Researchers can monitor brain activity associated with perception, pain, and awareness, although it remains unclear which neural pathways underlie the origins of consciousness.
Now, a recent study from the Massachusetts Institute of Technology illustrates how a developing brain-stimulation method, transcranial focused ultrasound, could offer a possible way to address this mystery.
This method uses acoustic waves to modulate brain activity, enabling scientists to noninvasively target deep brain structures with millimeter-level accuracy. In a review published in Neuroscience and Biobehavioral Reviews, the authors present a roadmap for using this technology to test different theories of consciousness.
Manipulating the Brain
Transcranial focused ultrasound could be used to determine whether specific brain signals actually cause consciousness or just happen to occur at the same time. In this method, acoustic waves travel through the skull and focus on a specific area, temporarily altering activity in brain structures located several centimeters below the surface. By changing brain activity rather than just observing it, researchers can begin to identify cause-and-effect links. This is a key step for testing ideas about how conscious experience forms.
“Transcranial focused ultrasound will let you stimulate different parts of the brain in healthy subjects, in ways you just couldn’t before,” said Daniel Freeman, a researcher at MIT Lincoln Laboratory and a 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.”
“There are very few reliable ways of manipulating brain activity that are safe but also work,” added Matthias Michel, a philosopher at MIT who studies consciousness and co-authored the paper.
A Noninvasive Innovation
Unlike other noninvasive methods, such as transcranial magnetic or electrical stimulation, focused ultrasound can reach deeper into the brain and target smaller areas. This allows scientists to study subcortical regions involved in emotion, perception, and motivation, which are usually only accessible during surgery.
“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 said. “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.”
Competing Theories of Consciousness
The authors organize their roadmap around two main theories of consciousness. The first, called the cognitivist view, suggests that conscious experience depends on higher-level mental processes such as reasoning or self-reflection. These processes likely involve large-scale brain networks, especially in the frontal cortex, that bring information together into a single experience.
The other theory, known as the non-cognitivist view, suggests that consciousness could arise directly from certain patterns of brain activity, without requiring higher-level thinking. In this school of thought, the cortex or subcortical structures located in the back of the brain may be key to the creation of conscious experience.
“Transcranial focused ultrasound gives us a solution to that problem,” Michel said. By altering activity in specific brain regions, researchers can determine whether disrupting the frontal cortex affects conscious perception or whether consciousness exists when only posterior or subcortical regions are active.
From Vision to Pain
The authors identify visual perception as an initial area for testing. While techniques such as electroencephalography can reveal which neurons respond to visual stimuli, they do not show whether these responses actually produce conscious visual experience.
“It’s one thing to say if these neurons responded electrically,” Freeman said. “It’s another thing to say if a person saw light.”
Pain is another area to explore. People often react to harmful stimuli before they are aware of pain, which raises questions about where and how the feeling of pain begins. Focused ultrasound could help show whether pain starts in the cortex, in deeper brain structures, or through interactions between them.
“It’s a basic science question, how is pain generated in the brain,” Freeman said. “Pain could stem from cortical areas, or it could be deeper brain structures. That’s a hypothesis. But now we have a tool to examine it.”
The authors emphasize that the technology is still new and that many questions remain about how reliably it can modulate neural circuits involved in consciousness.
“It’s a new tool, so we don’t really know to what extent it’s going to work,” Michel said. “But I feel there’s low risk and high reward. Why wouldn’t you take this path?”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration, a Bachelor of Science in Business Administration, and a Data Analytics certification. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.