The Search for Where Consciousness Lives in the Brain

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IIn 1998, neuroscientist Christof Koch bet with philosopher David Chalmers that within 25 years, scientists would discover the neural correlates of consciousness. He was certain that we were close to solving the so-called hard problem: how the physical flesh of the brain gives way to the daily streams of feelings, sensations, and thoughts that make up our waking experience.

That gamble didn’t work out for Koch: A few years ago, he paid off by delivering a case of fine wine to his opponent on a conference stage in New York.

But many scientists still believe that the scientific keys to the realm of consciousness are within their reach. Lately, some are focusing their attention on a new technology called focused transcranial ultrasound, in which sound waves are transmitted through the skull into interior tissues. These waves can be used to stimulate specific target areas as small as a few millimeters and monitor the resulting changes.

Now two MIT researchers have identified specific ways to use technology to solve the difficult problem of consciousness. Because transcranial ultrasound offers a powerful, non-invasive way to change brain activity, it will allow scientists to track cause and effect for the first time, they say. In a new article, published in Neuroscience and Biobehavioral Reviewsthey are preparing a series of experiments that will aim to answer how consciousness appears.

“Focused transcranial ultrasound will allow you to stimulate different parts of the brain in healthy subjects in a way that you couldn’t before,” Daniel Freeman, a researcher at MIT and co-author of the paper, explained in a statement. “It’s a tool that is not only useful for medicine or even basic science, but could also help solve the difficult problem of consciousness. It can probe where in the brain are the neural circuits that generate a sensation of pain, a sensation of vision or even something as complex as human thought.”

Read more: “What’s so difficult about understanding consciousness?»

Most existing brain monitoring technologies, such as MRI, EEG, or other forms of ultrasound, provide imaging of existing activity in neuronal tissue but cannot change it, so they can only measure correlation. Other forms of brain stimulation, such as transcranial magnetic or electrical stimulation, have been around for a long time, but they are blunt instruments and can only affect large areas of the cortex. Targeted transcranial ultrasound reaches the deep brain tissues that many theories of consciousness propose to be crucial and produces images with much higher resolution.

Aside from brain stimulation, the only way scientists can modify and monitor activity deep in brain tissue is through surgical interventions, which are both risky and invasive, presenting ethical challenges for studying healthy brains. “There are very few reliable ways to manipulate brain activity that are safe but also work,” said Matthias Michel, an MIT philosopher who studies consciousness and co-author of the new paper.

“This is truly the first time in history that we can modulate activity deep in the brain, just centimeters from the scalp, by looking at subcortical structures with high spatial resolution,” Freeman added. “There are a lot of interesting emotional circuits deep in the brain, but until now you couldn’t manipulate them outside of the operating room.”

Freeman and his colleagues are particularly interested in resolving a fundamental disagreement about the origins of conscious experience: Does it require higher-order mental processes, such as reasoning or self-reflection, linking the activity of different areas of the brain into a coherent whole? Or do specific patterns of localized activity – particularly in subcortical structures located at the back of the brain or at the back of the cortex – give rise to particular subjective experiences without the need for such interpretive overlays?

The new technology could answer a series of smaller questions that would help solve this larger puzzle. For example: what roles do the prefrontal cortex and subcortical structures play in conscious perception? Is consciousness local or distributed across networks? If it extends to distant brain regions, how are the different perceptions linked to produce a unified experience?

Some of the MIT researchers’ upcoming experiments will focus specifically on stimulating the visual cortex and higher-level areas of the frontal cortex. “It’s one thing to know whether these neurons were responding electrically,” Freeman argued. “It’s another thing to say whether a person has seen the light.”

Focused ultrasound won’t solve this difficult problem overnight, but it can help us start to see the light.

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Main image: Mahesh Patil / Shutterstock

• Kristen French

  Published on January 14, 2026

  Kristen French is an associate editor at Nautilus. She has worked in science journalism since 2013, reporting and writing articles and news for publications such as Wired, Backchannel, The Verge, And New York Review. She holds a master’s degree in science journalism from Columbia University.