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| Caption:Transcranial focused ultrasound, a noninvasive brain imaging tool depicted in the illustration, may help researchers gain knowledge about human consciousness. Image Credit: MIT News; figure courtesy of the researchers (CC BY-NC-ND 4.0) |
Scientific Frontline: "At a Glance" Summary
- Main Discovery: MIT researchers have established transcranial focused ultrasound (tFUS) as a breakthrough tool for studying consciousness, publishing a comprehensive "roadmap" to identify the neural substrates of subjective experience.
- Methodology: The technique utilizes focused ultrasound waves to non-invasively stimulate deep brain regions with millimeter-scale precision, modulating neural activity centimeters from the scalp without the need for surgical implants.
- Specific Detail: Unlike prior methods, tFUS allows for the manipulation of subcortical structures and "emotional circuits" deep within the brain, enabling researchers to causally link specific neural circuits to subjective sensations like pain, vision, or thought.
- Key Comparison: The technology offers significantly higher spatial resolution and depth penetration compared to traditional non-invasive methods like transcranial magnetic stimulation (TMS) or direct current stimulation (tDCS), which are limited to cortical surfaces or lack precision.
- Significance: The tool provides a practical means to test competing theories of consciousness—specifically distinguishing between "cognitivist" theories (requiring higher-level prefrontal cortex processing) and "non-cognitivist" theories (localized to posterior or subcortical regions).
- Future Application: Immediate experiments will focus on stimulating the visual cortex to map the causal chain of perception, potentially resolving the "hard problem" of how physical matter generates conscious experience.
Consciousness is famously a “hard problem” of science: We don’t precisely know how the physical matter in our brains translates into thoughts, sensations, and feelings. But an emerging research tool called transcranial focused ultrasound may enable researchers to learn more about the phenomenon.
The technology has entered use in recent years, but it isn’t yet fully integrated into research. Now, two MIT researchers are planning experiments with it, and have published a new paper they term a “roadmap” for using the tool to study 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 a new paper on the subject. “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.”
Transcranial focused ultrasound is noninvasive and reaches deeper into the brain, with greater resolution, than other forms of brain stimulation, 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 new work.
The paper, “Transcranial focused ultrasound for identifying the neural substrate of conscious perception,” is published in Neuroscience and Biobehavioral Reviews. The authors are Freeman, a technical staff member at MIT Lincoln Laboratory; Brian Odegaard, an assistant professor of psychology at the University of Florida; Seung-Schik Yoo, an associate professor of radiology at Brigham and Women’s Hospital and Harvard Medical School; and Michel, an associate professor in MIT’s Department of Philosophy and Linguistics.
Pinpointing causality
Brain research is especially difficult because of the challenge of studying healthy individuals. Apart from neurosurgery, there are very limited ways to gain knowledge of the deepest structures in the human brain. From the outside of the head, noninvasive approaches like MRIs and other kinds of ultrasounds yield some imaging information, while the electroencephalogram (EEG) shows electrical activity in the brain. Conversely, with transcranial focused ultrasound, acoustic waves are transmitted through the skull, focusing down to a target area of a few millimeters, allowing specific brain structures to be stimulated to study the resulting effect. It could therefore be a productive tool for robust 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.”
Crucially, the technology may help researchers determine cause-and-effect patterns, precisely because its ultrasound waves modulate brain activity. Many studies of consciousness today may measure brain activity in relation to, say, visual stimuli, since visual processing is among the core components of consciousness. But it’s not necessarily clear if the brain activity being measured represents the generation of consciousness, or a mere consequence of consciousness. By manipulating the brain’s activity, researchers can better grasp which actions help constitute consciousness, or are byproducts of it.
“Transcranial focused ultrasound gives us a solution to that problem,” says Michel.
The “roadmap” laid out in the new paper aims to help distinguish between two main conceptions of consciousness. Broadly, the “cognitivist” conception holds that the neural activity that generates conscious experience must involve higher-level mental processes, such as reasoning or self-reflection. These processes link information from many different parts of the brain into a coherent whole, likely using the frontal cortex of the brain.
By contrast, the “non-cognitivist” idea of consciousness takes the position that conscious experience does not require such cognitive machinery; instead, specific patterns of neural activity give rise directly to particular subjective experiences, without the need for sophisticated interpretive processes. In this view, brain activity responsible for consciousness may be more localized, at the back of the cortex or in subcortical structures at the back of the brain.
To use transcranial focused ultrasound productively, the researchers lay out a series of more specific questions that experiments might address: What is the role of the prefrontal cortex in conscious perception? Is perception generated locally, or are brain-wide networks required? If consciousness arises across distant regions of the brain, how are perceptions from those areas linked into one unified experience? And what is the role of subcortical structures in conscious activity?
By modulating brain activity in experiments involving, say, visual stimuli, researchers could draw closer to answers about the brain areas that are necessary in the production of conscious thought. The same goes for studies of, for instance, pain, another core sensation linked with consciousness. We pull our hand back from a hot stove before the pain hits us. But how is the conscious sensation of pain generated, and where in the brain does that happen?
“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 ahead
Freeman and Michel are not just abstractly charting a course for others to follow; they are planning forthcoming experiments centered on stimulation of the visual cortex, before moving on to higher-level areas in frontal cortex. While methods of recording brain activity, such as an EEG reveal areas that are visually responsive, these new experiments are aiming to build a more complete, causal picture of the entire process of visual perception and its associated brain activity.
“It’s one thing to say if these neurons responded electrically. It’s another thing to say if a person saw light,” Freeman says.
Michel, for his part, is also playing an active role in generating further interest in studies of consciousness at MIT. Along with Earl Miller, the Picower Professor of Neuroscience in MIT’s Department of Brain and Cognitive Sciences, Michel is a co-founder of the MIT Consciousness Club, a cross-disciplinary effort to spur further academic study of consciousness, on campus and at other Boston-area institutions.
The MIT Consciousness Club is supported in part by MITHIC, the MIT Human Insight Collaborative, an initiative backed by the School of Humanities, Arts, and Social Sciences. The program aims to hold monthly events, while grappling with the cutting edge of consciousness research.
At the moment, Michel believes, the cutting edge very much involves transcranial focused ultrasound.
“It’s a new tool, so we don’t really know to what extent it’s going to work,” Michel says. “But I feel there’s low risk and high reward. Why wouldn’t you take this path?”
Published in journal: Neuroscience & Biobehavioral Reviews
Title: Transcranial focused ultrasound for identifying the neural substrate of conscious perception
Authors: Daniel K. Freeman, Brian Odegaard, Seung-Schik Yoo, and Matthias Michel
Source/Credit: Massachusetts Institute of Technology | Peter Dizikes
Reference Number: ns011226_01