal cortex. While tools like EEG can show when neurons respond to visual input, these new studies aim to establish a clearer link between brain activity and what a person actually experiences.
“It’s one thing to say if these neurons reponded electrically. It’s another thing to say if a person saw light,” Freeman says.
Michel is also helping build a broader research community around consciousness at MIT. Along with Earl Miller, the Picower Professor of Neuroscience in MIT’s Department of Brain and Cognitive Sciences, he co-founded the MIT Consciousness Club. The group brings together scholars from multiple disciplines and hosts monthly events focused on advances in consciousness research.
The MIT Consciousness Club receives partial support from MITHIC, the MIT Human Insight Collaborative, an initiative backed by the School of Humanities, Arts, and Social Sciences.
For Michel, transcranial focused ultrasound represents a promising direction for the field.
“It’s a new tool, so we don’t really know to what extent it’s going to work,” he says. “But I feel there’s low risk and high reward. Why wouldn’t you take this path?”
The research described in the paper was supported by the U.S. Department of the Air Force.
