A mouse runs through a maze, learning its twists and turns. It pauses, explores, makes decisions, and improves with practice. There is just one problem: This mouse has no cortex, the outer layer of the brain that many neuroscientists believe is responsible for conscious experience.
What, then, is it like to be that mouse? Most leading theories of consciousness claim that this mouse experiences nothing at all, existing as a sort of robot, behaving intelligently yet feeling nothing. If this seems counterintuitive to you, you’re not alone. In fact, the question of what this mouse experiences has become a flash point in the field of consciousness, and it has raised a deeper question: Are we looking for consciousness in the wrong place? New ultrasound-based technologies may offer a way to directly test this question in the human brain. But to understand how this technology fits in, we first need to step back and examine the problem more closely.
Locating Experience
On the central question of where conscious experience arises in the brain, there are generally two camps. On the one side is the traditional view, which says that consciousness arises from neural activity in the cortex. Many researchers in this camp view experience as being intertwined with cognition, which enables not only our ability to think abstractly but also to reflect on ourselves. These cognitive functions are widely understood to depend on the cortex. I could not write this sentence without a cortex, on that we all agree.
On the other side are those who argue that, yes, it is true that the cortex performs the brain’s heavy lifting, but maybe cognition itself is largely subconscious. In this view, the cortex may perform complex processing, but it does not, in and of itself, produce any felt experience. Rather, electrical signals are sent from the cortex to deeper brain structures, and it is in these dark recesses of the brain that neural activity produces our subjective experience, including everything from feelings of thirst to ruminations about democracy.
The appeal of these deep brain structures, the argument goes, is that their basic neural architectures are strikingly similar across vertebrates, having evolved over 500 million years ago. This was the period when our ancient aquatic ancestors transitioned from a relatively passive, bottom-dwelling organism to active participants in a dynamic predator-prey environment. Animals exhibiting this kind of behavior certainly look like they’re experiencing something, but then again, so does that mouse.
To move beyond speculation, we have to turn to humans. Unlike mice, human subjects can report what they feel, allowing us to directly link neural activity to subjective experience. Nearly a century ago, neurosurgeon Wilder Penfield pioneered this approach, electrically stimulating the exposed brains of awake patients during surgery. By applying small currents to different regions, he could evoke vivid sensations, memories, and percepts. He was mapping the brain not just in terms of function but in terms of experience itself.
Notably, Penfield observed that stimulation of deeper brain structures often produced more immediate and intense experiential effects than stimulation of the cortex. And yet, these insights stopped short of a definitive answer as to the neural locus of experience because the brain is such an extraordinarily interconnected system. In other words, it is difficult to disentangle cause from downstream effects.
Noninvasive Deep Stimulation
Today, a new technology may offer a way forward. Transcranial focused ultrasound technology allows us to noninvasively stimulate regions that are deep in the human brain, without the need for surgery. This is not a form of imaging. Rather than measuring brain activity, focused ultrasound delivers brief, targeted acoustic pulses that interact with neural tissue, producing changes in electrical activity within the stimulated region. The spatial resolution is roughly a few millimeters, comparable in size to a grain of rice. To achieve such precise stimulation, each subject’s brain anatomy is mapped using MRI, while CT or specialized MRI sequences are used to account for the effects of the skull on the ultrasonic beam.
With this tool, we can now noninvasively and systematically probe virtually any location in the human brain and ask what the subject experiences. Does such stimulation alter ongoing visual processing? Can we temporarily turn off part of the auditory system to prevent one from hearing their own inner monologue? If so, what does this tell us about the location in the brain that produces these percepts? Focused ultrasound represents a modern extension of Penfield’s vision: We can begin to map the neural basis of experience without all the fuss of brain surgery. Moreover, these tests can be integrated into modern experimental designs, drawing on tools from cognitive neuroscience, such as temporally precise masking paradigms. This may allow us to finally disentangle cause from effect in the brain’s complex networks.
If these experiments succeed, they could reshape one of the oldest questions in science: Where in the brain does experience arise? The answer may confirm the central role of the cortex, or it may point us toward deeper, more ancient structures that have been hiding in plain sight. Either way, the implications extend far beyond theory, touching on the physical basis of human joy and suffering. Perhaps, at last, we will understand what it is like to be that mouse.
— Daniel Freeman, PhD, is a technical staff member in the Advanced Materials and Microsystems Technologies Group at MIT Lincoln Laboratory. His research, using Openwater’s low-intensity focused ultrasound platform, focuses on the neural mechanisms and physical processes that underlie subjective experience.