The thalamus is a gateway, shuttling select information into consciousness.
How consciousness emerges in the brain is the ultimate mystery. Scientists generally agree that consciousness relies on multiple brain regions working in tandem. But the areas and neural connections supporting our perception of the world have remained their grasp.
A new study, published in Science, offers a potential answer. A Chinese team recorded the neural activity of people with electrodes implanted deep in their brains as they performed a visual task. Called the thalamus, scientists have long hypothesized the egg-shaped area is a central relay conducting information across multiple brain regions.
Previous studies hunting for the brain mechanisms underlying consciousness have often focused on the cortex—the outermost regions of the brain. Very little is known about how deeper brain structures contribute to our sense of perception and self.
Simultaneously recording neural activity from both the thalamus and the cortex, the team found a wave-like signal that only appeared when participants reported seeing an image in a test. Visual signals specifically designed not to reach awareness had a different brain response.
The results support the idea that parts of the thalamus “play a gate role” for the emergence of conscious perception, wrote the team.
The study is “really pretty remarkable,” said Christopher Whyte at the University of Sydney, who was not involved in the work, to Nature. One of the first to simultaneously record activity in both deep and surface brain regions in humans, it reveals how signals travel across the brain to support consciousness.
The Ultimate Enigma
Consciousness has teased the minds of philosophers and scientists for centuries. Thanks to modern brain mapping technologies, researchers are beginning to hunt down its neural underpinnings.
At least half a dozen theories now exist, two of which are going head-to-head in a global research effort using standardized tests to probe how awareness emerges in the human brain. The results, alongside other work, could potentially build a unified theory of consciousness.
The problem? There still isn’t definitive agreement on what we mean by consciousness. But practically, most scientists agree it has at least two modes. One is dubbed the “conscious state,” which is when, for example, you’re awake, asleep, or in a coma. The other mode, “conscious content,” captures awareness or perception.
We’re constantly bombarded with sights, sounds, touch, and other sensations. Only some stimuli—the smell of a good cup of coffee, the sound of a great playlist, the feel of typing on a slightly oily keyboard—reach our awareness. Others are discarded by a web of neural networks long before we perceive them.
In other words, the brain filters signals from the outside world and only brings a sliver of them into conscious perception. The entire process from sensing to perceiving takes just a few milliseconds.
Brain imaging technologies such as functional magnetic resonance imaging (fMRI) can capture the brain’s inner workings as we process these stimuli. But like a camera with slow shutter speed, the technology struggles to map activated brain areas in real time at high resolution. The delay also makes it difficult to track how signals flow from one brain area to another. Because a sense of awareness likely emerges from coherent activation across multiple brain regions, this makes it more difficult to decipher how consciousness emerges from neural chatter.
Most scientists have focused on the cortex, with just a few exploring the function of deeper brain structures. “Capturing neural activity in the thalamic nuclei [thalamus] during conscious perception is very difficult” because of technological restrictions, wrote the authors.
Deep Impact
The new study solved the problem by tapping a unique resource: People with debilitating and persistent headaches that can’t be managed with medication but who are otherwise mentally sharp and healthy.
Each participant in the study already had up to 20 electrodes implanted in different parts of the thalamus and cortex as part of an experimental procedure to dampen their headache pain. Unlike fMRI studies that cover the whole brain with time lag and relatively low resolution, these electrodes could directly pick up neural signals in the implanted areas with minimal delay.
Often dubbed the brain’s Grand Central Station, the thalamus is a complex structure housing multiple neural “train tracks” originating from different locations. Each track routes and ferries a unique combination of incoming sensations to other brain regions for further processing.
The thalamus likely plays “a crucial role in regulating the conscious state” based on previous theoretical and animal studies, wrote the team. But testing its role in humans has been difficult because of its complex structure and location deep inside the brain. The five participants, each with electrodes already implanted in their thalamus and cortex for treatment, were the perfect candidates for a study matching specific neural signals to conscious perception.
Using a custom task, the team measured if participants could consciously perceive a visual cue—a blob of alternating light and dark lines—blinking on a screen. Roughly half the trials were designed so the cue appeared too briefly for the person to register, as determined by previous work. The participants were then asked to move their eyes towards the left or right of the screen depending on whether they noticed the cue.
Throughout the experiment the team captured electrical activity from parts of each participant’s thalamus and prefrontal cortex—the front region of the brain that’s involved in higher level thinking such as reasoning and decision making.
Unique Couplings
Two parts of the thalamus sparked with activity when a person consciously perceived the cue, and the areas orchestrated synchronized waves of activity to the cortex. This synchronized activity disappeared when the participants weren’t consciously aware of the cue.
The contributions to “consciousness-related activity were strikingly different” across the thalamus, wrote the authors. In other words, these specific deep-brain regions may form a crucial gateway for processing visual experiences so they rise to the level of perception.
The findings are similar to results from previous studies in mice and non-human primates. One study, tracked how mice react to subtle prods to their whiskers. The rodents were trained to only lick water when they felt a touch but otherwise go about their business. Each mouse’s thalamus and cortex sparked when they went for the water, forming similar neural circuits as those observed in humans during conscious perception. Other studies in monkeys have also identified the thalamus as a hot zone for consciousness, although they implicate slightly different areas of the structure.
The team is planning to conduct similar visual experiments in monkeys to clarify which parts of the thalamus support conscious perception. For now, the full nature of consciousness in the brain remains an enigma. But the new results offer a peek inside the human mind as it perceives the world with unprecedented detail.
Liad Mudrik at Tel Aviv University, who was not involved in the study, told Nature it is “one of the most elaborate and extensive investigations of the role of the thalamus in consciousness.”
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