While the technology has existed for a number of years, it hasn't yet become a staple in neuroscience studies. Recently, two researchers from MIT are set to embark on new experiments utilizing this technique and have released a comprehensive paper that acts as a detailed "roadmap" for its application in consciousness research.
"Transcranial focused ultrasound enables the stimulation of various brain areas in healthy individuals, in ways previously unachievable," states Daniel Freeman, an MIT researcher and co-author of the paper. "This tool holds potential not only for medical applications or basic science but could also contribute to unraveling the intricate problem of consciousness. It allows us to investigate where in the brain the neural circuits responsible for sensations like pain, vision, or even the complexity of human thought reside."
In contrast to other brain stimulation techniques, transcranial focused ultrasound is non-invasive and can precisely target deeper regions of the brain more effectively than methods such as transcranial magnetic or electrical stimulation.
"There are very few dependable methods to safely manipulate brain activity," remarks 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," is published in Neuroscience and Biobehavioral Reviews. The authors include Freeman, Michel, 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.
The Challenges of Understanding the Brain
Grasping the complexities of the human brain is particularly challenging as researchers often cannot conduct invasive experiments on healthy subjects. Outside of neurosurgery, options for probing deep brain structures are limited. Imaging technologies like MRIs and various ultrasound modalities can visualize anatomy, while electroencephalograms (EEG) capture electrical signals across the brain. However, these methods primarily observe activity rather than actively influencing it.
Transcranial focused ultrasound operates differently. It transmits acoustic waves through the skull, focusing them on a specific target, sometimes only a few millimeters wide. This capability allows researchers to stimulate particular brain regions and monitor the outcomes, making it a promising tool for meticulously controlled experiments.
"For the first time in history, we can modulate activity deep within the brain, just centimeters from the scalp, examining subcortical structures with remarkable spatial resolution," Freeman explains. "There are intriguing emotional circuits located deep in the brain that we couldn't manipulate outside of an operating room until now."
Investigating Cause and Effect in Consciousness
A significant advantage of this technology is its potential to clarify cause-and-effect relationships in the brain. Many current consciousness studies depend on observing brain activity as individuals process visual stimuli or engage in tasks related to awareness. While these studies reveal correlations, they don't always clarify whether a brain signal induces a conscious experience or merely follows it.
By actively altering brain activity, transcranial focused ultrasound may assist researchers in identifying which neural processes are crucial for consciousness and which are merely secondary effects.
"Transcranial focused ultrasound provides us with a solution to this challenge," Michel asserts.
Exploring Competing Theories of Consciousness
In their paper, the researchers discuss how this technology could be employed to test two overarching theories of consciousness. The cognitivist approach posits that conscious experience relies on higher-level mental processes such as reasoning, reflection, and information integration across the brain, often highlighting the frontal cortex's role.
The non-cognitivist perspective, on the other hand, argues that consciousness does not necessitate complex cognitive mechanisms. Instead, specific brain activity patterns may directly generate particular experiences. From this viewpoint, consciousness might emerge from more localized brain areas, including regions at the back of the cortex or deeper subcortical structures.
The researchers intend to utilize focused ultrasound to investigate questions regarding the prefrontal cortex's role in perception, whether awareness is contingent on local brain activity or broader networks, how distinct brain regions integrate information into a unified experience, and the contribution of subcortical structures to conscious awareness.
Insights from Pain and Vision
Experiments involving visual stimuli could shed light on which brain regions are essential for conscious perception. Similar methodologies could be applied to pain, a fundamental aspect of conscious experience. For instance, individuals often withdraw their hand from a hot surface before consciously feeling pain, prompting inquiries into where and how the sensation of pain originates.
"It's a fundamental scientific question: how is pain generated within the brain?" Freeman says. "It's surprising how much uncertainty exists... Pain could originate from cortical areas or deeper brain structures. While I am interested in therapeutic applications, I'm also intrigued by the possibility that subcortical structures might play a more significant role than previously recognized. This could suggest that the physical manifestation of pain is subcortical. Now, we have the means to investigate this hypothesis."
Future Experiments and Growing Interest at MIT
Freeman and Michel are not just proposing ideas for future research; they are actively organizing experiments that will start with stimulation of the visual cortex and subsequently progress to higher-level regions in the frontal cortex. While tools like EEG can indicate when neurons respond to visual stimuli, these new studies aim to establish a clearer connection between brain activity and an individual's actual experiences.
"It's one thing to determine if these neurons respond electrically; it's another to confirm if a person perceives light," Freeman states.
Michel is also fostering a broader research community focused on consciousness at MIT. He co-founded the MIT Consciousness Club alongside Earl Miller, the Picower Professor of Neuroscience in MIT's Department of Brain and Cognitive Sciences. The club brings together scholars from diverse fields and organizes monthly events centered on advancements 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 symbolizes a promising direction for the field.
"It's a new tool, and we don't fully understand its potential yet," he remarks. "However, I believe the risks are low while the rewards could be substantial. Why not explore this avenue?"
The research outlined in the paper was supported by the U.S. Department of the Air Force.